3.6 south minch, fu12 reports/expert group...south minch due to the weather and small boats are...

ICES WGCSE REPORT 2010 | 185

3.6 South Minch, FU12

Type of assessment in 2010

The assessment and provision of advice through the use of the UWTV survey data and other commercial fishery data follows the process defined by the benchmark WG and described in Section 2.2.

3.6.1 Ecosystem aspects

The South Minch Functional Unit 12 is located mid way down the west coast of Scot-land (Figure 3.5.1).

Owing to its burrowing behaviour, the distribution of Nephrops is restricted to areas of mud, sandy mud and muddy sand. Within the South Minch Functional Unit these substrates are distributed according to prevailing hydrographic and baythmetric conditions. The area is characterised by numerous islands of varying size and sea lochs occur along the mainland coast. These topographical features create a diverse habitat with complex hydrography and a patchy distribution of soft sediments. A more continuous extensive area of sediment suitable for Nephrops occurs further off-shore to the southwest. Figure 3.6.4 shows the distribution of sediment in the area.

Additional information on ecosystem aspects can be found in the Stock Annex.

3.6.2 The fishery in 2009

Two distinct fleets continued to operate in the South Minch during 2009, landing into the two main ports of Oban and Mallaig. Inshore, a large fleet of smaller vessels in-cluding creel boats operated throughout the year, whilst some larger twin riggers fished slightly further afield. 90% of boats are thought to fish for Nephrops at some time. Around 15 to 20 vessels are resident to Mallaig throughout the year. The local fleet has declined over the years. Approximately ten of these vessels are ‘day boats’, and approximately five are 17–19 m long twin riggers. Trips were typically of 1–3 days usually operating within about two hours steaming distance.

Traditionally east coast vessels (mainly twin riggers from Fraserburgh) visit Mallaig in March or April, but in the last years there was a significant reduction in effort from visiting vessels. During the winter months, fishing activity is severely reduced in the South Minch due to the weather and small boats are often restricted to trawling in the sheltered sea-lochs. There is increasing overlap of the areas exploited by trawl and creel fishing (This is described further in the quality of assessment section illustrate the extent of trawling by some vessels). Boats on the west coast of Scotland are oper-ating in accordance with the Scottish Conservation Credits Scheme and during 2009 were also required to fit 120 mm square meshed panels in accordance with the west coast emergency measures (Council Reg. (EU) 43/2009).

3.6.3 ICES advice for 2009 and 2010

ICES advice for 2009 based on Exploitation boundaries in relation to precautionary considerations was as follows:

“The current fishery appears sustainable. Therefore, ICES recommends that Nephrops fisheries should not be allowed to increase relative to the past two years (2006–2007). This corresponds to landings of no more than 5000 tonnes for the South Minch stock.”

186 | ICES WGCSE REPORT 2010

ICES advice for 2010 based on Single-stock exploitation boundaries was as follows:

“ICES advises on the basis of exploitation boundaries in relation to high long-term yield and low risk of depletion of production potential that the Harvest Rate for Nephrops fisheries should not exceed F2008. This corresponds to landings of no more than 4126 t for the South Minch stock.”

3.6.4 Management applicable in 2009 and 2010

Management applicable to this stock is included in management for Division VIa as a whole, and is described in Section 3.5.1.

3.6.5 Assessment

No specific concerns were raised in relation to the assessment method or data during the review of the 2009 assessment.

Approach in 2010

The assessment in 2010 is based on a combination of examining trends in fishery in-dicators and underwater TV using an extensive data series for the South Minch FU12.

The assessment of Nephrops through the use of the UWTV survey data and other commercial fishery data follows the process defined by the benchmark WG and de-scribed in the Stock Annex.

The provision of advice in 2010 develops the process defined by the Benchmark WG. Section 2.2 outlines the WG approach to integrate WKFRAME recommendations in the provision of FMSY proxies for Nephrops. The approach was developed based on intersessional work carried out by participants of the Benchmark and involving col-laboration between WGNSSK and WGCSE.

Previous TV based assessments have derived predicted landings by applying a har-vest rate approach to populations described in terms of length compositions from the trawl component of the fishery. Creel fishing is important in the South Minch and increasingly operates across similar areas to the trawl fishery. For this reason the as-sessment is performed using combined length compositions from these fisheries.

Data available

An overview of the data provided and used by the WG is shown in Table 2.1.

Commerical catch and effort data

Official catch statistics (landings) reported to ICES are shown in Table 3.5.2. These relate to the whole of VIa of which the South Minch is a part. Landings for FU12 pro-vided through national laboratories are presented in Table 3.6.1, broken down by country and by gear type. Landings from this fishery are predominantly reported from Scotland, with low levels reported from the rest of the UK in the mid-1990s, and low levels more recently reported for Ireland. Total international reported landings in 2009 was 4282 tonnes, consisting of 3347 tonnes landed by trawlers and 900 tonnes landed by creel vessels. These estimates for total landings show a reduction from the high values in the previous two years to landings more typical of the late 1980s. The high landings of 2006–2008 are thought to have arisen through a combination of good recruitment in the mid-2000s feeding into the fished population, increased catching opportunities and to the introduction of the “buyers and sellers” regulations in the UK in 2006 which have increased the reliability of landings information. Landings


from creel vessels have remained relatively stable over the last four years, at close to 1000 tonnes, the highest level in the time-series.

Reported effort (given in days fished rather than hours since this is thought to be more reliable).by all Scottish Nephrops trawlers has fluctuated without trend in the most recent years after reaching a peak in the early 1990s. (Figures 3.6.1 and 3.6.2).

Sex ratio in the South Minch shows some variation but males consistently make the largest contribution to the annual landings. This occurs because males are available throughout the year and the fishery is also prosecuted in all quarters. Females on the other hand are mainly taken in the summer when they emerge after egg hatching (Figure 3.6.2).

Discarding of undersized and unwanted Nephrops occurs in this fishery, and quar-terly discard sampling has been conducted on the Scottish Nephrops trawler fleet since 2000. Discarding rates average around 21% by number in this FU (Table 3.6.5).

Studies (Guéguen, J. and Chareau, A., 1975; Sangster et al., 1997; Wileman et al., 1999) suggest that some Nephrops survive the discarding process, an estimate of 25% sur-vival is assumed for this FU in order to calculate removals

Length compositions

(landings + dead discards) from the population. The discard rate adjusted to account for some survival was es-timated at the Benchmark Workshop (WKNEPH 2009) to be 16.7% (taking a three year average 2005–2007) and according to the agreed benchmark protocol this value is used in the provision of landings options for 2011.

Length compositions of landings and discards are obtained during monthly market sampling and quarterly on-board observer sampling respectively. Quarterly landings and discards-at-length data were available from Scotland and these sampling levels are shown in Table 3.5.4. Length compositions for the creel fishery are of landings only since the small numbers of discards survive well and are not considered to be removed from the population. Although assessments based on detailed catch analysis are not currently possible, examination of length compositions can provide a prelimi-nary indication of exploitation effects.

Figure 3.6.3 shows a series of annual length frequency distributions for the period 1979 to 2009. Catch (removals) length compositions are shown for each sex along with the mean size for both. In both sexes the mean sizes have been fairly stable over time. Examination of the tails of the distributions above 35 mm (the length beyond which the effects of recruitment pulses and discarding are considered to be negligi-ble) shows no evidence of reductions in relative numbers of larger animals.

The observation of relatively stable length compositions is further confirmed in the series of mean sizes of larger Nephrops (>35 mm) in the landings shown in Figure 3.6.1 and Table 3.6.2. This parameter might be expected to reduce in size if overexploita-tion were taking place but there is no evidence of this. The mean size of smaller ani-mals (


Research vessel data

Underwater TV surveys using a stratified random approach are available for this stock since 1995. Underwater television surveys of Nephrops burrow number and dis-tribution, reduce the problems associated with traditional trawl surveys that arise from variability in burrow emergence of Nephrops. TV surveys are targeted at known areas of mud, sandy mud and muddy sand in which Nephrops construct burrows.

The numbers of valid stations used in the final analysis in each year are shown in Ta-ble 3.6.4. On average, 35 stations have been considered valid each year, and then raised to a stock area of 5072 km2. In 2009 station numbers were the second lowest in the time-series, owing to time constraints on the RV survey and the presence of creels at a number of the planned station locations.

Data analyses

Exploratory analyses of survey data

Full details of the UWTV approach can be found in the Stock Annex and the Report of (WKNEPH) in 2009 (ICES, 2009).

A re-working of the UWTV survey abundance series for Division VIa was presented to the Nephrops Benchmark Workshop (WKNEPH) in 2009 (ICES, 2009) and further details of the technical changes to the camera can be found in the report of that work-shop. The revised abundance estimates for FU12 from 1999 onwards were presented for the first time at WGCSE 2009 and are slightly higher than the previous values due to the field of view being smaller than previously calculated.

Table 3.6.3 shows the basic analysis for the three most recent TV surveys conducted in FU12. The table includes estimates of abundance and variability in each of the strata adopted in the stratified random approach. Due to the fact only one station was surveyed in the mud sediment type in 2008, it was not possible to calculate a sample variance for this area in the usual way. Instead an average of the three previous years was taken. Results in 2009 were typical of previous years.

Figure 3.6.4 shows the distribution of stations in recent TV surveys (2004–2009), with the size of the symbol reflecting the Nephrops burrow density. The most recent sur-vey suggests higher abundance in the southeast part of the functional unit.. Table 3.6.4 and Figure 3.6.5 show the time-series estimated abundance for the TV surveys, with 95% confidence intervals on annual estimates.

The review of the use of the UWTV surveys for Nephrops in the provision of advice was extensively reviewed by WKNEPH (ICES, 2009). A number of potential biases were highlighted including those due to edge effects, species burrow mis-identification and burrow occupancy. The cumulative bias correction factor esti-mated for FU12 was 1.32 meaning that the TV survey is likely to overestimate Neph-rops abundance by 32%.

Final assessment

The underwater TV survey is presented as the best available information on the South Minch (FU12) Nephrops stock. This survey provides a fishery independent es-timate of Nephrops abundance. The details of the 2009 survey is shown in Table 3.6.3 and compared with the 2007 and 2008 outcomes. At present it is not possible to ex-tract any length or age structure information from the survey and it therefore only provides information on abundance over the area of the survey.


The 2009 TV survey data presented at this meeting shows that the abundance is more or less the same as in 2008, although the confidence limits are quite large around the estimate.

The TV survey results reported here do not cover the sea loch areas adjacent to the main South Minch grounds and should therefore be considered underestimates of the overall abundance. The sea lochs support an unknown but significant part of both the trawl and creel fishery. This issue is discussed further under quality of assess-ment.

3.6.6 Historic stock trends

The TV survey estimates of abundance for Nephrops in the South Minch show that the population has fluctuated without obvious trend over the period of the survey. The recently observed upturn gives an abundance which is just below the long-term aver-age (2200 million animals). The bias adjusted abundance estimates from 1999–2009 (the period over which the survey estimates have been revised) is shown in Table 3.6.5. The stock is estimated to now be at 2035 million individuals as shown in Table 3.6.4.

Table 3.6.5 also shows the estimated harvest ratios over this period. These range from 7–27% over this period. (It is likely that prior to 2006, the harvest ratios are un-derestimates of the actual harvest ratios due to under-reporting of landings).

3.6.7 MSY considerations

A number of potential Fmsy proxies are obtained from the per-recruit analysis for Nephrops and these are discussed further in Section 2.2 of this report. The analysis assumes the same input parameters (exploitation, discard ogive and biological pa-rameters) as used at the Benchmark meeting in 2009. The complete range of the per-recruit Fmsy proxies is given in the table below and the process for choosing an appro-priate Fmsy proxy is described in Section 2.2. Note that all Fmsy proxy harvest rate val-ues are considered preliminary and may be modified following further data exploration and analysis.

For this FU, the absolute density observed on the UWTV survey is intermediate (av-erage of just over 0.43 m-2) suggesting the stock has moderate productivity. In addi-tion, the fishery in this area has been in existence since the 1960s and the population has been studied numerous times (Afonso-Dias, 1998; Howard and Hall, 1983). His-torical harvest ratios in this FU have been variable but generally around the F35%SpR.. The WG concluded that combined sex F35%SpR is an appropriate F proxy for South Minch FU12 Nephrops. This is slightly below Fmax in males and is predicted to result in about 27% SPR for males; in excess of the 20% considered precautionary lower bound outlined in Section 2.2.


Fbar(20–40 mm)

HR (%)

SPR (%)

Fmult M F M F T

F0.1

M 0.22 0.13 0.06 7.8 40.9 60.8 48.5

F 0.44 0.27 0.12 13.8 23.8 43.7 31.4

T 0.25 0.15 0.07 8.7 37.4 57.7 45.2

Fmax

M 0.42 0.25 0.12 13.3 24.8 44.8 32.5

F 1.1 0.67 0.31 26.8 9.9 23.6 15.2

T 0.54 0.33 0.15 16.1 19.8 38.7 27.1

F35%SpR

M 0.28 0.17 0.08 9.6 34.5 54.9 42.3

F 0.64 0.39 0.18 18.3 16.9 34.8 23.8

T 0.38 0.23 0.11 12.3 27.0 47.3 34.8

The Btrigger point for this FU (bias adjusted lowest observed UWTV abundance) is cal-culated as 1016 million individuals.

3.6.8 Landings forecasts

A landings prediction for 2011 was made for the South Minch (FU12) using the ap-proach agreed at the Benchmark Workshop and outlined in the Section 2.2. The text table below shows landings predictions at various harvest ratios, including a selec-tion of those equivalent to the per-recruit reference points discussed in Section 2 of this report and the harvest ratio in 2009 using the input parameters agreed at WKNEPH (ICES 2009). The landings prediction for 2011 at the Fmsy proxy harvest ratio considered appropriate for the South Minch (i.e. 12.3%) is 3809 tonnes. Since current harvest rate is above the Fmsy proxy, the transition scheme towards the ICES MSY framework applies and would result in a landings estimate of 3995 tonnes for 2011.

The inputs to the landings forecast were as follows:

Mean weight in landings (07–09) = 23.8 g

Discard rate (by number) = 15.7%

Survey bias = 1.32.

Harvest rate Survey Index (adjusted)

Implied fishery

Retained number Landings (tonnes)

Fmsy 12.3% 1542 160 3809

Fmsy transition 12.9% 1542 168 3995

F0.1(M) 7.8% 1542 101 2416

F0.1(T) 8.7% 1542 113 2694

F35%SpR(M) 9.6% 1542 125 2973

F35%SpR(T) 12.3% 1542 160 3809

F2009 13.0% 1542 169 4026

Fmax (M) 13.3% 1542 173 4119

Fmax (T) 16.1% 1542 209 4986

F0.1(M,T) : Harvest ratio equivalent to fishing at a level associated with 10 % of the slope at the origin on the male or combined sex YPR curve.


F35%SPR(M,T): Harvest ratio equivalent to fishing at a rate which results in male or com-bined SPR equal to 35% of the unfished level.

Fmax (M, T) : Harvest ratio equivalent to fishing at a rate which maximises the male or combined YPR.

A discussion of Fmsy reference points for Nephrops is provided in Section 2.2.

3.6.9 Biological reference points

Precautionary approach biological reference points have not been determined for Nephrops stocks.

3.6.10 Quality of assessment and forecast

The length and sex composition of the landings data is considered to be well sam-pled. Discard sampling has been conducted on a quarterly basis for Scottish Nephrops trawlers in this fishery since 1990, and is considered to represent the trawl fishery adequately. In this assessment combined trawl and creel length compositions are used to account for the fact that the creel fishery accounts for over 20% of the land-ings, increasingly operates over similar areas to trawling, and exhibits a length com-position composed of larger animals.

There are concerns over the accuracy of historical landings and effort data prior to 2006 when Buyers and Sellers was introduced and the reliability began to improve. Because of this the final assessment adopted is independent of official statistics. In-corporation of creel length compositions has also improved estimates of harvest ra-tios.

Underwater TV surveys have been conducted for this stock every year since 1995. The number of valid stations in the survey has remained relatively stable throughout the time period. Confidence intervals around the abundance estimates are on average greater during the most recent years, when abundance estimates have been slightly higher. The overlap of confidence intervals makes it difficult to determine which population changes are significant. Results suggest the population has fluctuated without trend.

There is a gap of 18 months between the survey and the start of the year for which the assessment is used to set management levels. It is assumed that the stock is in equi-librium during this period (i.e. recruitment and growth balance mortality) although this is rarely the case. The effect of this assumption on realised harvest rates has not been investigated.

The cumulative bias estimates for FU12 are largely based on expert opinion (See An-nex). The precision of these bias corrections cannot yet be characterised.

The survey should be considered as a minimum estimate. Overall area of the ground is estimated by contoured sediment data. New VMS data linked to landings (from queries of the Scottish FIN database) suggest that not all areas are being considered in the current UWTV approach and as such, the absolute abundance estimate for this ground is likely to be an underestimate. Figure 3.6.6 illustrates differences between the British Geological Survey based sediment approach to estimating area and the activity of >15 m trawlers– inclusion of smaller vessels would likely further modify this. Work is in progress to refine the area estimate.

The landings forecast for 2011 (equivalent to fishing at F35%SpR) is 4009 tonnes. This is very close to the reported landings in 2009. In the provision of catch options based


on the absolute survey estimates additional uncertainties related to mean weight in the landings (which in this case are very stable) and the discard rates also arise. A three year average (2005–2007) of discard rate (adjusted to account for some survival of discarded animals) have been used in the calculation of catch options. The recent observed discard rate has however shown a 50% decline in 2009.

3.6.11 Status of the stock

The UWTV survey indicates that the population declined from a record high in 2004 to record low in 2007 but is at a higher level again in 2009. The stable mean sizes in the length compositions of catches (of individuals >35 mm CL) and recent fall in es-timated harvest ratios (removals/TV abundance) to the equivalent of the FMSY proxy suggests that the stock is now being exploited sustainably.

3.6.12 Management considerations

The WG, ACFM and STECF have repeatedly advised that management should be at a smaller scale than the ICES Division level. Management at the Functional Unit level could confer controls to ensure effort and catch were in line with resources available.

Creel fishing takes place in this area but overall effort in terms of creel numbers is not known and measures to control numbers are not in place. There is a need to ensure that the combined effort from all forms of fishing is taken into account when manag-ing this stock.

There is a bycatch of other species in the area of the South Minch and STECF contin-ues to estimate that discards of whiting and haddock are high in VIa generally. It is important that efforts are made to ensure that unwanted bycatch is kept to a mini-mum in this fishery. Current efforts to reduce discards and unwanted bycatches of cod under the Scottish Conservation Credits scheme and the West of Scotland emer-gency measures (Council Reg. (EU) 43/2009), include the implementation of larger meshed square meshed panels (120 mm) and real time closures to avoid cod.

The implementation of buyers and sellers legislation in the UK in 2006 is improving the reliability of fishery statistics but the transition period is accompanied in some cases by large changes in landings which produce significant changes in the lpue and cpue series that cannot be completely attributed to changes in stock. Until a sufficient time-series of reliable data has built up, use of fishery catch and effort data in the as-sessment process should be avoided.


3.6.13 References

Afonso-Dias, M. S. 1998. Variability of Nephrops norvegicus (L.) populations in Scottish waters in relation to the sediment characteristics of the seabed. PhD Thesis University of Aber-deen. 282 pp.

Council Reg. (EU) 43/2009.

Guéguen, J. and Chareau, A. 1975. Essai de détermination du taux de survie des langoustines hors taille rejetées lors des opérations de pêche commerciale. ICES, Doc. Shellfish Comm., CM 1975/K:12 (mimeo).

Howard F.G. and Hall, W.B. 1983. Some observations on the biometrics of Nephrops norvegicus (L.) in Scottish waters. ICES, Doc.ShellfishComm.,CM1983/K:36.

Sangster, G.I., Breen, M., Bova, D.J., Kynoch, R., O’Neill, F.G., Lowry. N., Moth-Poulsen, T. Hansen, U.J., Ulmestrand, M., Valentinsson, D., Hallback, H., Soldal, A.V., and Hod-devik, B. 1997. Nephrops survival after escape and discard from commercial fishing gear. Presented at ICES FTFB Working Group, Hamburg, Germany 14–17 April, 1997, ICES CM 1997 CM/B.

Wileman, D.A., Sangster, G.I., Breen, M., Ulmestrand, M., Soldal, A.V. and Harris, R.R. (1999) Roundfish and Nephrops survival after escape from commercial fishing gear. EU Contract Final Report. EC Contract No: FAIR-CT95-0753.


Table 3.6.1. Nephrops, South Minch (FU12), Nominal Landings of Nephrops, 1981–2009, as offi-cially reported.

Year

UK Scotland

Other UK Ireland Total

Nephrops trawl

Other trawl

Creel Sub-total

1981 2965 254 432 3651 0 0 3651

1982 2925 207 420 3552 0 0 3552

1983 2595 361 456 3412 0 0 3412

1984 3228 478 594 4300 0 0 4300

1985 3096 424 488 4008 0 0 4008

1986 2694 288 502 3484 0 0 3484

1987 2927 418 546 3891 0 0 3891

1988 3544 364 555 4463 10 0 4473

1989 3846 338 561 4745 0 0 4745

1990 3732 262 436 4430 0 0 4430

1991 3597 341 503 4441 1 0 4442

1992 3479 208 549 4236 1 0 4237

1993 3608 193 649 4450 5 0 4455

1994 3743 265 404 4412 3 0 4415

1995 3442 716 508 4666 14 0 4680

1996 3107 419 468 3994 1 0 3995

1997 3519 331 492 4342 3 1 4345

1998 2851 340 538 3729 0 0 3730

1999 3165 359 513 4037 0 14 4051

2000 2939 312 699 3950 0 2 3952

2001 2823 393 767 3983 0 9 3992

2002 2234 315 742 3291 0 14 3305

2003 2812 203 858 3873 0 6 3879

2004 2865 104 880 3849 0 19 3868

2005 2810 46 953 3809 1 31 3841

2006 3569 19 922 4510 9 35 4554

2007 4436 8 958 5402 19 30 5451

2008 4432 5 895 5332 2 13 5347

2009 3347 20 900 4267 4 11 4282

* provisional na = not available.


Table 3.6.2. Nephrops, South Minch (FU12): Mean sizes (CL mm) above and below 35 mm of male and female Nephrops in Scottish catches and landings, 1981–2009.

Year

Catches Landings

< 35 mm CL < 35 mm CL > 35 mm CL

Males Females Males Females Males Females

1981 28.2 26.4 29.6 27.5 41.5 38.0

1982 27.8 27.1 28.7 28.8 41.7 41.3

1983 28.6 26.5 29.3 27.6 39.5 37.6

1984 27.9 26.3 28.4 27.0 39.8 38.0

1985 27.9 27.5 28.6 28.5 40.0 37.6

1986 28.4 27.9 29.3 28.9 39.5 37.3

1987 28.3 26.6 29.2 28.1 39.8 37.6

1988 29.3 27.7 30.4 29.7 39.5 38.6

1989 28.6 28.1 29.8 29.4 39.5 38.4

1990 28.0 27.5 29.3 29.0 39.4 38.5

1991 29.4 27.5 29.9 27.9 39.0 38.5

1992 29.6 28.6 31.0 29.8 39.5 38.0

1993 29.0 27.8 30.0 28.5 39.5 38.0

1994 29.8 28.0 30.8 29.2 39.3 38.1

1995 29.5 28.2 30.0 28.4 39.4 38.0

1996 28.9 28.5 30.4 29.8 39.9 38.1

1997 29.3 28.7 30.6 29.6 39.8 37.8

1998 28.6 27.6 30.4 28.7 39.1 38.0

1999 28.6 27.7 30.0 29.5 39.4 38.3

2000 28.9 28.3 30.9 30.0 39.7 38.5

2001 27.7 27.3 29.7 28.8 39.6 38.1

2002 29.1 27.8 30.4 29.0 39.5 38.8

2003 29.0 28.1 30.4 29.5 39.8 38.4

2004 28.8 28.1 30.1 29.8 39.5 38.8

2005 28.1 27.8 30.4 29.5 39.8 38.6

2006 29.2 28.0 30.5 28.8 39.5 38.1

2007 29.7 28.2 29.9 28.2 40.0 38.3

2008 28.6 27.5 29.4 28.5 39.6 38.1

2009 28.7 27.8 29.7 28.6 40.0 38.3

* provisional na = not available.


Table 3.6.3. Nephrops South Minch (FU12). Results by stratum of the 2007–2009 TV surveys. Note that stratification was based on a series of sediment strata (M – Mud, SM – Sandy mud, MS – Muddy sand).

Stra

tum

Are

a (k

m²)

Num

ber

of

Stat

ions

Mea

n bu

rrow

de

nsit

y

(no.

/m²)

Obs

erve

d va

rian

ce

Abu

ndan

ce

(mill

ions

)

Stra

tum

va

rian

ce

Prop

orti

on

of t

otal

va

rian

ce

2007 TV Survey

M 303 3 0.21 0.01 65 372 0.008

SM 2741 15 0.30 0.07 822 33184 0.727

MS 2028 21 0.24 0.06 490 12092 0.265

Total 5072 39 1377* 45647 1

2008 TV Survey

M 303 1 0.58 0.05 176 4593 0.037

SM 2741 18 0.45 0.19 1227 78145 0.636

MS 2028 14 0.36 0.14 718 40157 0.327

Total 5072 33 2121* 122895 1

2009 TV Survey

M 303 2 0.135 0.004 41 186 0.001

SM 2741 13 0.447 0.207 906 65406 0.373

MS 2028 10 0.397 0.146 1088 109660 0.626

Total 5072 25 2035 175252 1

*Note: abundance estimates for these years based on figures prior to the 2009 revision of the dataseries. Differences between these figures and the revised figures shown on Table 3.6.4 are small.

Table 3.6.4. Nephrops, South Minch (FU12): Results of the 1995–2009 TV surveys.

Year

Stations Mean density Abundance

95% confidence interval

burrows/m² millions millions

1995 33 0.30 1520 331 1996 21 0.38 1945 700 1997 36 0.28 1434 244 1998 38 0.38 1916 306 1999 37 0.28 1433 343 2000 41 0.48 2447 460 2001 47 0.53 2689 606 2002 31 0.49 2507 749 2003 25 0.56 2847 998 2004 38 0.67 3377 625 2005 33 0.57 2914 977 2006 36 0.48 2436 789 2007 39 0.26 1341 205 2008 33 0.42 2123 548 2009 25 0.40 2035 837


Table 3.6.5. Nephrops, South Minch (FU12): Adjusted TV survey abundance, landings, discard rate proportion by number) and estimated harvest rate.

Adjusted survey (millions)

Landings (tonnes) Discard rate (%) Harvest ratio*

1999 1086 4051 0.15 0.16

2000 1854 3952 0.19 0.09

2001 2037 3992 0.28 0.11

2002 1899 3305 0.18 0.07

2003 2157 3879 0.21 0.08

2004 2558 3868 0.24 0.07

2005 2208 3841 0.26 0.08

2006 1845 4554 0.14 0.11

2007 1016 5451 0.22 0.27

2008 1608 5347 0.25 0.17

2009 1542 4282 0.12 0.13

*harvest rates previous to 2006 are unreliable


Figure 3.6.1. Nephrops, South Minch (FU12), Long-term landings, effort, lpue and mean sizes.


Figure 3.6.2. Nephrops, South Minch (FU12), Landings, effort and lpues by quarter and sex from Scottish Nephrops trawlers.


Figure 3.6.3. Nephrops. South Minch (FU12). Catch length frequency distribution and mean sizes (red line) for Nephrops in the South Minch, 1979–2009.

10 20 30 40 50 60 70

1980

1985

1990

1995

2000

2005

2010

Males

Length (mm)

Yea

r

10 20 30 40 50 60 70

1980

1985

1990

1995

2000

2005

2010

Females

Length (mm)


Figure 3.6.4. Nephrops, South Minch (FU12), TV survey station distribution and relative density (burrows/m2), 2004–2009. Shaded green and brown areas represent areas of suitable sediment for Nephrops. Bubbles in this figure are all scaled the same. Red crosses represent zero observations.


Figure 3.6.5. Nephrops, South Minch (FU12), Time-series of revised TV survey abundance esti-mate (not adjusted for bias), with 95% confidence intervals, 1995–2009.


Figure 3.6.6. Nephrops, South Minch (FU12), comparison of area of Nephrops ground defined by BGS sediment distribution (upper plot) and by distribution of VMS pings (shown in red) re-corded from Nephrops trawlers >15 m length (lower panel).

3.6 South Minch, FU123.6.1 Ecosystem aspects3.6.2 The fishery in 20093.6.3 ICES advice for 2009 and 20103.6.4 Management applicable in 2009 and 20103.6.5 Assessment3.6.6 Historic stock trends3.6.7 MSY considerations3.6.8 Landings forecasts3.6.9 Biological reference points3.6.10 Quality of assessment and forecast3.6.11 Status of the stock3.6.12 Management considerations3.6.13 References

3.6 south minch, fu12 reports/expert group...south minch due to the weather and small boats are...

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