Not to be cited without permission ICES / CIEM International Council for the Exploration of the Sea ICES CM2007/Q:22

Estimation of catch efficiency in a new angler fish survey trawl

D.G.Reid, R.J. Kynoch, I. Penny & K. Peach.

FRS Marine Laboratory Aberdeen

Abstract A new series of targeted angler fish (Lophius spp.) bottom trawl surveys were launched by FRS in 2005. The surveys used a new survey net based on a commercial trawl and adapted for survey purposes in collaboration with the fishing industry. The aim of the surveys was to use swept area abundance estimates to provide an absolute abundance estimate for these species in waters around Scotland. Before this was possible it was necessary to determine the capture efficiency of the net using the efficiency equations developed by Somerton et al (1999). Two components were examined: herding by the sweeps, and losses under the footrope. Herding was reported in a previous paper. The present paper presents the results of a series of trials using bags below the fishing line to quantify the proportion of fish at length that escaped below the gear. The results show clear length dependency with smaller fish more likely to be lost under the gear. Below 30cm, approximately 75% of the angler fish were found in the ground gear bags, while for the larger fish the proportion was 25%. The paper presents the catching efficiency of the net using both the sweep herding and losses under the ground gear. In brief, the combination of the two components indicated that the net efficiency was approximately 0.7. That is, that the net would be expected to catch approximately 70% of the fish that were originally in the path of the net between the wings. Keywords: Angler fish, trawl efficiency, ground gear Corresponding author: Dave Reid, Fisheries Research Services, Marine Laboratory Aberdeen, Victoria Road, PO Box 101, Aberdeen, AB11 9DB, Scotland, UK. Tel: +44 1224 295363, Fax: +44 1224 295511, Email: reiddg@marlab.ac.uk

Introduction The Northern Shelf and North Sea angler fish (Lophius piscatorius & L. budegassa) fisheries are of considerable commercial importance to Scotland. Current quotas are set at around 9,800 tonnes, worth around €24.5m. By weight, it is the second most abundant demersal species landed in Scotland. However, the assessment for these species has been difficult for some years (see ICES Working Group on the Assessment of Northern Shelf Demersal Stocks (WGNSDS) reports (ICES 2005 & 2006)). The assessment depended on information on effort and landings from the fishery and on fishery independent data from bottom trawl surveys. Effort and landings data were considered unreliable, and WGNSDS also examined the available survey data (from IBTS surveys in ICES areas IV and VIa), and found conflicting signals. Based on this the ICES Advisory Committee on Fisheries Management (ACFM) recommended the establishment of a suitable survey that could provide reliable abundance estimates for these species. The surveys were initiated by Fisheries Research Services, Aberdeen (FRS) in the autumn of 2005, using both research and commercial vessels. The surveys used a newly designed trawl gear based on commercial trawls and modified for research purposes. The aim was to use the surveys to establish absolute abundance using swept area estimates of fish density and raising these to strata (Sparholt 1990). The conversion of trawl CPUE to biomass estimates requires an estimation of the swept area covered by the trawl and an estimate of the survey trawl catchability Q (Somerton et al 1999). Somerton et al divided catchability into three components; vertical herding, horizontal herding and escapement under the footrope. In a species like angler fish the type of vertical herding described by Godø and Totland (1996) is unlikely to occur, as these fish tend to stay on the seabed. Horizontal herding along the sweeps and bridles has been investigated in this net, and been shown to be relatively small (Reid et al – in press). This study showed that approximately 4% of the fish encountering the wires would be likely to be caught. That leaves escapement under the footrope (Engås and Godø 1989; Walsh 1992) as a possible modifier for swept area abundance estimates. The present study was designed to investigate the extent to which angler fish escape under the footrope in the new survey trawl. The experiments used additional collecting bags mounted underneath the fishing line. The design was based on that developed by Engås and Godø (1989) and used on similar rock-hopper gear by Langeland et al (in press). The experimental approach involved a series of repeat tows to quantify the catch in both the cod end and in the collecting bags. Data were analysed to provide a figure for the proportion caught by length categories. This in turn was used in the catchability equation described by Somerton et al (1999) and parameterised for horizontal avoidance in this gear by Reid et al (in press).

Material & Methods Main gear The survey trawl used in this survey and for the stock estimation surveys was based on standard commercial gear used in the anglerfish fishery. The design was based on discussion between FRS and industry representatives and the specification included the following key points;

• Ground gear length to be 45.7m. • Rockhopper discs in the centre to be 400mm diameter. • The rockhoppers to be rigged on 19mm chain. • To ensure no Anglerfish pass over the headline the design must incorporate a

‘ballooned’ top sheet (approximately 20% more) similar to that already supplied to the fleet.

• Because it is important to ensure small Anglerfish and megrims are retained the mesh size in the lower wings must be 120mm.

• High tenacity twine should be used throughout the trawls construction. • Both headline and footrope to be wrapped with rope and include selvedge

ropes. • Design must incorporate measures to give added strengthening to weak points

around the mouth and belly of the trawl. This strengthening to be similar as that which would normally be built into commercial scraper trawls (i.e. top and bottom guard meshes and tearing strips etc).

• Must include a tickler chain of 19mm chain as per standard length to suit this gear.

• Sweepline rig to consist of 73.2m x 26mm single sweep plus 18.3m x 22mm chain, 36.5m x 18mm wire top bridle, 36.5m x 19mm chain lower bridle.

• The otterboards were Morgere Ovalfoils weighing 1700kg and with a surface area of 5.82m2.

A drawing of the gear is presented in Figure 1, the ground gear in Figure 2, and the towing rig in Figure 3. Collection bags The initial design concept for the FRS ground gear collection bags were similar to a rig developed by the Institute of Marine Research, Bergen. The design incorporated three separate bags consisting of a port, starboard and centre bag (Figure 4). Each bag was similar to a small low opening trawl with their headlines rigged to the fishing line of the anglerfish survey trawl (Figure 5).The centre bag was intended to collect escapees from under the trawls bosom ground gear section. The port/starboard bags were to collect escapees going under the quarter sections of the trawls ground gear. Model tests on the design aspects of the new collection bags were carried out at the Seafish Flume tank, Hull during June 2006. To ensure the model observations were representative of the full-scale trawl 1/15 scale models were used during the trials. The mesh size used in the construction of the bags was scale at 100mm full-scale. On observation the initial design the side panels were found to have too many meshes and therefore causing slack netting to hang down around the belly netting. It was further

noted that the cut used in the top sheet wing panels of the outer bags did not follow the trawls fishing line correctly and therefore caused the bags to fall back and become tangled with the centre bag. After minor modification the bags were then found to be orientated correctly with no slack netting noted down the side panels and all three were fishing symmetrically. The trials concluded with observations of the rig with the tickler chain added to the trawl. It was concluded that the bags would be in a position to collecting escapees going under the trawls ground which had also encountered the tickler chain. The full-scale collection bags were constructed from 100mm x 4mm single high tenacity PE twine with all leading edges strengthened with sections of double twine. The codends were constructed from 2mm single PE twine and rigged with strengthening bags. The ground gear was constructed from 100mm rubber discs on 20mm combination with 16mm chain tacked along its full length to give added weight and ensure good contact with the seabed. A full net plan specification is given in Figures 6 and 7. The headline of each bag was laced to the fishing line of the anglerfish trawl. A 2m bridle rope was used to attach the wing tips of both port/starboard wing bags to the trawls rockhopper ground gear. This allowed the tension in the wing bags to be adjusted by increasing or decreasing the length of the bridle rope. It should be noted that the codend on the main gear was 100mm mesh constructed from double 6mm braid. The cod end mesh in bags was 50mm, single 2.8mm twisted braid. Data collection The survey was carried out on the FRS research vessel FRV Scotia from 19th to 27th October 2006. The trawl locations (see figure 8) were based on information from the commercial fishery and were chosen to provide clear tows with good expected catches of Angler fish and over a range of depths. A total of 29 trawl stations were completed, with the ground gear collection bags deployed on 21 tows. Tow speed was approximately 4 knots and durations of between 30 and 60 minutes Gear performance was monitored during the tows using Scanmar trawl surveillance equipment to provide: depth; headline height; door spread; wing spread, bridle angle, bottom contact and average speed over the ground. Catches were recorded separately for the main cod end and the three bags. In each case the entire catch was worked up, with all fish being scored for length, weight and sex.

Results As described above, the ground gear bags were deployed on 21 tows, and recovered undamaged on 14. Of the remainder, some had all 3 bags damaged while some had on or two damaged. The tow duration ranged from 30 to 60 minutes with a mean of approximately 40 minutes. A grand total of 245 angler fish were caught, 163 in the main net and 82 in the ground gear bags. For the main gear, this represents a catch rate of approximately 18 fish per hour, which is comparable with commercial rates in the same area. The length frequency distribution of these fish is shown in figure 9. Over the complete size range this translates into 66.5% captured and 33.5% lost under the footrope. For the collection bags, there were 25 fish caught in the centre bag, 29 & 28 in the port and starboard bags respectively. Given the relatively low capture rate the analysis was carried out on all the hauls pooled together, and collated in 5cm classes. Percentage catch rates relative to the total catch (codend and bags combined) in each class, and in the bags or the codend were calculated. The data are presented in figure 10. As in figure xx, there is a clear mode between 5 and 15 cms. Based on age length keys from the stock assessment surveys these are likely to be “o” group fish. There is a second mode at 25-30 cms, probably representing group 1 fish, and then the remainder representing older fish. The small fish were much better represented in the bags. This is very clear in the cumulative plots presented in figure 11, with much better catch rates for the fish smaller than 30cm, suggesting that there is some mesh selection in to 100mm mesh of the main gear codend. Based on this observation the proportions of angler fish caught in the cod end versus the bags was calculated separately for fish below and over 30cm. For the smaller sizes, 75% of the total catch was taken in the bags and 26% in the codend. For the larger fish, the proportions were reversed with 24% in the bags and 76% in the codend. Four of the fourteen hauls were taken in the hours of darkness. The same calculations were performed for night and day separately. These results are summarised in Table 1. Day/Night % Fish <

30cm in codend

% Fish > 30cm in codend

Number of hauls

Number of Fish caught

Day 25 82.3 10 156 Night 27 66 4 89 All 26 76 14 245 Table 1 Catches in the codend as a percentage of total catch for day and night hauls and split below and above 30cm. Number of hauls and number of fish caught are also included.

Combination of sweep and ground gear escape analyses As detailed above the three main components of catch efficiency defined by Somerton et al (1999) were vertical herding, horizontal herding and escapement under the footrope. Vertical herding has been taken as minimal in this angler fish, so the main components would be vertical herding by the sweeps and bridles and footrope escape. The catch efficiency equation defined by Somerton et al was

( )AAeheQ 12+= Where: Q = Trawl efficiency e = proportion of fish caught in the net area h = proportion of fish caught in the area between the wing ends and the doors A2 = Area between the wing ends and the doors A1 = Area swept by the net Based on the analysis presented in Reid et al (in press), the proportion of fish caught in the area between the wing ends and the doors (h) was 0.017. Based on the current analysis, the proportion of fish caught in the net area (e), across all sizes would be 0.665. Based on data from the study reported in Reid et al, the average value for wing spread was 24.6m, and door spread was 87.8m. For a two hour tow at 4 knots. This translates to an A1 of approximately 160,000m2 and an A2 of approximately 410,000m2. Entering these values into the equation, the resultant Q value is 0.71. So any estimate of angler fish abundance based on the net swept area should be corrected by a multiplier of 1.41. Given the evidence for cod end mesh selection in the fish below 30cm (figure 11) an alternative efficiency can be calculated using an “e” value of 0.76. This provides an efficiency value of 0.80, and a multiplier of 1.24, for fish above 30cm length. There was some evidence for a relationship between gear efficiency and fish length. Table 2 shows that there was a steadily increasing proportion of fish taken in the main net with size class. It is important to not that in the largest size class only two fish were caught Size class <20cm 20-30cm 30-40cm 40-50cm 50-60cm 60-70cm % total catch in the cod end

13%

37.5%

70%

84%

92%

100%

Table 2. Percentage of total catch taken in the code end by10cm size classes Bottom contact The ground gear bags used in this study were relatively fragile compared to the main net. As a result all the successful tows with the collection bags were carried out in an area characterised by smooth, fine sand. Similar tows were attempted in other substrates by resulted in damage to the collection bags. Television cameras were mounted on the net during some of the deployments, and these were used to confirm

the nature of the seabed. Similar TV observations were carried out during the 2005 survey, and clearly showed that many of the “clear tows” were much rougher than those used in 2006. In some cases the tows included large boulders (<2m diameter) and rock outcrops. The ground gear used on this net is a Rockhopper design, where the gear will ride over obstructions. In the experimental tows, the seabed was relatively flat, and the gear tended to dig in to the soft substrate, so the bottom contact was good throughout the tows. Two bottom contact sensor data sets are presented in Figure 12. The top plot shows a typical sensor record for the collection bag tows, and the lower plot shows a record taken during the 2005 survey of the type of bottom contact data regularly seen in other areas. The ground gear was clearly in good contact throughout in the top panel, but much less so in the lower. While it is impossible to confirm, it is reasonable to assume that this would minimise the escape opportunities for the angler fish in these trials in comparison to the more rugged seabeds often seen on the surveys. This would suggest that the efficiency estimate is probably conservative, and that possibly more fish would escape on the harder ground

Discussion The present study has shown that there is clear evidence that angler fish are able to escape capture in the new trawl by escapement under the footrope. Combined with the information from the previous study (Reid et al, in press) this can be used to provide a catch efficiency of 0.71. There are a number of caveats to be considered before using this efficiency value. Firstly, the value is based on ALL size classes. There is persuasive evidence that the smallest size classes (<30cm) were not fully retained in the cod end. Given the 100mm cod end mesh used in the main net this may not be surprising. An alternative would be to use the Q value derived for fish over 30cm of 0.80. Furthermore, there was some evidence of size related efficiency. The larger the fish, the greater the proportion caught in the cod end versus the ground gear bags. Thus it may be possible to use length specific Q values to correct the swept area estimates. Size related escape under the survey ground gear (Engås and Godø 1986, Ehrich 1987, Walsh 1989, Walsh 1992, Langeland et al in press) has been reported for cod and haddock, although not for angler fish. Similar results have also been reported for commercial nets (Ingólfsson and Jørgensen, 2006). The size range of fish caught during the trials was less than that seen during the actual survey. The bulk of the fish found on the survey were between 2 and 7 years old. Mean size of 2 year olds was 30.6cm, and for 7 year olds was 69.3, the modal ages were 4 and 5 year old with mean lengths of 44.3 & 51.7 respectively. On the trials the modal size class was 25-30 cm, probably representing 1 & 2 year olds. Reasonable numbers were caught up to 50cm probably representing fish up to 5 years old. The main component missing in the trials would then be fish of 6 years of age or more. Given that the calculated efficiency for fish over 50cm was high in these trials, it is important to confirm this for these larger fish, and ensure that the analysis is more representative of the fish seen in the stock estimation surveys. A short experimental survey with stronger collection bags is planned for the autumn of 2007, and this will be targeted to include the larger fish. A second issue was the type of seabed substrate encountered in these experiments. As detailed in the results section, the trials were carried out on soft, flat seabeds, while much of the stock assessment survey would have been on much rougher seabeds. The implication from this would be that the net would be operating at it’s most efficient in these situations. So the estimate of fish escaping under the ground gear could be considered as a minimum. On the rougher grounds, where the ground gear loses contact more often, the opportunity for escape will be greater, and consequently the efficiency reduced further. Engås et al (2001) showed similar excursions of the ground gear from the seabed and postulated that this would impact on catch efficiency. Weinberg et al (2002) showed that reduced bottom contact with increasing trawl speed reduced the capture efficiency of a similar survey net for most benthic fish species.

There were some indications that the efficiency of the net was different between night and day. The proportion of fish over 30cm caught in the codend was greater during the day than at night. This would suggest that there may be a visual element to the behaviour of the fish in relation to the gear. Day night differences in escape have also been reported for cod and haddock (Langeland et al in press). Laboratory based studies of the swimming behaviour of this species would be needed to confirm this. The value of experiments using ground gear collection bags has been established by a range of studies (Walsh 1989, Godo and Walsh 1992, Dahm & Weinbeck 1992, 1996, Dahm 1997, Munro et al 1997). Dahm (2000) addressed the concern that bags would affect the performance of the gear and concluded that this was not the case. The method has also been used recently to examine the performance of a newly designed survey trawl (Langeland et al, in press). It has not been possible, so far, to apply the efficiency findings to the stock estimation. A modelling approach using the data collected on herding by the wires and the losses under the footrope has been proposed, and this will be carried out in Spring 2007. These results will be reported separately. In conclusion, the present study has provided clear evidence that angler fish are able to escape under the footrope of a survey trawl. Using collection bags under the ground gear, it has been possible to quantify this effect. It has also been possible to demonstrate a length dependence in this catchability. Some evidence of day night effects were also seen.

References Dahm, E. (2000). Changes in the length composition of some fish species as a

consequence of alterations in the groundgear of the GOV trawl. Fisheries Research, 49: 39-50.

Dahm, E. (1997). Escape strategies of different marine species under the footrope of survey trawls. ICES CM 1997/W:22

Dahm, E. and Weinbeck, H. (1992). Escapement of fish underneath the groundrope of a standard bottom trawl used for stock assessment purposes in the North Sea. ICES CM 1992/B:20

Dahm, E. and Weinbeck, H. (1996). New facts on the efficiency or total gear selectivity of German survey bottom trawls – possible effects on stock assessment and stock protection. ICES CM 1996/B:8

Engås, A. and Godø, O. R. 1989. Escape of fish under the fishing line of a Norwegian sampling trawl and its influence on survey results. Journal Conseil International pour l’Exploration de la Mer, 45: 269-276.

Engås, A., Ona E. and Ramberg, K. (2001). In situ determination of bottom trawl ground gear contact. ICES CM 2001/Q:07

Ehrich, S (1987) The portion of young cod escaping under the G.O.V.-trawl rigged with a heavy bobbin footrope. ICES CM 1987/B:28

Godø, O.R. and Walsh, S.J. 1992. Escapement of fish during bottom trawl sampling – implications for resource assessment. Fisheries Research, 13: 281-292.

Godø, O. R., and Totland, A. 1996. A stationary acoustic system for monitoring undisturbed and vessel affected fish behaviour. ICES CM 1996/B:12.

ICES (2006). Report of the Working Group on the Assessment of Northern Shelf Demersal Stocks, 10-19 May 2005 (ICES CM 2006/ACFM:13).

ICES (2005). Report of the Working Group on the Assessment of Northern Shelf Demersal Stocks, May 2004 (ICES CM 2005/ACFM:01).

Ingólfsson, O.A. and Jørgensen, T. 2006. Escapement of gadoid fish beneath a commercial bottom trawl – Relevance to the overall trawl selectivity. Fisheries Research 79; 303-312.

Langeland, M.R., Engås, A., Huse, I. and Jørgensen, T. (in press). Escapement of fish under a survey trawl – The effect of ground gear configuration. ICES Journal of Marine Science.

Munro, P.T., Weinberg, K.L., and Somerton, D. (1997). Estimating size dependent capture rates at the footrope of two different kinds of survey trawls. ICES CM 1997/W:06

Reid, D.G., Allen, V.J., Bova, D.J., Jones, E.G., Kynoch, R.J., Peach, K.J., Fernandes, P.G. & Turrell, W.R. (in press). Angler fish catchability for swept area abundance estimates in a new survey trawl. ICES Journal of Marine Science.

Somerton, D., Ianelli, J., Walsh, S., Godø, O. R., and Ramm, D. 1999. Incorporating experimentally derived estimates of survey trawl efficiency into the stock assessment process: a discussion. ICES Journal of Marine Science. 56: 299-302.

Sparholt, H. 1990. An estimate of the total biomass of fish in the North Sea. J. Cons. Cons. Int. Explor. Mer, 46: 200–210.

Walsh, S. J. 1989. Escapement of fish underneath the footgear of a ground fish survey trawl. ICES CM 1989/B:21 20 pp.

Walsh, S.J. 1992. Size-dependent selection at the footgear of a groundfish survey trawl. North American Journal of Fisheries Management, 12: 625-633.

Weinberg, K.L., Somerton, D.A., and Munro, P.T. (2002) The effect of trawl speed on the footrope capture efficiency of a survey trawl. Fisheries Research. 58, 303-313.

Figure 1. Net drawing of the adapted commercial trawl used in this study, and in the abundance surveys

Figure 2. Illustration of the towing rig used for the trawl. Observations were carried

out along the bridles and the wire sweeps.

Figure 3. Details of the ground gear of the survey trawl.

Figure 4 – Collection bag concept

Figure 5 – Attachment of collection bags to the trawl fishing line

Figure 6 – Net plan for centre collection bag

All rope lengths worked out at a mesh size of 100mm inside mesh or 105mm knot to knot

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Wing lines of panel 1.05m-- 8 flys at 1T 2B at 131 mm spaces To fit the bosom or between quarters of anglerfish survey Trawl

Figure 7 – Net plan for port/starboard collection bag

. Port side bag is exactly the same but a mirror image.

Headline and footropes the same with 60 fly meshes at 1drop 2 bars hung at 131mm apart (half a bar stretch) a total length of 60 at 131mm plus 2 bars at 52.5mm each equals 7.96m (26ft) Wing lines of panel 1.05m-- 8 flys at 1T 2B at 131 mm spaces

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Figure 8. Map showing haul location during experimental trials.

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Figure 3. Length frequency plot for angler fish caught in the 14 haul with intact collection bags

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Figure 11. Cumulative catch as percentage of total by size class in the cod end and in the collection bags.

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Figure 12. Top panel: Bottom contact sensor record from one of the hauls in the Scalloway deeps using the ground gear bags. Bottom panel: Bottom contact sensor record from one of the hauls in the 2005 trials survey

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