acoustic measurements of cascade plateau orange roughyroughy (kloser et al. 2002). in order to...

Acoustic measurements ofCascade Plateau orange roughy

R04/1086 January 2007

Protecting our fishing futurewww.afma.gov.au

Box 7051, Canberra Business Centre, ACT 2610 Tel (02) 6272 5029 Fax (02) 6272 5175 AFMA Direct 1300 723 621

• R. J. Kloser • T. E. Ryan • G. J. Macaulay • M. E. Lewis

CSIRO Marine and Atmospheric Research

Acoustic measurements of Cascade Plateau orange roughy

R. J. Kloser, T. E. Ryan, G. J. Macaulay and M.E. Lewis

Final Report to AFMA (R04/1086)

January 2007

Enquiries should be addressed to: R. J. Kloser: CSIRO Marine Research, PO Box 1538, Hobart 7001, Australia. Email: [email protected]

ISBN: 978 1 921232 46 6

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EXECUTIVE SUMMARY

Management of Cascade Plateau orange roughy is based on an absolute estimate of spawning biomass using industry vessel acoustics. Large errors can occur when quantifying orange roughy with vessel mounted acoustic systems (Kloser, 1996; Kloser et al., 2002; McClatchie and Coombs, 2004) and are a major concern with the 2003-2005 Cascade Plateau biomass estimates. This report demonstrates that these biomass estimates are sensitive to assumptions of species identification that can not be resolved with the commercial trawling currently used. An in-house designed self contained two-frequency acoustic probe, lowered from industry vessels, showed that the region surveyed contained a high number of fishes with small and large gas bladders. Therefore rigorous non trawl based species identification are required at the Cascade Plateau if robust and repeatable estimates of relative and absolute biomass are to be used for management purposes. This report reinforces earlier work on orange roughy schools off the east coast of Tasmania (e.g. Kloser et al., 2002) where deeply towed multiple frequency systems have been used for quantitative biomass estimates. It is recommended that similar methods be considered for future Cascade Plateau acoustic surveys using existing multifrequency towed bodys (e.g. MUFTI-2 optic fibre based) or the development of a battery powered acoustic system purpose built for towing at speed from industry vessels.

To convert the acoustic survey data to orange roughy biomass requires the expected echo (target strength, TS) from a single orange roughy. An acoustic probe lowered to within 50 m of schools was unable to resolve single orange roughy targets. The volume reverberation frequency difference 38 120Sv −Δ of orange roughy in schools was -4 to -5 dB and was used as the basis of classifying both schools and single tracked targets. The density of orange roughy in schools was well in excess of the horizontal resolution of the acoustic system and single fish targets found outside schools were not classified as orange roughy based on the frequency difference selection criteria.

The finite-difference model of orange roughy backscatter has been able to demonstrate a frequency difference 12038−ΔTS that is in agreement with experimental observations of schooling orange roughy. This is a major advance for the modelling of orange roughy and gives more confidence in using models to estimate orange roughy target strength. There remains some uncertainty with the model results due to calibration and pressure and temperature differences in material properties. Orange roughy are composed of wax ester and changes in material properties of these wax esters will have a large impact on target strength (Kloser and Horne 2003). To the extent possible the adjustment for the material properties within the model is consistent with experimental data on the changes of wax esters with depth.

Prior to this study the target strength uncertainty for a 43 cm standard length orange roughy was -47.9 dB to -51.5 dB, approximately a factor of 2.3 (Table 4.1). All previous measurements were based on extrapolated measurements of smaller < 36 cm orange roughy. Based on modelling of the large Cascade Plateau orange roughy a target strength of -49.4 dB re 1 m2 at 38 kHz is proposed for a 43 cm standard length fish. Using this updated target strength measurement the current Cascade Plateau biomass assessments will reduce by 10 %. Potential error sources with the model target strength estimate are bounded by a factor of 0.71 to 1.64 effect on biomass estimates (Table 4.1). Further work is outlined below that would reduce this uncertainty.

Uncertainties in the modelled target strength measurements used in this report could be reduced by:

o More fish run through the model of various lengths and sex.

o Calibration of the model with more standard targets of known reflectance.

o Sensitivity of the model to pressure and temperature material properties assumptions.

In situ measurements are required to validate the model target strengths at specific tilt angles with:

o Higher resolution acoustic system to observe single targets within schools. Based on a preliminary analysis a 3 degree acoustic system at the -10 dB power levels would be suitable.

o Video recording of in situ acoustic targets to obtain identification, length and orientation measurements. This could be obtained by placing an acoustic and video package on the headline of a trawl.

The biomass and density of prey species could be estimated from the multi-frequency target strength data. This information would be useful for longer term ecosystem based fisheries management objectives.

CONTENTS

1. INTRODUCTION................................................................................................................. 7

1.1 Background and need ............................................................................................ 7

1.2 Objectives................................................................................................................ 7

2. TARGET STRENGTH OF ORANGE ROUGHY................................................................. 8

2.1 Introduction ............................................................................................................. 8

2.2 Modelling ................................................................................................................. 9

2.3 Conclusion ............................................................................................................ 15

2.4 In situ target strength measurements ................................................................ 27

3. SPECIES IDENTIFICATION............................................................................................. 35

3.1 Introduction ........................................................................................................... 35

3.2 Material and Methods ........................................................................................... 36

3.3 Results ................................................................................................................... 38

3.4 Discussion............................................................................................................. 46

4. DISCUSSION AND CONCLUSION ................................................................................. 48

4.1 Management implications.................................................................................... 52

4.2 Recommended future work. ................................................................................ 52

5. REFERENCES.................................................................................................................. 53

INTRODUCTION 7

1. INTRODUCTION

1.1 Background and need The Cascade Plateau orange roughy fishery is the largest orange roughy fishery in Australian waters with a total allowable catch in 2004 of 1500 tonnes reducing to 500 tonnes in 2007. It is situated 260 km south east of Tasmanai (43:56.44S, 150:27.37E) where it rises from the deep ocean to a minimum depth of around 600 m. A recent stock assessment of the resource based on an industry funded acoustic survey, catch records, length and age data estimated that the pre-fishery population (1989) is not large, ~50,000 t (Wayte 2006). The greatest influence on the stock assessment result was the acoustic biomass estimate from 2005 (Ryan 2006). The acoustic snapshot biomass survey carried out in 2005 needed correction for the fishes acoustic target strength and proportion of the population in schools at the time of the survey. Estimates of these parameters were based on the length of the spawning season and corrections used at the Eastern Zone and NZ assessments. The largest uncertainty was the target strength of the larger orange roughy at the Cascade Plateau compared to the Eastern Zone orange roughy ( Ryan 2006). Incorrect species identification may result in a significant positive bias due to inclusion of gas bladdered fish within the region of schooling orange roughy (Kloser et al. 2002).

In order to resolve the uncertainty in the assessment and move from a more precautionary management response in the short term, improvements in both species identification and measurement of orange roughy target strength are required. Traditionally, obtaining target strengths of deep-water species has required the use of expensive equipment using deep towed bodies and large vessels. Recently CSIRO Marine Research has developed a portable battery powered device (Drop TS) that can be lowered from commercial vessels into fish schools greatly reducing the need for dedicated vessels. The device (Drop TS) contains two frequencies 38 kHz and 120 kHz that can be used for both target strength determination and species discrimination. These frequencies have been proven to be effective in discriminating orange roughy from other species in the Eastern Zone orange roughy fishery (Kloser et al., 2002).

Removing the uncertainty surrounding species identification and the fishes target strength would help the short term (5 years) management decisions of the fishery. The low cost technique developed could be applied to other schooling fisheries where industry acoustic surveys are being used or considered (e.g. eastern and southern zone orange roughy, blue grenadier and small pelagics).

1.2 Objectives o Determine the target strength of Cascade Plateau orange roughy.

o Estimate the species composition of orange roughy surrounding the schools.

8 TARGET STRENGTH OF ORANGE ROUGHY

2. TARGET STRENGTH OF ORANGE ROUGHY

2.1 Introduction Life-history characteristics of exploited deep-water (> 600 m) fish species such as orange roughy (Hoplostethus atlanticus) differ from most continental-shelf species: long lived, late age of maturity, slow growth, and low fecundity (Koslow et al., 2000). Orange roughy fisheries have existed in New Zealand and Australian waters for the last two decades and new orange roughy fisheries have developed off Namibia, Chile and in high-seas regions (Branch, 2001). Acoustic surveys are used in Australia, New Zealand, and Namibia on spawning aggregations to quantify stock abundance (Kloser et al., 1996; McClatchie et al., 1999; Boyer and Hampton, 2001). In the Australian Eastern Zone fishery, a relative acoustic index is used to monitor population trends (Bax, 1999), but an absolute biomass estimate is being used in the Cascade zone fishery as no surveys index is available. Accurate, absolute abundance estimates rely on accurate target strength (TS) measures when using the echo-integration technique (Dragesund and Olsen, 1965).

TS measurements of fish can be divided into ex situ (i.e. tethered experimental) and in situ (i.e. split-beam field) methods (Foote, 1991). Ex situ measurements of orange roughy TS are uncertain due to entrained air bubbles within the body and changes in the density and sound speed of lipids when fish are measured at the surface (Kloser et al., 1997; McClatchie et al., 1999; Barr, 2001). The direct in situ method of TS measurement is reported to be the best (Ehrenberg, 1983), although there are numerous acoustic and biological sampling uncertainties that need to be resolved. In situ TS measurements require a single fish be located within the pulse-resolution volume. This is traditionally obtained by lowering the transducer to within 50 – 100 m of the targets (Kloser et al., 1997) and using short pulse lengths. Even at close range, multiple targets can be accepted by the single-fish classification algorithms (Soule et al., 1995). To minimize single-target classification error, TS measurements are compared to volume reverberation to filter multiple-target acceptances (e.g. Sawada et al., 1993; Gauthier and Rose, 2001). Single- and split-beam fish tracking can also be used to ensure that single fish are being resolved (Demer et al., 1999).

In situ TS measurements need to reflect fish size, sex ratio, and maturity stage characteristics of the surveyed population. It is best to obtain in situ TS measurements at the same time and location as the acoustic survey. Are target strength measurements of fish at the periphery of schools representative of the species and sizes of fish within schools? This problem is exaggerated in the deep ocean where schools of orange roughy are often associated with deep scattering layers of micronekton (i.e. small fishes, crustaceans, squid). Lantern fishes (Myctophidae) are the most numerous component of the micronekton. Small individuals (<10 cm total length (TL)) have the same TS at 38 kHz as orange roughy (35 cm standard length (SL)) due to resonant scattering of the gas-filled swimbladder at depth (Kloser et al., 2002). To date all target strength measurements have been obtained from orange roughy of mean standard length between 33 and 36 cm (Kloser et al., 2000, Barr and Coombs 2001, Kloser and Horne 2003). Conversion of these measurements to Cascade orange roughy mean standard length of 43 cm needs to assume a length or weight conversion factor (Doonan and Bull 2001, Hampton and Soule 2002, Honkalehto and Ryan 2003).

Based on ex situ surface measurements of 16 “live” orange roughy (28 to 37 cm SL) a tilt averaged target strength, <TS>=16.374 log10 Standard Length - 71.621 where <TS> is in dB re 1m2 and Standard Length is in cm was estimated (Mclatchie et al., 1999). These ex situ surface measurements of orange roughy that usually live at 800 m depth may have been affected by micro bubbles, changes in material properties and orientation assumptions (e.g. Kloser and Horne 2003). Therefore this target strength relationship has been adjusted by

TARGET STRENGTH OF ORANGE ROUGHY 9

Doonan and Bull (2001) and Hampton and Soule (2002) base on in situ measurements by Barr and Coombs (2001) and Kloser et al. (2000) respectively. In situ multi-frequency target selection criteria measurements have also not supported the ex situ measurements (Kloser and Horne 2003). A summary of the in situ measurements and formula for 35 cm and 43 cm standard length orange roughy are shown in Table 4.1. Based on the currently used target strength to length relationship (Honkalehto and Ryan 2003) the biomass could vary by a factor of 0.56 to 1.64 (Table 4.1). The largest uncertainties in the in situ target strength data for orange roughy are with species identification and targets being representative of the population.

To resolve species identification difficulties, broadband or multi-frequency methods can be used to classify dominant acoustic target groups (Barr, 2001; Kloser et al., 2002, Barr and Coombs 2005). Matching TS samples with pelagic and demersal trawling in time and space is difficult due to selectivity the depth (600 to 1200 m) and ruggedness of the seabed (Kloser et al., 1997; McClatchie et al., 2000). Echograms show that orange roughy schools extend up to 150 m in the water column but are difficult to capture in midwater, presumably due to diving avoidance (Koslow et al., 1995b). The effect of diving on in situ target strengths can lower the average TS by 3 dB (Kloser et al., 2000). What is less certain is the effect of deep-towed acoustic sensors on in situ TS measurements. Fish may change their tilt angle without downward movement in the presence of the lowered transducer. This “passive” avoidance may differ from natural schooling-fish orientations encountered during acoustic surveys.

The uncertainty in frequency-dependent TS measurements, introduced by variability in fish identification, size, species composition, and tilt distribution, can be explored using backscatter models (e.g. Love, 1978, Clay and Horne, 1994; Macaulay 2002). Interpretation of model results relies on knowing the dominant scattering species, their size, orientation, anatomy, and material properties. The sensitivity of backscatter models to sound-speed and density changes may also influence accuracy of model predictions (Chu et al., 2000, Kloser and Horne 2003). Kirchoff Ray Mode (KRM) modelling of simple orange roughy shapes have not been able to reproduce the ensemble volume reverberation scattering strength dB difference between 38 kHz and 120 kHz (Kloser and Horne 2003). This has placed in doubt the validity of those simple models to accurately characterise orange roughy scattering and sensitivity to orientation and material properties. We propose to use computer aided tomography (CAT) scans to obtain fine scale measure of density changes in the fish at atmospheric pressures and room temperatures. It is then proposed to estimate the changes in these material properties at orange roughy depths of 800 m and 5 degrees temperature. This fine scale material properties data will then be used in a finite difference model that is able to incorporate the complexities of the scattering within the non gas bladdered orange roughy (Macaulay 2002).

To determine the target strength of orange roughy at the Cascade Plateau spawning fishery acoustic measurements of single and multiple orange roughy together with backscatter model predictions at two frequencies are explored.

2.2 Modelling

Based on contribution by Gavin Macaulay CSIO6301 NIWA November 2006


2.2.1 Methods

Theoretical basis The propagation of sound waves through a fish and surrounding water were modelled using the equations of motion for a lossless fluid with variable sound speed and density ;

ρρ∇⋅∇

−∇=∂∂ pp

tp

c2

2

2

21 , (Eq. 2.1)

where c is sound speed, p acoustic pressure, t time, and ρ density. Sound speed and density vary with space, while the pressure varies with space and time.

Solution of the equations of motion A three-dimensional finite-difference time-domain (FDTD) algorithm was used to solve equation 2.1. The differencing equation used was a centred second-order spatial and temporal derivative scheme. All simulations were run with a Courant-Friedrichs-Levy (CFL) number of 0.5. The FDTD region was surrounded by 12 perfectly matched layers (PML) to prevent reflections from the boundaries of the domain. The formulation of Liu & Tao was used.

The model as implemented does not allow for shear waves caused by directionally dependent material properties (for example, as found in bone), or for acoustic attenuation. For the frequency range and propagation distances of interest here, attenuation is insignificant. The effect of not modelling directionally dependent material properties is unknown.

Density and sound speed from CT scans Computed tomography scans were taken from two orange roughy – details on each fish are given in Table 2.1. Both fish were caught at the Cascade Plateau in July 2005. The CT scanner used was a Siemens Somatom Sensation 16 located at Royal Hobart Hospital, Hobart, Australia. Both scans were done on 11 November 2005. A scan was taken every 2 mm, with a slice thickness of 2 mm. Exposure was 3 seconds per slice, with a tube voltage of 120 kV and current of 15 mA. The field of view was 243 mm, giving a transverse image resolution of 0.475 mm (the 3rd dimension of the resolution volume is the slice thickness, 2 mm). The ambient temperature of the fish and room was ~21ºC. A proprietary processing algorithm was used to produce the CT images from the raw x-ray data. The data output from the scanner were one HU (Hounsfield) number for each resolution volume. Conversion from these data to physical density was achieved via

2.100606.1 +×= dρ , (Eq. 2.2)

where ρ is the tissue density in kg/m3 and d the HU number from the scanner. The relationship between HU number and density is particular to each CT scanner, but no scans of objects of known density were available for the scanner, and a generic relationship based on measurements from other CT scanners were used instead. Such a relationship is valid for at least the range , corresponding to densities of 873 to 1225 kg/m3 and covers most of the soft tissue in the fish. Note that this range does not include the density of the wax-ester filled swimbladder in orange roughy, nor bone, but the calculated linear relationship between HU number and density is expected to be valid for these lower and higher densities .

207125 ≤≤− d

Sound speed was calculated using the density to sound speed relationship given by Aroyan . Few detailed measurements of sound speed and density exist for orange roughy. Measurements of sound speed at a range of pressures and temperatures in lipid extracted from orange roughy gave a value of 1470 m/s at 12oC and atmospheric pressure. This lies within the range of values obtained using Aroyan’s relationship on orange roughy soft tissue. A


more precise comparison of these results was not possible because the lipid was obtained from the entire fish and does not represent any identifiable part of an orange roughy. Phleger & Grigor measured the density of lipids in orange roughy and obtained a value of 903 kg/m3 at 6 ºC, and found the source of the lipid (swimbladder, skin, bone, etc) made little difference to the density. This compares to the range of values obtained from the orange roughy swimbladder from CT scans of 840-930 kg/m3 ºC. The average of fish flesh density given by Shibata compares well to the average density obtained from the CT scans (1050 kg/m3 and 1052 kg/m3 respectively).

As in Aroyan , all points with an HU value in the range 150–300 were given a constant sound speed of 1730 m/s, and all points above 300 were given a sound speed of 3450 m/s and taken to represent bone. In addition, all HU values less than –200 were taken to be air and the sound speed and density were set to that of seawater. This scheme attempts to account for the effect of density blurring in the transition from bone to soft tissue, and soft-tissue to air, and also re-immerses the fish in seawater (if scanned in air).

The CT scan data from each fish were re-sampled using linear interpolation to give a grid spacing of 1.25 mm in each dimension. This was sufficient to model frequencies up to about 120 kHz, with 10 or more grid points per acoustic wavelength. The same grid was used for the simulations at 38 and 120 kHz.

The CT scans from the two fish contained, on occasion, voids in the fish flesh, with a density similar to that of air. The mechanism for the formation of these voids is unknown. If left in the model they would generate strong acoustic reflections and degrade the accuracy of the simulation results. To avoid this, each void was filled in with a material having the same properties as the surrounding fish tissue. Orange roughy also have cavities in external bone structure which are thought to contain oil, but did not when the fish were scanned. These pockets of oil were reintroduced in the model by identifying such cavities and replacing the CT scan measurements with those of a fluid with density 990 kg/m3 and sound speed 868 m/s. These two alterations to the CT scan data were an attempt to correct for perceived damage to the fish between their natural state and the state during the CT scans. Areas of water were also present in CT scan data and these were replaced by a fluid with properties close to that of air (and consequently converted to seawater during the simulation).

Simulations The fish data were each placed in a computation region that was 6.25 mm larger than the fish and extended for approximately 112 mm above and below the fish – see Figure 2.1. The sound speed and density in this region, and in the region immediately surrounding the fish, were set to those of seawater with a salinity of 35.0 ppt, temperature of 6.0 ºC and a pressure of 10 MPa (nominally 1000 m), giving a density of 1032 kg/m3 as calculated from the UNESCO equation of state given by Gill and sound speed of 1491 m/s, calculated from an algorithm given by Fofonoff & Millard .

The CT scan data comprising each fish were rotated to a range of angles from –40 to +40 degrees, at 2 degree intervals, and a separate simulation run for each angle (negative angles denote a head down attitude). This gave reflectivity estimates for a range of fish tilt angles.

It was important to ensure that any reflections from the computational region boundaries were less than those expected from the fish. Many absorbing boundary conditions available in the literature have reflection coefficients in the range –10 to –40 dB, and for fish with target strengths in the range –30 to –50 dB this can cause echoes from the fish to be contaminated with echoes from the boundaries to the extent that the results are meaningless. For this


reason, perfectly matched layers (PML) were used to absorb outgoing waves. In addition, the normal component of particle velocity on the outermost layer of the PML was also set to zero, simulating a hard boundary. The fish were also placed in the centre of the computation region with sufficient distance between the fish and the top and bottom boundaries to minimise overlap between the reflections from the fish and the boundaries. The distance above the fish must be sufficient to allow the incident pulse to be clear of the receivers before the first part of the fish reflection arrives. In general this requires that the distance from the receivers to the fish be at least half the length of the incident pulse. Another constraint on the distance is the requirement to keep the computation region at a manageable size for computation time and computer memory reasons. Reflections from the fish that then hit the nearby vertical boundaries are not a problem because their amplitude is already considerably reduced through reflection off the fish, and the resulting boundary reflection is then considerably less than the fish reflection itself and does not markedly affect the original fish reflection. Note that when the fish was rotated the vertical extent of the fish increased, resulting in larger computation regions for larger tilt angles. This causes the incident pulse reflection from the bottom boundary to arrive at the receivers at different times for different tilt angles.

A pulse of length 0.32 ms was introduced into the domain, travelling in the positive x-direction and located 5 grids in from the PML boundary. The model was time-stepped until a steady state had been reached – in general the steady state was a pressure of zero throughout the domain, indicating that all of the incident and reflected pulses had reached the absorbing boundaries and been absorbed.

The pressure at a range of points in the domain was recorded. These data were decimated from the FDTD model sample rate of 5.52 MHz to 394.29 kHz using the Matlab decimate command (which applies an eighth-order lowpass Chebyshev Type I filter and then resamples the smoothed signal). They were then quadrature demodulated using the Matlab demod command (which multiplies the signal by a cosine and sine of the modulation frequency and then applies a fifth-order Butterworth lowpass filter). The final step was to apply a filter to duplicate some of the filtering applied in a real echosounder. In this instance a raised cosine finite impulse response lowpass filter with a cutoff frequency of 10 kHz, and transition bandwidth 6 kHz was used. The effect of the transmitter and transducer on the transmitted and received pulse was not explicitly modelled, but would be partially accounted for by the last filter in the processing chain.

These operations gave the amplitude and phase of the reflected pulse as a function of time at a specific point near the fish. The backscattered target strength (TS) of the fish was then calculated via

⎟⎟⎠

⎞⎜⎜⎝

⎛=

ipprTS 10log20 , (Eq. 2.3)

where p is the peak of the demodulated and filtered pressure envelope at range r from the centroid of the fish and pi the peak of the demodulated and filtered pressure envelope incident upon the fish. This gives a target strength estimate at specific incident and reflection angles. Multiple simulations at a range of tilt angles are then required to estimate tilt-averaged target strength. For orange roughy the centroid position was taken to be the topmost part of the swimbladder when the fish was level. The ranges at which the fish echoes were recorded varied from 0.3 to 0.55 m. This is likely to be close to the near-field of the fish and will contribute some uncertainty to the target strength estimates. However, observation of the development and propagation of the scattered waves indicated that by a range of about 0.3 m


the scattering field was reasonably stable in time and was representative of the far-field pattern. To remove some of the variability due to the highly directional nature of the backscattered wave field and the potential for near-field effects, the average of the echo amplitude at all recording points within 5 wavelengths of the normal axis was used in the calculation of the backscattered target strength.

The accuracy of the model was tested by simulating the reflection from a fluid sphere immersed in a fluid with properties identical to the seawater described earlier. A sphere of radius 25 mm and density 1324.2 kg/m3 and sound speed 1730 m/s was placed in the centre of a computation domain (of size 500x250x125 mm) with density 1032.11 kg/m3 and sound speed 1491.22 m/s (seawater). The backscattered target strength at 38 and 120 kHz at a range of 243.75 mm from the sphere centre was calculated in the same manner as for the orange roughy simulations and compared to a theoretical solution obtained from the formulae in Anderson .

Images of the scattered wave field are presented in various figures in this report. The colours in these images represent acoustic pressure relative to ambient pressure – red is high, blue is low and green is ambient. The fish outline is indicated by the black shape. The black rectangle indicates the boundary between the PMLs and the interior. As waves travel into the PML region they slow down and are absorbed, and generate minimal reflections back into the interior. The t value given in each figure is the time in ms since the simulation started.

2.2.2 Results

The backscattered target strength results at 38 kHz and 120 kHz are given in Figure 2.2 for fish or7 and or8. As has been observed from ex-situ experiments on orange roughy, the echo amplitude is relatively constant with tilt angle and the model results are consistent with this. There is also a large difference in target strength between 38 and 120 kHz, with the 120 kHz values higher than the 38 kHz. This is consistent with the deductions in Kloser et al., , although the difference reported there was 4.6 dB, while the difference in the model results is 6–8 dB.

The results from the backscattering simulations on a fluid sphere are given in Table 2.2 and are in reasonable agreement with the theoretical estimates. This simple test case indicates that the model target strength at 38 kHz could be about 1.3 dB higher than actual and 0.8 dB higher than actual at 120 kHz, giving a relative difference of 0.5 dB in favour of 38 kHz. Further comparisons against more complicated objects, and for which theoretical or proven solutions are available, would be required for increased confidence in the model results.

Images of the scattered wave field from fish or8 at 0 and 40 degrees tilt at 38 kHz are given in Figure 2.3 and Figure 2.4 respectively and Figure 2.5 and Figure 2.6 for 120 kHz. Similar images from fish or7 are given in Figure 2.7 to Figure 2.10. To provide some context for these images, an x-ray style image (derived from the CT scans) of each fish is given in Figure 2.11 to allow the identification of the major body parts in each fish and the relationship to the scattering characteristics. The acoustic impedance throughout the fish can also be calculated from the CT data and this provides insight into which parts of the fish are likely to scatter strongly – Figure 2.13 contains the impedance plot for fish or8. Regions where the impedance changes rapidly generate strong reflections, while areas with slow or small impedance change generate weak reflections.

Several observations can be made from the wave field images. At 38 kHz the scattering is very directional, with much of the scattered energy travelling out in relatively few directions.


At 120 kHz the scattered energy travels out in many more directions, and hence is more evenly distributed over the computational domain. There also appears to be more energy reflected at 120 kHz than 38 kHz, as is clear from the backscattered target strengths plots in Figure 2.2. Strong scatter occurs from the head of the fish at both frequencies, presumably due to the dense bone, otoliths and eyes.

Some interesting shadows form below the fish, particularly at 120 kHz. There is a small area of ambient pressure below the ventral spine, which is probably caused by the dense spine casting a shadow. A larger shadow region also exists below the head, although this appears to be caused by a combination of acoustic shadowing and destructive interference of the forward scattering. The tail scatters more sound that would be expected from such a small object, presumably because it contains high density material.

The form of the wave field inside the fish is different to that outside the fish mainly because waves are travelling into and out of the page when inside the fish, whereas most of the waves outside the fish are travelling roughly parallel to the page.

From observations of time-series movies of the wave field, it is clear that the scattering reaches a steady state, as would be expected from the rule-of-thumb that at least 10 wavelengths are required to achieve this – the 0.32 ms pulse at 38 kHz is 12 wavelengths long, and the 120 kHz is 38 wavelengths long. In particular, the complex backscattered wave field pattern at 120 kHz remains constant while the incident pulse is travelling through the fish.

After the incident wave has passed through the fish there is a residual reverberation inside the fish, particularly at 120 kHz (see Figure 2.12). Note also how the character of the wave field is different inside and outside the swimbladder. This reverberation gives rise to a distinct tail in the backscattered pulses, an example of which is given in Figure 2.14. This is for fish or8 at 120 kHz and shows the backscattered pulses at 5 different tilt angles at the point directly above the centroid position used when calculating the target strength. The incident pulse was 0.32 ms long, while the backscattered pulses last for up to 0.7 ms. Similar plots for or8 at 38 kHz are given in Figure 2.15. The reflection of the incident pulse off the bottom boundary is also present in these plots as a pulse at the 1–1.2 ms time, particularly in the bottom pane of Figure 2.15. The position of this pulse varies somewhat due to the variable distance from the receiver to the bottom boundary.

The 38 kHz images show a high frequency artefact that is removed by the band-pass filtering when calculating the backscattered target strength (but is not removed in the wave field images). It is not present in the 120 kHz mainly because the grid size is insufficient to allow for frequencies much above 120 kHz.

The wave field in Figure 2.4 shows an interesting change in phase in the wave field below the swimbladder. The sound speed in the swimbladder is less than in the rest of the fish and it is likely that this phase change is generated at the merging of the delayed and non-delayed waves. A similar, but less noticeable situation is also visible in Figure 2.3.

The scattering models of the fish detailed in this report are computationally intensive, with the shortest simulation taking 43 hours of computer time and the longest 105 hours on AMD64 X2 4600 dual core processors with 4GiB of RAM. In total, the simulations took 7200 hours of computer time and were performed on a cluster of eight computers. Individual model runs required between 1.0 and 3.6 GiB of computer RAM to store the model variables (pressure and particle velocity) on the FDTD grid.


2.3 Conclusion

An acoustic scattering model has been applied to two Australian orange roughy at 38 and 120 kHz and a range of tilt angles from –40 to +40°. Estimates of the target strength at these angles have been calculated and are broadly in line with in-situ estimates. The models indicate that the target strength at 120 kHz is considerably higher than at 38 kHz, which are consistent with the in-situ estimates. Images of the scattered wave field at various stages in the scattering event are presented, and aspects of the field discussed in relation to the scattering processes. Table 2.1. Details on the fish that were CT scanned. Lengths are standard length.

Fish Length (cm) Sex Stage Weight (g) Voxel size (mm) OR7 46.4 F 5 3272 0.475, 0.475, 2.0 OR8 37.9 M 4 1745 0.475, 0.475, 2.0

Table 2.2. The theoretical and model backscattered target strength (dB re 1m2) for a fluid-filled sphere of radius 25 mm.

Frequency (kHz)

Theoretical TS (dB re 1m)

Model TS (dB re 1m)

Difference (dB re 1m)

38 –49.3 –48.0 1.3 120 –50.3 –49.5 0.8


Figure 2.1. The simulation geometry indicating the location of the fish. The arrow indicates the direction of travel of the incident wave, and the dots the positions of receivers in the model. The six boundary surfaces absorb incident waves using the PML technique. The dimensions of the volume are varied to suit the size and rotation angle of the fish. The orange roughy drawing is from Paul .


Figure 2.2. Target strength to tilt-angle relationships for OR7 (upper panel) and OR8 (lower panel) at 38 kHz (solid line) and 120 kHz (dotted line) from the model. Dots indicate the angles at which model results were available.

-40 -30 -20 -10 0 10 20 30 40-52

-50

-48

-46

-44

-42

-40

Tilt angle (°)

TS (d

b re

1m

2 )

-40 -30 -20 -10 0 10 20 30 40-54

-52

-50

-48

-46

-44

-42

-40

-38

Tilt angle (°)

TS (d

b re

1m

2 )


Figure 2.3. Scattered field from fish or8 at 0º tilt and 38 kHz at 0.46 ms into the simulation.



Figure 2.7. Scattered field from fish or7 at 0 º tilt, 38 kHz, and 0.49 ms into the simulation.



Figure 2.11. X-ray style images of or8 (upper) and or7 (lower). The colours represent x-ray density (red is high, blue is low).


Figure 2.12. An example of the internal reverberation that remains after the incident wave has passed through the fish. Fish is or7 and frequency is 120 kHz.


Figure 2.13. Impedance of fish or8 relative to the impedance of seawater. The red and yellow areas are bone and the otoliths. The swimbladder is the dark blue region below the spine.


Figure 2.14. Backscattered pulses from fish or8 at 120 kHz and a range of tilt angles. The blue line is the raw backscattered amplitude and the red line the filtered backscattered amplitude. The incident pulse was 0.32 ms in length. The x axis has an arbitrary zero, and the y axis has an arbitrary scale proportional to acoustic pressure in Pa.

0 0.5 1 1.50

5000

10000α = -40°

0 0.5 1 1.50

5000

10000α = -20°

0 0.5 1 1.50

5000

10000α = 0°

0 0.5 1 1.50

5000

10000α = 20°

0 0.5 1 1.50

5000

10000α = 40°

Time (ms)


Figure 2.15. Backscattered pulses from fish or8 at 38 kHz and a range of tilt angles. The blue line is the raw backscattered amplitude and the red line the filtered backscattered amplitude. The incident pulse was 0.32 ms in length. The x axis has an arbitrary zero, and the y axis has an arbitrary scale proportional to acoustic pressure in Pa.

0 0.2 0.4 0.6 0.8 1 1.2 1.40

1000

2000 α = -40°

0 0.2 0.4 0.6 0.8 1 1.2 1.40

1000

2000 α = -20°

0 0.2 0.4 0.6 0.8 1 1.2 1.40

1000

2000 α = 0°

0 0.2 0.4 0.6 0.8 1 1.2 1.40

1000

2000 α = 20°

0 0.2 0.4 0.6 0.8 1 1.2 1.40

1000

2000 α = 40°

Time (ms)


2.4 In situ target strength measurements

2.4.1 Methods In situ target strength measurements were collected using a specially fabricated battery operated Drop TS probe. The Drop TS probe houses split beam transducers at 2 frequencies 38, and 120 kHz using Simrad EK or ES60 transceivers and motion measurement from a Microstrain 3DM-GX1. The acoustic and motion data were logged to a PC and the system powered by batteries for 3 to 5 hours duration. The specifications of the two split beam transducers (38 and 120 kHz) and calibration constants are outlined in Table 2.3. Figure 2.16 shows the Drop TS being deployed from the vessel. Figure 2.16. Drop TS system

Main pressure case containingEK60 transceivers (38 and 120kHz)plus onboard PC

Battery housing

38kHz transducer 120kHz transducer

Tracklink positioning beacon

To calibrate the system a 38.1 mm target sphere was suspended 11.6 m below the transducers and the Drop TS probe lowered to depth. Sphere target strength was calculated at the nominal continuous frequency using MacLennan (1981) with a correction applied depending on the pulse length (Table 2.3). The sound speed and absorption were calculated using the formula from MacKenzie (1981) and Francois and Garrison (1982b) respectively on data from a temperature probe and salinity from a WOCE model.


Table 2.3. Transducer specifications and calibration constants used to analyse the data.

Parameter Channel 1 Channel 2 Units

Frequency 38 120 kHz Transducer type Simrad split beam –

ES38DD Simrad split beam

ES120-7DD

Pulse length 0.512 0.512 ms Power 2000 500 W

Beamwidth -3dB power (along/athwart)

6.9/7.1 6.7/6.5 degrees

Equivalent beam width -20.70 -21.30 dB re 1 steradian Nominal Absorption 0.01 0.034 dB m-1

Nominal Sound speed

1500 1500 m s-1

Transducer gain 23.9 23.6 dB SA correction -0.92 -0.55 dB

Angle sensitivity 21.9 23 Angle offsets (along/athwart) 0/0 0/0 degrees

Calibration Sphere target strength (38.1 mm)

-42.4 -39.5 dB re 1 m2

The beam compensation, B, of the split beam transducers were optimised for a flat response over target along α and athwart β acceptance angles of +/- 4o by adjusting the half power beam opening, φ , and along, oα and athwart oβ offset angles in degrees (Eq. 2.4).

.1 .1 .1 .1 (dB) 2 2 2 20 00 0- -- -B( , )= 6 [( + ( - 0.18 ( ( ]) ) ) )/2 /2 /2 /2

β βα β α βα αα βφ φ φ φ

Eq. 2.4

To obtain in situ measurements, the Drop TS probe with suspended calibration sphere was lowered over areas of interest and the vessel allowed to drift with the range on the EK60 transceivers set to 200 m allowing maximum ping rate of nominally 3 to 4 pings per second. Within EchoView the raw target strength data were determined using the parameters outlined in Table 2.4. These parameters were optimised visually to obtain the maximum amount of targets whilst rejecting returns from high density regions. These single targets were then used to obtain fish tracks to explore uncertainties in the single target detection process and sensitivities to detection angles off axis and noise as described in Kloser and Horne (2003). This process was applied to both frequencies and the data exported tagged with time, vessel position, Drop TS depth, pitch and roll. Table 2.4. Target strength selection criteria used in EchoView to obtain fish tracks and export summary data to Matlab.

Parameter Channel 1 Channel 2 Units Frequency 38 120 kHz

TS threshold -80 -80 dB Pulse length determination level 12 12 dB

Min normalised pulse length 0.6 0.6 Max. normalised pulse length 1.8 1.3 Maximum beam compensation 12 12 dB

Maximum phase deviation minor axis 1 1 degrees Maximum phase deviation major axis 1 1 degrees

The tracked fish data from each drift experiment and depth zone were obtained from a time and depth range based on information about orange roughy school location from the vessel sounder. A minimum and maximum threshold was set according to the region identified to


exclude spurious bottom targets and short fish tracks. The sensitivity of the target strength estimates to threshold levels was explored.

The identification of orange roughy targets was based on school location, frequency difference measurements and school behaviour (Kloser et al., 2002). The species identification experiments confirmed that for orange roughy schools (mean standard length 43 cm) there was a volume reverberation scattering strength difference Sv38 -120 of -4 to -5 dB (see Chapter 3 for details). This compares with a 12038−ΔSv of -5 dB +/- 2.5 dB for 35 cm standard length orange roughy (Kloser et al., 2002). Using the principle of linearity in acoustic (MacLennan and Simmonds 1992) the difference in volume reverberation measurements requires a similar dB difference in ensemble target strengths. We hypothesise that ensemble measurements from tracked fish are probable orange roughy when,

i) the target strength is between –2.5 dB to - 7.5 dB and 12038−ΔTS

ii) occur in regions associated with orange roughy schools and

iii) the schools show a characteristic avoidance response to sampling gears,

based on Kloser et al. (2002).

2.4.2 Results In total 10 drift measurements were carried out targeted at vessel mounted acoustic echo marks of interest (Appendix B). At these locations 14 demersal trawls were targeted with ~32 tonnes of orange roughy retained and a sub sample from each catch measured to obtain biological parameters of sex, length, weight, and gonad stage (Table 2.5, Appendix B).

Table 2.5. Mean standard length, weight and sex ratio of orange roughy.

Sex Mean length with stdev (mm)

Mean weight with g (stdev)

Total no. samples

Sex ratio

Female 451 (34) 3353 (666) 646 69% Male 420 (34) 2460 (509) 296 31%

The catch composition of the trawls contained a high proportion of orange roughy (mean 97% s.d. 3.3%) with the small (mean 3%) bycatch dominated by gas bladdered species with high target strength such as oreos, whiptails, morid cods and misc_high (Figure 2.17, Appendix B).


Figure 2.17. Proportion of orange roughy and by-catch mix (by weight). Note Y axis scale starts at 90% to allow the low proportions of by catch to be better visualised.

90%

92%

94%

96%

98%

100%

1 (5

000)

2 (4

500)

3 (2

500)

4 (6

000)

5 (6

800)

6 (1

000)

7 (2

000)

8 (4

18)

10 (2

000)

11 (1

000)

12 (1

000)

13 (7

000)

Shot number and total catch in kg, in brackets

Per

cent

age

of fi

sh (b

y w

eigh

t)

Misc_LowMisc_ HighWhiptailsMorid CodsSharksOreosProportion OR

Of the 10 successful drift experiments, drift number 11 contained a clear example of an orange roughy school that satisfied all the selection criteria. Figure 2.18 shows the orange roughy school targeted with the TS probe and how the school disperses as the probe is lowered closer to the school. The school of orange roughy is defined from the amplitude difference frequency mixing of the 38 kHz and 120 kHz data (Figure 2.18c, Kloser et al., 2002).


Figure 2.18. Example of operation 11 volume reverberation (Sv) compensated echogram for 7 minutes of data drifting at 580 m depth for 4 minutes over an orange roughy school and then lowered to 700 meters at 12:16 on the school for the a) 38 kHz frequency, b) 120 kHz frequency and c) the amplitude mixed frequency. The major features of the calibration sphere and weight and seabed are highlighted.

seafloor

calibration sphereweight

(a)

(b)

(c)

12:11 12:12 12:13 12:14 12:15 12:1612:10

12:11 12:12 12:13 12:14 12:15 12:1612:10

12:11 12:12 12:13 12:14 12:15 12:1612:10

Both frequencies were calibrated using a 38.1 mm tungsten carbide sphere suspended below the transducer during the deployment. Adjusting for the transducer beam width and along and athwart offset angles (Eq. 2.4) reduced the calibration error of the 38 kHz transducer from 0.49 dB (s.d. 0.43 dB) to -0.01dB (s.d. 0.39 dB) and the 120 kHz system from -0.59 dB (s.d. 0.38) to 0.00 (s.d. 0.21dB) (Figure 2.19). The 120 kHz system calibration was sensitive to transducer depth with an ~1 dB change measured when moving from 580 m to 700 m depth.


Figure 2.19. Calibration deviations for angles off axis for the original a) 38 and b) 120 kHz transducers and after beam compensation (Eq.2.4) c) 38 kHz and b) 120 kHz.

0 2 4 6-2

-1

0

1

2tsc 38 s + -42.4 vs angle

calib

ratio

n er

ror

dB

off axis angle

0 2 4 6-2

-1

0

1

2tscomp 38 s + -42.4 vs angle

calib

ratio

n er

ror

dB

off axis angle

0 2 4 6-2

-1

0

1

2tsc 120 s +-39.5 vs angle

calib

ratio

n er

ror

dB

off axis angle

0 2 4 6-2

-1

0

1

2tscomp 120 s -39.5 vs angle bw = 6.85 aloff = -0.2

calib

ratio

n er

ror

dB

off axis angle

Based on the school of orange roughy highlighted in Figure 2.18c as blue the Sv histogram (5 m averages, n = 769) for both frequencies are normally distributed with a trimmed (5%) mean

of -4 dB and a mode of -4.5dB (38 120Sv −Δ Figure 2.20b).

Figure 2.20. Frequency distribution of a) volume reverberation Sv dB re m-1 for an orange roughy school highlighted as blue in Figure 2.18c at 38 kHz solid and 120 kHz dashed; b) the frequency distribution of the dB difference between the 38 and 120 kHz ( 38 120Sv −Δ ) with mean difference of 4 dB highlighted.

-80 -75 -70 -65 -60 -55 -50 -450

2

4

6

8

10

12

14

Sv (dB re 1m)

Per

cent

age

38 kHz120 kHz

-10 -5 0 5 100

2

4

6

8

10

12

14

16

Sv dif ference 38-120 kHz (dB)

Per

cent

age

mean of 38 kHz minus 120 kHz

-80 -75 -70 -65 -60 -55 -50 -450

2

4

6

8

10

12

14

Sv (dB re 1m)

Per

cent

age

38 kHz120 kHz

-10 -5 0 5 100

2

4

6

8

10

12

14

16

Sv dif ference 38-120 kHz (dB)

Per

cent

age

mean of 38 kHz minus 120 kHz

(a) (b)

The apparent fish density within the orange roughy school is approximated assuming a mean standard length orange roughy being 43 cm with target strength of -49.4 and -44.4 dB at 38 and 120 kHz respectively (Figure 2.21, Table 4.1). The median density of orange roughy within the school is 5.5 fish per 100 m3 or 1 fish per 18 m3. The sampling width and volume


at the half power points for the 7 degree 38 kHz and 120 kHz frequencies, 0.512 ms pulse length system at 100 m depth is approximately 12.2 m and 45 m3 respectively. To satisfy the selection criteria of the target strength algorithm that a single target is tracked between the -3 dB power points of the beam and the selection threshold is -12 dB implies one fish is required within a beam angle of at least 10 degrees (assuming a conical beam transducer) and associated horizontal resolution of 17.5 m and sample volume of 92 m3 at 100 m depth. At 100 m range the median density of orange roughy to satisfy the horizontal and vertical density requirements would need to be less than 0.0015 fish m-3 and 0.01 fish m-3 respectively. The apparent mean orange roughy density at 100 m range (0.055 fish m-3) within a school is well in excess (factor of 37) of the horizontal resolution required (Figure 2.21). Figure 2.21. Apparent fish density of orange roughy within the school assuming a simplistic conversion factor for a 43 cm orange roughy of target strength -49.4 dB at 38 kHz (solid) and -44.4 dB at 120 kHz (dashed).

10-3 10-2 10-1 1000

2

4

6

8

10

12

14

fish m-3

Per

cent

age

38 kHz120 kHz

In situ target strength measurements were obtained from 3029 tracked targets (23004 targets) using the120 kHz frequency as the masking frequency for the region of interest in operation 11 ( Figure 2.22).


Figure 2.22. In situ target strength data extracted from tracks of single fish identified in the data shown in Figure 2.18 based on the selection criteria of Table 2.4. All target strength values from tracked targets (#23004) for the, a) 38 kHz and b) 120 kHz frequency. Linear mean tracked targets (#3029) of the 23004 targets identified for c) 38 kHz and d) 120 kHz target strengths. The red box outlines the region of

12038−ΔTS

12038−ΔTS that is associated to orange roughy schools and the expected maximum and minimum range of target strengths for orange roughy.

a) b)

-10 -5 0 5 10 15 20-70

-65

-60

-55

-50

-45

-40

-35

-30

-25

-20TS 38 kHz vs dB diff 38-120 > 22 m

TS 3

8 kH

z dB

dB diff 38-120-10 -5 0 5 10 15 20

-70

-65

-60

-55

-50

-45

-40

-35

-30

-25

-20TS 120 kHz vs dB diff 38-120 > 22 m

TS 1

20 k

Hz

dB

dB diff 38-120

c) d)

-10 -5 0 5 10 15 20-65

-60

-55

-50

-45

-40

-35

-30

dBdiff 38-120 kHz

38 k

Hz

targ

et s

tren

gth

dB

-10 -5 0 5 10 15 20-65

-60

-55

-50

-45

-40

-35

-30

dBdiff 38-120 kHz

120

kHz

targ

et s

tren

gth

dB

Only 4 tracked targets with the criteria of 12038−ΔTS of -2.5 to -7.5 dB and with a target strength range of -45 to -55 dB for 38 kHz and -40 and -50 dB for 120 kHz were identified. Of these 4 tracked targets 3 were located within 10 m of the seafloor but at a range of ~150 m. This low number of targets is insufficient to enable any statistical robust statement about orange roughy in situ target strength. It would require a large number of tracked targets (100s) to provide a robust analysis of orange roughy in situ target strength.

SPECIES IDENTIFICATION 35

3. SPECIES IDENTIFICATION

3.1 Introduction

The Cascade Plateau seamount is located 260 km south east of Tasmania (43:56.44S, 150:27.37E) where it rises from the deep ocean to 600 m depth. In late June an aggregation of orange roughy forms for about 10 days for their annual spawning event. Since 1998 acoustic surveys of the spawning event have been carried out annually from industry vessels. The first 5 years of acoustic surveys were based on digitisation of the uncalibrated vessel’s 28 kHz echosounder (Prince and Diver, 2002; Prince, 2004). These surveys gave insight into the variable nature of the spawning event but due to intrinsic calibration shortcomings, produced no quantitative biomass estimates. Since 2003 Simrad ES60 echosounders have been used for the vessel based acoustic surveys. These sounders were calibrated using the suspended sphere method (Foote et al., 1987) enabling quantitative biomass estimates to be produced for the years 2003, 2004 and 2005 (Honkalehto and Ryan, 2003; Honkalehto and Ryan, 2004; Ryan 2006). These results are used as absolute estimates by a Bayesian stock assessment model to determine the recommended biological catch (Wayte, 2006).

Large errors can occur when quantifying orange roughy biomass with vessel mounted acoustic systems (Kloser, 1996; Kloser et al., 2002; McClatchie and Coombs, 2004) and are a major concern with the 2003-2005 Cascade Plateau biomass estimates. Orange roughy have a low acoustic backscatter compared to many of the other fish species that share their habitat (Kloser et al., 1997; Barr, 2001; McClatchie and Coombs, 2004). Therefore inadvertent inclusion of even a relatively small numbers of fish with high acoustic reflectivity can lead to a significant positive bias in biomass estimates. This type of bias can occur through echogram misinterpretation or if other species are within the large acoustic sampling volume of the vessel-based systems (typically approx. 70 m diameter by 3 m height at orange roughy depths, or 12,000 m3).

The 2003-2005 biomass estimates were based on echointegration of echogram regions that were considered by the analyst to contain 100% orange roughy (Honkalehto and Ryan, 2003; Honkalehto and Ryan, 2004; Ryan 2006). However interpretation of the echograms from the vessel mounted acoustics is not always straightforward for the Cascade Plateau. While some school marks have clearly defined orange roughy characteristics (i.e. shape, size, depth, intensity) others are difficult to distinguish from the general backscatter. To capture interpretation uncertainty the 2003-2005 analysis used a simple hierarchical classification scheme (Honkalehto and Ryan, 2003, see also methods section). There is a level of subjectivity in this classification method such that robustness and accuracy our classifications need to be tested by an independent means. For example in the 2005 analysis certain scenarios resulted in biomass estimates that were a factor of 9 greater than the base case estimates (Ryan, 2006).

Many of the error sources associated with vessel mounted systems can be eliminated or significantly reduced by use of deeply deployed acoustic systems (Kloser, 1996). Deep deployment can bring the system close to the orange roughy to overcome the range limitations of higher frequencies (Kloser et al., 2002; McClatchie and Coombes, 2004) to allow simultaneous ensonification at multiple frequencies. Kloser et al., 2002 described a method of visualising three frequency data to empirically distinguish regions of orange roughy from two other groupings of fish species, namely small gas bladdered fish (Myctophids) and large gas bladdered fish (Macrourids and Mourids). A deep deployed

36 SPECIES IDENTIFICATION

system can also be used to make in situ measures of orange roughy target strength (Kloser et al., 1997) (see Chapter 2).

A survey from a scientific research vessel using our existing four frequency deep-towed body, MUFTI-2, would from a technical perspective, be an ideal way of quantifying the uncertainty in orange roughy target strength and species identification at the Cascade Plateau. However a number of factors made this a non-favoured option. The MUFTI-2 system is an expensive sophisticated instrument that requires specialised technical staff plus a suitable back deck arrangement to facilitate safe deployment and retrieval of the system as well as accommodate its 7 tonne optic fibre winch. Normally a dedicated scientific research vessel would be required but the cost if this can be high and availability at this very specific time of year difficult to secure.

An alternative strategy was to develop a dual frequency acoustic system designed for vertical deployment from a commercial orange roughy trawler. The system, referred to as Drop TS system from now on, was designed for simple at-sea operation so that specialist technical staff were not required. A key aspect was that it should have both self contained power and data logging thus avoiding the need for specialised and expensive data and power transmission cables and their associated winch systems. This enables the Drop TS system to be attached to any suitably strong wire, in this case the vessel’s trawl warp. In 2005 a survey program that used the Drop TS as the primary sampling tool was carried out on an orange roughy trawler (FV Riba1) during the annual survey of orange roughy at the Cascade Plateau.

This chapter discusses the design and application of the Drop TS system with regard to its ability to inform our understanding of species identification at the Cascade Plateau and to test the validity of the hierarchical classification scheme that was applied to the 2003-2005 vessel-based biomass estimates (Ryan, 2006).

3.2 Material and Methods

3.2.1 Acoustic Equipment The Drop TS system consisted of a cylindrical aluminium frame 1 m in diameter and 1.5 m high that housed Simrad 38 kHz ES38D and 120 kHz ES120-7DD deepwater transducers, a pressure case for sealed lead-acid batteries and the main pressure case that housed Simrad 38 kHz and 120 kHz EK60 transceivers, power control circuitry and a 600 MHz industrial PC running Simrad ER60 software. Platform motion was monitored by Microstrain 3DM-GX1 sensors whose output was formatted to allow input and logging by the Simrad ER60 software. Seawater temperature and system depth were also measured by ancillary sensors. To provide a calibration reference a 38.1 mm tungsten carbide sphere was suspended 16 m vertically below the system during deployment.

3.2.2 Interpretation and classification of vessel-mounted echograms In an effort of quantify the uncertainty involved in assigning a species composition to regions of echograms, Honkalehto and Ryan (2003) used a simple hierarchical classification system to assign confidence level to their interpretations of echogram regions. This method is summarised here to give context to the discussion about how the Drop TS data was able to improve our understanding of species composition at the Cascade Plateau. The 2003-2005 surveys were carried out by industry vessels fitted with Simrad 38 kHz ES60 echosounders. The analyst would review the vessel-mounted echograms and consider each significant school mark and determine an appropriate classification. Classification criteria for orange


roughy included school depth, location (with respect to known “hot-spots”), school shape and intensity, trawl catch information and temporal and spatial relationship with other classified school marks. These criteria were considered in combination by the analyst to determine a final classification status. Classification types were i) high confidence orange roughy (OR1), ii) medium confidence orange roughy (OR2), iii) unknown species composition with high Sv (“Unknown-hi”) and iv) unknown species composition with low Sv (“Unknown-Lo”). Only those echogram regions that were classified as OR1 were used in the biomass estimates considered by the stock assessment model (Ryan, 2006). Sensitivity to interpretation was tested by running scenarios that assumed regions classified as OR2, “Unknown-hi” and “Unknown-Lo” contained 100% orange roughy.

3.2.3 Acoustic sampling The Drop TS deployments were carried out from the deepwater trawler FV Riba1. The deployments had dual objectives of measuring backscatter from individual fish targets and characterising the species composition of the Cascade Plateau ecosystem. The vessel’s Simrad 38 kHz ES60 acoustic system was used to locate school marks that were then targeted by the Drop TS system for a detailed investigation. The system was deployed vertically from the vessel’s trawl warp with either the vessel drifting or moving at very low speed (< 2 knots). The objective of the deployment was to lower the system to be within 50-150 metres of the targeted school mark. The position of the Drop TS system was monitored using a Tracklink 1500 USBL acoustic positioning system. During deployment it was found useful to keep the vessel’s echosounder running. This gave information about the depth under the vessel while sidelobe reflections from the Drop TS frame allowed the system’s depth to be monitored in real time with particular attention to its proximity of the seafloor.

3.2.4 Biological sampling Biological samples were obtained from trawls using the vessel’s demersal trawl net fitted with a fine-mesh cod end liner (Ryan, 2006). A second vessel, FV Petersen, was at the Cascade Plateau to conduct a vessel-based acoustic biomass survey and also targeted trawl shots as part of that program. The objectives of the trawls were to obtain representative samples of orange roughy and to investigate the species composition of school marks. For each trawl sub-samples of 100 orange roughy were processed to obtain metrics of fish length, weight, sex and stage. Bycatch species were sorted according to their “acoustic group” (Kloser et al., 1997) and measures of their length and weight taken for each fish.

3.2.5 Species identification using the Drop TS system The Drop TS data provided a number of indicators that were considered in combination to inform our concluding interpretation of species identity. These were i) behavioural response, ii) amplitude differences between species and across frequencies, iii) absolute values of mean volume backscatter, Sv and iv) absolute values of individual fish target strength, TS. These are now discussed in turn.

Behavioural response

The scare reactions of orange roughy to foreign objects have been well documented for other spawning locations both in New Zealand and Australia (Koslow et al., 1995b; Kloser et al., 2000; Kloser et al., 2001). The avoidance reaction attributed to orange roughy was observed


on the echogram as a layer of high signal backscatter close to the seafloor with a “clear” area of low backscatter immediately above. The “clear” area is thought to be due to the region having just been vacated by orange roughy (Kloser et al., 2002). A non-avoidance response by school regions was interpreted to indicate species other than roughy.

Amplitude differences between species and across frequencies

Differences in Sv between 38 kHz and 120 kHz frequencies ( 12038−ΔSv ) were used as a key to distinguishing regions of orange roughy from other species. Kloser et al., 2002 described how a of ~ +12dB was associated with regions dominated by myctophid species while a

12038− of ~ -2 to -4 dB was associated with regions of orange roughy. These differences were visualised using the colour mixed synthetic echogram method described by Kloser et al., 2002 (replicated in Echoview 3.40 using virtual variables) and measured directly using the relative mean dB graph tool in Echoview 3.40 (Echoview 3.4 help file; Korneliussen and Ona, 2003)).

12038−ΔSvΔSv

Absolute values of mean volume backscatter

During the 2005 vessel-mounted acoustic surveys, small but very high backscatter school marks were observed (Ryan, 2006). These regions were classified as “Unknown-hi”. The absolute values of Sv measured by the Drop TS within “Unknown-hi” regions were noted and considered with all other indicators of species type, to see if our interpretation of these classification types could be better understood.

Absolute values of individual fish TS

Absolute and relative values of backscatter from individual fish (TS and 12038−ΔTS respectively) were also used as a key to species type. Single TS values of orange roughy were expected to have values of approximately -50 dB at 38 kHz and -46 dB at 120 kHz (Kloser and Horne 2003). While single TS values of orange roughy were difficult obtain (see Chapter 2), those of other fish species were abundant. Myctophid species will have similar TS to orange roughy at 38 kHz (Kloser et al., 2002) but have a comparatively much weaker signal at 120 kHz (approx. 12 dB less than orange roughy), (Kloser et al., 2002). Large-gas bladdered fish such as whiptails, blue-eye trevalla and oreos are expected to have TS values at 38 kHz that are 10 to 20 dB higher than orange roughy (e.g. Kloser et al. 2002, Barr and Coombs 2005). Histogram plots of TS distributions from within regions of interest at 38 kHz and 120 kHz frequencies were made. The absolute modal TS values and their differences across the two frequencies were used to infer the dominant scattering species within regions.

These four indicators of species type were used in combination to inform our concluding interpretations of species composition.

3.3 Results

3.3.1 Biological sampling A summary of the biological sampling is given in section 2.4.2 and Figure 2.17. Of relevance to this chapter is the species composition of the trawl catch. The data showed how demersal trawl shots were dominated by orange roughy with trawl shots of 5 tonnes or greater containing at least 98% of orange roughy by weight. By-catch included large size, high acoustic reflectivity species such as blue eye trevalla and oreos. Smaller gas bladdered fish (myctophids) were not well represented, most likely because of the large mesh size of the commercial trawl net system.


3.3.2 Acoustic survey program

Over the 10 day survey period 20 vessel-mounted acoustic surveys were completed by the two participating vessels, FV Riba 1 and FV Petersen (Ryan 2006). 29 targetted demersal trawl shots were completed to obtain biological measures of orange roughy length, sex, weight and stage as well as species composition information (Ryan 2006). 9 Drop TS deployments targeted echogram observed school mark regions in an attempt to obtain measures of individual fish target strength and to investigate the species composition of the deepwater fish community (Ryan 2006), Figure 3.1.

Figure 3.1. Map of the main survey area at Cascade Plateau (43:56.44S, 150:27.37E) showing the location of classified school regions, an example of the survey transect lines, the start/stop location of 13 targeted demersal trawl shots the 9 Drop TS deployments completed by FV Riba 1.

Legend

Regions of OR1

Transect lines

Demersal trawl shots

Drop TS deployments

Regions of "Unknown Hi"

Regions of "Unknown Lo"

Cascade Plateau seamount

Map showing classified regions and locationof demersal trawls and Drop TS deployments

2000 m

1400 m

700 m

1200 m

1800 m

1600 m1500 m

800 m

1300 m

1900 m

0 2.5 5

nautical miles

3.3.3 Improving interpretation of vessel-mounted acoustic using the Drop TS system – three examples

During the course of the voyage nine successful Drop TS deployments were made. Three examples are given to demonstrate how the Drop TS system was able to inform our understanding of species composition.


Example 1 – validation of regions of orange roughy The first example demonstrates the ability of the dual frequency system to distinguish regions of orange roughy from the diffuse backscatter. Figure 3.2a shows a school mark observed on the echogram of the vessel-mounted acoustics. This region was considered highly likely to contain orange roughy and was therefore classified as OR1. The Drop TS system targeted this region to obtain detailed acoustic information. The Drop TS Sv echograms show a distinct layer above the seafloor that was stronger on 120 kHz than the 38 kHz by an average of 5 dB (Figure 3.2b, Figure 3.2c and Figure 3.2d). When viewed as a colour mixed echogram this layer is clearly distinct from the backscatter in the remainder of the acoustically observed water column (Figure 3.2e). This dB difference and general appearance of the layer close to the seafloor support the inference that the region classified as OR1 on the vessel mounted echogram is dominated by orange roughy (Kloser et al., 2002). However the Drop TS data indicates that close to the orange roughy layer a significant amount of signal is originating from small gas bladdered species. Of note is the fact that neither demersal trawl data nor the single frequency vessel-mounted 38 kHz echogram had sufficient information to indicate the mix of both orange roughy and other species as was clearly suggested by the Drop TS system.

Example 2 – identification of schools of large gas bladder fish The second example explores the “Unknown-hi” marks that were observed during the FV Riba1’s 2005 voyage. School marks with this classification were generally observed at the southern end of the plateau, either at shallow (600 m) or deep (> 800 m) depths (Figure 3.1). An analysis of a survey that contained many “Unknown-hi” school marks found that if these marks did in fact contain orange roughy, the biomass estimate for that survey would be a factor of 9 greater than if the converse were true (Ryan, 2006). There is clearly potential for a large variation in biomass should our classification of school marks be in error. This motivated an examination of the Drop TS data to see what could be learnt about the species composition of these “Unknown-hi” marks.

Figure 3.3a shows a series of “Unknown-hi” school marks observed on the vessel-mounted 38 kHz echogram. The Drop TS system was deployed to target these marks. Three key pieces of information were used to conclude that the “Unknown-hi” marks did not contain orange roughy. Firstly, single target information shows the presence of a significant number of high value single targets in the vicinity of the “Unknown-hi” school marks (Figure 3.3b and Figure 3.3d). A bi-modal frequency distribution is observed. The lower mode (-48 dB at 38 kHz, -60 dB at 120 kHz) is interpreted as being due to the presence of the ubiquitous myctophid species while the higher mode (-38 dB at both 38 and 120 kHz) is thought to be due to the presence of large gas bladder fish (Kloser et al., 2002). The second indication that the “Unknown-hi” mark is not made up of orange roughy is the lack of avoidance to the Drop TS system (Figure 3.3c). The avoidance reaction of orange roughy observed at other fishing grounds (Koslow et al., 1995b; Kloser et al., 2000; Kloser et al., 2001) was observed by the Drop TS system on other occasions during the Cascade Survey but was notably absent when targeting the “Unknown-hi” schools. Scare reactions by orange roughy are typically observed when a system systems gets within 70-100 metres range. In this example the system comes within 30 metres of the school mark with no sign of reaction observed on the echogram. Thirdly, the absolute Sv value of the “Unknown-hi” regions were always approximately 10 dB higher than that of the OR1 classified regions; this means that if the “Unknown-hi” regions did contain orange roughy their packing density would be a factor of 10 higher than that of OR1 classified regions. Consideration of all indicators led to the conclusion that the


“Unknown-hi” marks are made up of a large gasbladder fish, possibly oreo or blue eye trevalla.

Example 3 – an example of a complex species mix The third example of how the Drop TS system was applied looks at the potential complexity of school marks observed by the vessel mounted acoustics. Figure 3.4a shows a section of echogram from the vessel-mounted 38 kHz system with three regions defined according to their backscatter: (i) strong, (ii) moderate and (iii) diffuse. The composition of each of these regions was unknown, however, based on the characteristics of the mark, its depth and location, the strong core and possibly moderate strength regions would be classified as OR1. If this mark was targeted with a demersal trawl net it would be fully expected to return a large catch of orange roughy with very little bycatch. The Drop TS targeted this mark, but by the time it was lowered into the position the strong core had either moved or dispersed as it was no longer observed by the vessel-mounted acoustics (Figure 3.4b). The Drop TS system explored the remaining two regions during its deployment (Figure 3.5).

Region 1 shown on Figure 3.5a was thought to correlate to the diffuse region shown on the vessel-mounted echogram in Figure 3.4, while Region 2 is thought to correlate to the moderate strength region shown in Figure 3.4. Region 1 is dominated by single targets that are approximately 10 dB stronger on 38 kHz compared to 120 kHz suggesting that they originate from small gas bladder fish (Kloser et al., 2002). Region 2 has a large number of targets at around -38 dB on both frequencies, indicating the presence of large gas bladder fish (Kloser et al., 2002). This example demonstrates how vessel-mounted echogram observed school marks can have a complex species composition that is not apparent when considering either the vessel-mounted acoustics or demersal trawling. Further, it showed that a substantial portion of the mark was unlikely to contain orange roughy. The example shows that misclassification echogram observed school marks from the vessel-mounted system is a real possibility at the Cascade Plateau.


Figure 3.2. Visualisation of Sv echogram data for Drop TS deployment 13, 26/06/2005 12:20. Figure 3.2a is an image from the vessel’s 38 kHz echosounder showing the school mark which was targeted by the Drop TS system. Figure 3.2b and Figure 3.2c are echograms from the Drop TS 38 kHz and 120 kHz frequencies respectively. The image’s pixel colours are governed by the Sv (or backscatter) values according to the scale bar on the right hand side of the image. The colour scaled pixels indicates that the distinct layer above the seafloor is stronger on 120 kHz while the diffuse scatter in the water column is stronger on 38 kHz. This is observed as an absolute dB difference between the two frequencies (Figure 3.2d). Figure 3.2e combines the 38 kHz and 120 kHz Sv data allowing the user to tune a mixing of two colour palettes (green and blue for 38 kHz and 120 kHz respectively) to maximize the contrast between different echogram regions. It shows a distinct layer above the seafloor (blue pixels) which is stronger on 120 kHz than 38 kHz and thus is attributed to orange roughy (Kloser et al., 2002). The remainder of the water column is stronger on 38 kHz and is inferred to consist mainly of small gas bladder species (Kloser et al., 2002).

DropTS depth/time profile

Probable orange roughyschool mark targetedby the DropTS system

(a) ES60 vessel-mounted echogram (c) DropTS 38 kHz echogram

(b) DropTS 120 kHz echogram DropTS colour mixed echogram. blue = higher on120 kHz, green = higher on 38 kHz

(e)

Orange roughy layer

Myctophid layer

-34 dB

-34 dB

-70 dB

-70 dB

Seafloor

0

1

2

3

4

5

6

38 kHz 120 kHz

(d) relative mean dB difference between 38 kHzand 120 kHz for selected echogram region

Rel

ativ

e m

ean

dB

Selectedechogramregion

Selectedechogramregion


Figure 3.3. Use of Drop TS system to investigate the species composition of the “Unknown-hi” school marks during Drop TS deployment 6 (23-Jul) Figure 3.3a shows school marks observed on the vessel-mounted 38 kHz echogram that were classified as “Unknown-hi”. These marks were targeted by the Drop TS system. Figure 3.3b and Figure 3.3d shows the distribution of detected single targets from regions located close to the “Unknown-hi” school marks. The higher mode of the bi-modal distribution (-38 dB at both 38 kHz and 120 kHz) suggests the presence of large gas-bladder species such as morid cods and whiptails. Figure 3.3c is an echogram image from the Drop TS 38 kHz system showing a high-signal school mark that showed no indication of avoidance as would be expected by an orange roughy school.

(b) Distribution of single targets at 38-kHz

10

-75 dB -25 dB-50 dB -38 dB-75 dB -25 dB-50 dB -38 dB-75 dB -25 dB-50 dB -38 dB-75 dB -25 dB-50 dB -38 dB

% o

f tar

gets

Lower mode

Upper mode

-75 dB -25 dB-50 dB -38 dB-75 dB -25 dB-50 dB -38 dB-75 dB -25 dB-50 dB -38 dB-75 dB -25 dB-50 dB -38 dB

(d) Distribution of single targets at 120-kHz

10

% o

f tar

gets

Upper mode

Lower mode

(a) 38 kHz echogram from vessel-mounted acoustics

Strong marks observed byvessel-mounted acoustics

seafloor

(c) "Unknown-hi" school mark observedon the DropTS 38-kHz echogram

20 m

40 m

Calibrationsphere

Weight

seafloor

high signal region froma school of "unknown-hi"fish species


Figure 3.4. Section of echogram from vessel-mounted 38 kHz system showing complex school mark observed on the 24th of June at 20:00. The mark was made up of three regions, (i) a strong core region close to the seafloor, (ii) a moderate strength region associated to the bottom-referenced strong core and (iii) a diffuse region “streaming” away from the moderate strength region. After observing this school the vessel turned around, deployed the Drop TS system and slowly drifted back through the mark (Figure 3.4b). The pink line shows the deployment profile of the Drop TS system in relation to the 38 kHz vessel mounted echogram. Note the strong core region had either moved or dispersed by the time the Drop TS was at depth and in position.


Figure 3.5. Visualisation of data from the Drop TS system as it passed through the moderate and diffuse backscatter regions as denoted on Figure 3.4. Figure 3.5a and Figure 3.5b show echograms of Sv data from 38 kHz and 120 kHz showing two regions, the upper region 1 having weaker backscatter than the lower region 2. Single targets detected within regions 1 and 2 are shown in Figure 3.5b and Figure 3.5e for 38 kHz and 120 kHz respectively. Figure 3.5c and Figure 3.5f show the distribution of single targets from both regions 1 and 2 at 38 kHz and 120 kHz respectively. A bi-modal distribution can be observed with peaks at -50 dB and -38 dB on 38 kHz and -64 dB and -38 dB for 120 kHz, suggesting the presence of both small and large gas bladder species.

Region 1, 38-kHz

Region 1, 38-kHz

Region 2, 38-kHz

Region 1, 120-kHz

Region 1, 120-kHz

Region 2, 120-kHz

Region 2, 38-kHz Region 2, 120-kHz

(a) 38-kHz Sv echogram (d) 120-kHz Sv echogram

(b) 38-kHz detected single targets (e) 120-kHz detected single targets

(c) 38-kHz distribution of single targets (f) 120-kHz distribution of single targets


3.4 Discussion

3.4.1 Outcomes and benefits of the Drop TS system

The 2005 Cascade Plateau biomass estimates were based on the echointegration of 38 kHz vessel mounted acoustics (Ryan 2006). Interpretation and classification of echogram regions that were thought to contain only orange roughy was a critical step in the process. A simple set of classification criteria (OR1, OR2, ‘Unknown-hi’ and ‘Unknown-Lo’) were used to guide the interpretation however there remained a significant level of subjectivity. The targeted demersal trawls were able to provide evidence of substantial numbers of orange roughy but could not provide a complete and unbiased characterisation of the dominant sources of deepwater acoustic backscatter. The use of a deeply-deployed two-frequency acoustic system (Drop TS) gave information that greatly improved our understanding of the Cascade Plateau deepwater ecosystem and our interpretation of the vessel-mounted acoustics leading to a more robust biomass estimate. Specifically the Drop TS information was able, by inference, to:

(i) Validate OR1 classified vessel-mounted school regions as containing orange roughy.

(ii) Characterise vessel-mounted school regions that had been classified as ‘Unknown-hi’ and consequently reject these regions from the vessel-based biomass estimates. In one survey, failure to reject these ‘Unknown-hi’ regions would have resulted in a biomass estimate that was higher by a factor of 9. These ‘Unknown-hi’ regions give an extreme example of the level of bias possible due to subjectivity in echogram interpretation. In practice an experience analyst would probably have rejected these regions based on inspection of vessel-mounted acoustic data alone. However, given the complexity of the species mix at the Cascade Platue, interpretation of vessel mounted acoustics is likely to always have a high level of subjectivity and an associated potential for error unless there is a non-trawl based means of verifying the species composition of observed echogram marks.

(iii) Demonstrate the complexity of school regions that, if not properly quantified, could lead to large positive biases in the vessel-based acoustic estimates. The information showed that acoustic backscatter from non-roughy sources, both small gas bladder (myctophid) and large gas bladder (blue-eye, morid cods, whiptails) commonly exists in and around regions of schooling orange roughy.

The Drop TS system has demonstrated the importance of accounting for all sources of acoustic backscatter at the Cascade Plateau if accurate biomass estimates are to be achieved.

3.4.2 Limitation of the Drop TS system

The objectives of this project were to obtain information on orange roughy target strength and to characterise the species composition of the deepwater community at the Cascade Plateau. The system was designed with these objectives in mind and, importantly, for vertical deployment from an industry vessel. The successful deployment of the Drop TS system represents a


significant advance in our development of low cost observation methods that can be executed from industry vessels. The self contained nature of the Drop TS system (i.e. power supply and digitised recording of acoustic signal) greatly simplified the logisitics and level of expertise required. The drawback is that no information is directly available at the surface, hence precise targeting of regions can be difficult and the positioning of the system cannot be tuned based on real-time observations. In general the combination of the vessel mounted acoustics and the Tracklink acoustic positioning system gave enough information to allow us to successfully deploy the Drop TS system for these particular experiments.

Our experiments suggest that two frequencies (38 and 120 kHz) are sufficient to discriminate orange roughy when they are in schools but this has not been demonstrated for lightly aggregated fish. Absolute measures of TS and Sv were used as an indicator of large gas bladder fish but these metrics may not be sufficient to distinguish orange roughy from fish with similary morphology (e.g. deepwater shark). The use of a lower frequency (12 or 18 kHz) has been used at other orange roughy grounds to distinguish large gas bladder fish from other species (Kloser et al., 2002) and would be a useful addition to the Drop TS system.

In its present form the system cannot be deeply towed at a speed suitable for survey (> 4 knots). Instead the mode of operation was to target significant school marks that had been previously observed on the vessel-mounted acoustics. While this operational mode was adequate to meet the objectives for the present project, it limits the potential to use the system to carry out other tasks such as a quantitative biomass survey.

3.4.3 Summary and recommendations for future work

The Drop TS system has demonstrated the complexity of the species composition, and hence the potential for positive bias in biomass estimates, at the Cascade Plateau. Future biomass estimation surveys at the Cascade Plateau should endeavour to reduce this potential for positive bias. The acoustic footprint of the vessel mounted system at orange roughy depths is very large (>70 metres diameter at 600 m) and is of particular concern due to inevitable inclusion of possibly a significant amount of signal from non-roughy species. A deeply towed acoustic system reduces this error and many of the other errors and problems associated with using vessel-mounted acoustics to quantify deep-water fish species (Kloser 1996). If such a system has multiple frequencies there is the potential to accurately distinguish orange roughy from other fish species (Kloser et al., 2002). This project has demonstrated that deployment of equipment such as the Drop TS is possible from industry vessels. It is recommended future quantitative biomass surveys be carried out using a deep-deployed multifrequency instrument. A self powered, self recording system such as the Drop TS, but repackaged to include a lower frequency (18 kHz), allow towing at survey speed (4 knots or better) and deployment from an industry vessel, may be the best way to achieve a higher survey precision. The information content would be similar to that obtained by our existing Multi-Frequency Towed Instrument (MUFTI-2) but operationally it would be far simpler, cheaper to implement and require less expertise to operate.

48 DISCUSSION AND CONCLUSION

4. DISCUSSION AND CONCLUSION

The 2 frequency lowered probe was able to classify orange roughy schools from other gas bladdered targets and provide a more robust biomass assessment for the Cascade Plateau orange roughy (Ryan 2006). The experimental results demonstrate that the larger ( 43sL cm= ) orange roughy at the Cascade Plateau have a similar volume reverberation dB difference ( = -4 to -5 dB) to that observed on the smaller Eastern Zone orange roughy (

12038−ΔSv35sL cm= ) and New

Zealand orange roughy ( 34sL c= m ) (Kloser et al., 2002). It appears that two frequencies are sufficient to robustly discriminate orange roughy in schools but this has not been demonstrated for lightly aggregated fish. Orange roughy are a weak scatterer of acoustic energy due to the lack of a gas filled bladder and they may also be similar in acoustic reflectivity to deepwater sharks that are a similar size and also lack a gas filled bladder. The addition of a lower frequency would improve the discrimination of large and small swim bladdered fish (Kloser et al., 2002). Extra frequencies may also be able to distinguish a wider range of targets that have similar morphologies such as sharks and orange roughy.

Due to avoidance and the high density of orange roughy within schools insufficient single target tracks were obtained to provide a measure of orange roughy in situ target strength. The in situ target strength measurements surrounding orange roughy schools where the 12038−ΔTS was 5 to 20 dB indicated the presence of small gas-filled swim bladdered fish (presumably 5 to 12 cm Myctophids). These small gas-bladdered fish have a similar target strength to orange roughy (Kloser et al., 2002). In situ target strength data also show that there are a high number of large gas bladdered fish with target strengths at 38 kHz of > -40 dB. These high target strength gas bladdered fish would cause a large positive bias on estimates of orange roughy biomass if included and reinforces the need to have rigorous species identification procedures (Kloser et al., 2002).

To improve on the experimental procedures used here it would be necessary to have a higher resolution system to resolve single targets in schools at ranges of 100 m. Based on a simple estimate of average fish density 0.05 fish m-3, an acoustic system of at least 5.3 m horizontal resolution at 100 m depth is required and is approximately equivalent to an angular resolution of 3o at the -10 dB power levels.

The finite-difference model of orange roughy backscatter has been able to demonstrate a 12038− that is in agreement with experimental observations. This is a major advance for the

modelling of orange roughy and gives more confidence in using models to estimate orange roughy target strength. There remains some uncertainty with the model results due to calibration and pressure and temperature differences in material properties. Orange roughy are composed of wax ester and changes in material properties of these wax esters will have a large impact on target strength (Kloser and Horne 2003). To the extent possible the adjustment for the material properties within the model is consistent with experimental data on the changes of wax esters with depth.

ΔTS

The model estimated that the 12038 dB was -6.5 dB and -4.8 dB for the 38 cm and 46 cm standard length orange roughy respectively assuming a normal tilt distribution (mean 0o and s.d. 15o) (

−ΔTS

Figure 4.1). At 38 kHz the model estimated that the normally distributed (mean 0o and s.d. 15o) tilt averaged target strength was -50.5 dB and -48.5 dB for the 38 cm and 46 cm orange roughy respectively (Figure 4.2). The target strength of a 35 cm and 43 cm orange roughy would be -50.8 dB and -49.4 dB respectively based on log10( )TS A L B= + where A = 16.37,

DISCUSSION AND CONCLUSION 49

B = -76.1 and L is the standard length in cm (Figure 4.2, Table 4.1). A is as derived by McClatchie et al., (1999) and B is the average of the two orange roughy modelled (Table 2.1).

Figure 4.1. Summary of model for the 46 cm (square) and 38 cm (diamond) orange roughy at variable normal distribution modes and sd of 5o (solid) and 15o dashed.

12038−ΔTS

-20 -15 -10 -5 0 5 10-8

-7.5

-7

-6.5

-6

-5.5

-5

-4.5

-4

Tilt distribution mode (degrees)

<TS

> dB

diff

38

-120

khz

Figure 4.2. Summary of model measurements of orange roughy target strength at 38 kHz (black and 120 kHz red for the 46 cm (square) and 38 cm (diamond) orange roughy tilt averaged with a normal distribution of variable mode and standard deviation of 5o (solid) and 15o (dashed).

Table 4.1 summarises the target strength values for orange roughy that have been measured and used to estimate orange roughy biomass at 38 kHz frequency. The new data in this report based on the larger Cascade Plateau orange roughy is within the bounds of other model and in situ studies.

50 DISCUSSION AND CONCLUSION

-20-52

-51

-50

-49

-48

-47

-46

-15 -10 -5 0 5 10

-45

-44

-43

-42

Tilt distribution mode (degrees)

<TS

> dB

DISCUSSION AND CONCLUSION 51

Table 4.1. Tilt averaged target strength at 38 kHz for orange roughy of the form <TS> = A* log10(Lsl) - B where <TS> is in dB and Lsl is the standard length in cm and A was determined by ex situ measurements (McClatchie et al., 1999) and B by both ex situ and in situ methods and transformed to a 35 cm and 43 cm standard length orange roughy. Currently biomass estimates of the Cascade Plateau use the Hampton and Soule (2002) relationship based on in situ measurements of Kloser et al., (2000) (Honkalehto and Ryan 2003).

Source Type A B Target

strength dB for 35 cm

Target strength dB for 43 cm

Biomass Factor Comments

Kloser et al., (1997) In situ 16.37 75.3 -50.0 -48.6 0.83 Based on in situ measurements Kloser et al., (1997) problems of

species identification raised.

McClatchie and Ye (2000) Ex situ 16.37 73.6 -48.3 -46.9 0.56

Based on ex situ measurements of 16 anesthetised fish in a tank at 1- 2 m depth and a scattering model (McClatchie et al., 1999). Problems with gas inclusions for surface measurements and changing material

proporties at depth with adjustment of ~2 dB included.

Kloser et al., (2000) In situ 16.37 76.5 -51.2 -49.8 1.10

Based on in situ measurements of Kloser et al., 2000 and length dependency (A = 16.37) of McClatchie et al., 1999. Problems of

species identification with in situ experiments. Doonan and Bull

(2001) In situ 16.15 74.3 -49.4 -47.9 0.71 Based on in situ measurements obtained from 38 kHz phase measurements - potential problems with species identification

Hampton and Soule (2002) 16.15 76.2 -51.3 -49.8 1.11

Based on in situ measurements of Kloser et al., (2000) and length dependency (A = 16.37) of Doonan and Bull (2001). Problems of

species identification with in situ experiments.

Kloser and Horne (2003) In situ 16.37 78.2 -52.9 -51.5 1.64

Only based on 9 tracked fish on the periphery of a school satisfying multi-frequency criteria Kloser and Horne (2003). Length dependency

(A = 16.37) based on McCLatchie et al., 1999

This report Model 16.37 76.1 -50.8 -49.4 1.00

Limitations due to number of fish scanned (2) as being representative of different lengths, sex and stages. Possible

calibration uncertainties. Due to low number of samples assume length relationship follows that of McClatchie et al., (1999) and B is average of the two normal tilt averaged target strengths (mean 0o

and sd 15o)

4.1 Management implications

This study has demonstrated the need for rigorous non trawl based species identification for both absolute and relative orange roughy acoustic biomass estimates. Orange roughy vessel-mounted acoustic biomass assessments are prone to positive bias due to inclusion of gas bladdered fishes within the region of schooling orange roughy. This study found that this bias could have been as high as a factor of 9 if the echograms had not been interpreted by an experience analyst. Application of deep-towed multifrequency systems in future surveys would greatly reduce the potential for positive bias due to incorrect echogram interpretation. This study reinforces earlier work on orange roughy schools off the East coast of Tasmania (e.g. Kloser et al., 2002).

Prior to the study the uncertainty for target strength of a 43 cm standard length orange roughy was -47.9 dB to -51.5 dB, approximately a factor of 2.3 (Table 4.1). All previous measurements were based on extrapolations of measurements of smaller <36 cm orange roughy. Modelling results of the larger Cascade Plateau orange roughy from this study show that -49.4 dB is more applicable and within the bound of uncertainty. The current Cascade Plateau biomass assessments will reduce by 10 % using this updated target strength measurement. The model target strength has potential error sources that are bounded by a factor of 0.71 to 1.64 (Table 4.1). Further work is outlined below that would reduce this uncertainty.

4.2 Recommended future work.

Future Cascade Plateau acoustic surveys need rigorous non trawl based species identification methods to ensure unbiased absolute and relative orange roughy acoustic biomass estimates.

To resolve uncertainties in the modelled target strength measurements used in this report there needs to be:

o More fish run through the model of various lengths and sex.

o Calibration of the model with more standard targets of known reflectance.

o Sensitivity of the model to pressure and temperature material properties assumptions.

In situ measurements are required to validate the model target strengths at specific tilt angles with:

o Higher resolution acoustic system to observe single targets within schools. Based on a first order analysis a 3 degree acoustic system at the -10 dB power levels would be suitable.

o Simultaneous ensonification of orange roughy with acoustics and video to obtain positive identification, length and orientation measurements. (e.g. Using an acoustic and video package on the headline of a trawl may provide this information.)

The multi frequency target strength data collected should enable a biomass and density of prey species to be obtained which would be useful for longer term ecosystem based fisheries management objectives.

APPENDIX A - 53

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acoustic measurements of cascade plateau orange roughyroughy (kloser et al. 2002). in order to...

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