Theme session L - Bringing collaborative science – industry research data into stock assessment and fishery management: evaluating progress and future options. ICES CM 2009/L:11 Not to be cited without prior reference to the author North Sea GOV ground gear bag trials Brian Harley, Sven Kupschus and Arnold Locker Abstract The UK fishing industry has argued that the international bottom trawl survey carried out in English waters on the RV Cefas Endeavour is not representative of the numbers of fish, especially cod, in the North Sea. They are particularly concerned with the set- up of the ground gear. During two periods in 2008 catch rate comparisons were made between a commercial fishing boat and the Cefas Endeavour using the GOV gear. The rate of escape of the main commercial species from the standard GOV trawl was assessed by fitting additional nets behind the ground gear. The aim was to determine not only the escape rate, but also the factors, if any, that influenced escape. In January 2008, the GOV and commercial trawls had similar overall catches of cod, and the catch rates of the nets were strongly correlated. In contrast, GOV catch rates of cod in September 2008 were only 8% of the catch rates of the commercial gear, and although catch rates were positively correlated, the correlation was weaker than in January. In contrast with cod, the relative performance of the two nets for catching haddock, whiting and plaice was more consistent between January and September. It was considered possible that with the greater water clarity and improved visibility of the gears and sediment plumes in September, the design and towing geometry (e.g. sweep angle) of the commercial trawl may have become more effective than that of the GOV for herding cod. Escapement of cod under the GOV net was 20% in January and 14% in September. A GAM model fitted to the escapement data for cod indicated effects of season (reduced escape rates in autumn), fish length (reduced escape rates in larger fish) and overall catch rate (increased escape rates in small catches). This could potentially cause the catchability characteristics of the GOV to drift over time in response to changes in the density distribution of cod in the survey area. This was demonstrated by applying the results of the GAM model to the data from the time series of Cefas IBTS Q3 surveys to estimate potential escape rates at each station. On average, the probability of a cod escaping underneath the standard GOV gear was predicted to be ~ 25% at low densities, decreasing to around ~15% at greater abundance, indicating that the relationship between abundance on the grounds and the survey catch rates is not linear, as assumed by the assessment. The effect of this trend on the ICES assessment of North Sea cod was found to be relatively minor. Keywords: North Sea cod, GOV trawl, IBTS, comparative trawling, Fisheries Science Partnership Brian Harley and Sven Kupschus, CEFAS Lowestoft Laboratory, Pakefield Road, Lowestoft, Suffolk, NR33 0HT, UK. Arnold Locker, Locker Trawlers Ltd, 39 Birch Grove, Sleights, Whitby, YO22 5DE, UK
Transcript
Theme session L - Bringing collaborative science – industry research data into stock assessment and fishery management: evaluating progress and future options.
ICES CM 2009/L:11
Not to be cited without prior reference to the author
North Sea GOV ground gear bag trials Brian Harley, Sven Kupschus and Arnold Locker
Abstract The UK fishing industry has argued that the international bottom trawl survey carried out in English waters on the RV Cefas Endeavour is not representative of the numbers of fish, especially cod, in the North Sea. They are particularly concerned with the set-up of the ground gear. During two periods in 2008 catch rate comparisons were made between a commercial fishing boat and the Cefas Endeavour using the GOV gear. The rate of escape of the main commercial species from the standard GOV trawl was assessed by fitting additional nets behind the ground gear. The aim was to determine not only the escape rate, but also the factors, if any, that influenced escape. In January 2008, the GOV and commercial trawls had similar overall catches of cod, and the catch rates of the nets were strongly correlated. In contrast, GOV catch rates of cod in September 2008 were only 8% of the catch rates of the commercial gear, and although catch rates were positively correlated, the correlation was weaker than in January. In contrast with cod, the relative performance of the two nets for catching haddock, whiting and plaice was more consistent between January and September. It was considered possible that with the greater water clarity and improved visibility of the gears and sediment plumes in September, the design and towing geometry (e.g. sweep angle) of the commercial trawl may have become more effective than that of the GOV for herding cod. Escapement of cod under the GOV net was 20% in January and 14% in September. A GAM model fitted to the escapement data for cod indicated effects of season (reduced escape rates in autumn), fish length (reduced escape rates in larger fish) and overall catch rate (increased escape rates in small catches). This could potentially cause the catchability characteristics of the GOV to drift over time in response to changes in the density distribution of cod in the survey area. This was demonstrated by applying the results of the GAM model to the data from the time series of Cefas IBTS Q3 surveys to estimate potential escape rates at each station. On average, the probability of a cod escaping underneath the standard GOV gear was predicted to be ~ 25% at low densities, decreasing to around ~15% at greater abundance, indicating that the relationship between abundance on the grounds and the survey catch rates is not linear, as assumed by the assessment. The effect of this trend on the ICES assessment of North Sea cod was found to be relatively minor. Keywords: North Sea cod, GOV trawl, IBTS, comparative trawling, Fisheries Science Partnership Brian Harley and Sven Kupschus, CEFAS Lowestoft Laboratory, Pakefield Road, Lowestoft, Suffolk, NR33 0HT, UK. Arnold Locker, Locker Trawlers Ltd, 39 Birch Grove, Sleights, Whitby, YO22 5DE, UK
Introduction The Fisheries Science Partnership (FSP) was established in 2003 to build relationships between UK fishermen and scientists, and to involve fishermen in the co-commissioning of science. The UK’s Department for Environment, Food and Rural Affairs (Defra) funds the FSP, and about ten projects per year have been carried out since 2003/04, comprising a mixture of time-series surveys and a variety of other projects addressing specific industry concerns such as fishing gear selectivity studies and examination of spatial patterns of catch compositions. Reports for FSP projects already completed are available on the FSP page of the Cefas website1 (the one pertaining to this investigation is titled “Project 24X: North Sea GOV gear trials continuation”, Kupschus et al. 2009). Industry proposals for FSP projects have typically been developed at a port/regional level, refined and agreed with Cefas and approved by the FSP Steering Committee. Charter vessels are selected through an open tendering process, and are given dispensations from the relevant quota and effort controls. The present study was established to address a general distrust by industry of the results of the ICES North Sea cod assessment, and specifically the survey data used for tuning the assessment. A specific concern was the presence of gaps between the fishing line and the ground gear on the survey GOV trawl which was considered by industry to facilitate escapement of cod. To address these concerns, Cefas scientists working in collaboration with fishermen on the NE coast of England devised a programme of comparative trawling on a variety of different grounds, to compare the catch rates and length structure of cod and other species taken using a GOV and a commercial trawl. The work was undertaken on a research vessel used in the IBTS survey of the North Sea (RV Cefas Endeavour) and on a North Sea whitefish trawler (MFV Our Lass II). The experiment was carried out initially in January 2008 and repeated in September 2008, closer to the time of the IBTS survey and also a time of year with better water clarity. Escapement of cod under the ground gear of the RV Cefas Endeavour was evaluated using bags fitted to the ground gear. As an adjunct to the main work, an underwater camera was fitted to the selvedge of the trawl to monitor the behaviour of different species in the mouth of the trawl, and to determine the potential routes of escape of the major commercial species. Some additional tows on nearby hard ground were carried out in January by a second trawler MFV Jubilee Quest. The agreed objectives of the programme were: 1) To better quantify the rate of escape of cod underneath the ground gear and through the gaps between the fishing line and ground gear of the GOV trawl as used in the North Sea groundfish survey (ground gear “A”, for soft-ground stations). Using additional bags underneath the gear to capture escapees, the aim is to develop quantitative models that describe the effects of length of cod, abundance of cod and a number of environmental co-variables including time of the year (taken as a proxy for water clarity) on the rate of escape from the GOV.
2) To calibrate the catch rates on the research vessel to commercial catch rates using commercial gear and practices. 3) To determine the means of escape visually from images recorded by a camera mounted on the gear, and to describe the ground contact and behaviour of the gear more generally. Data were collected for cod, haddock, whiting and plaice. This paper focuses on the results for cod although presents some catch-rate data for the other species. More results for the other species are given in Kupschus et al. (2009). The implications of the results of the FSP study for the ICES assessment of North Sea cod were evaluated by re-running the assessment using the English (Cefas) IBTS survey series on its own, and comparing runs using the original survey data to runs using the data adjusted to reflect the observed escapement patterns from the GOV during the FSP project. Methods Vessel, gear and experimental design The commercial trawler chartered for comparative trawling on “soft” ground was Our Lass II, a Whitby-based, steel-hulled twin rigger of 20.15 m registered length (21.5 m overall). Built in 2007, she is equipped with a 480 kW power unit and carried two trawls for the purpose of the experiment, a “prawn scraper net” and a “fish-tail hopper” net. The RV Cefas Endeavour is a Cefas-owned and Lowestoft-based multidisciplinary research vessel of 73.91 m (overall) powered by a 3240 kW diesel-electric plant. For the trials she was equipped with an IBTS standard GOV trawl net set up as for the annual Cefas IBTS survey (ICES 2006a), with three additional bags designed and built by Jackson Trawl (Peterhead) fitted to the fishing line of the GOV, but having their own independent ground gear. A second trawler, FV Jubilee Quest was also chartered in January 2008 to conduct tows on nearby hard ground using trawl gear rigged for such ground, at the same time as Our Lass II and Cefas Endeavour towing on soft ground. The present paper however focuses on the soft-ground experiment. The gears and vessels are of different design, and in addition to the different net designs they use different trawl doors, sweep lengths, towing angles, net sensor and ground-gear configuration. Hence there are many factors that may result in different catch rates. To reduce the number of variables, the ground gears used on Our Lass II and Cefas Endeavour were both set up with 8-inch rubber disks. All relevant net parameters (e.g. door spread, headline height, wire out) were recorded on both vessels using net monitoring equipment. The commercial trawlers used 80-mm mesh codends, and the Cefas Endeavour used a 50-mm mesh codend with a 20-mm mesh codend liner or “blinder” in order to keep catchability consistent with the IBTS standard. The escape bags fitted to the GOV ground gear were 1/3 of the length of the GOV net and were constructed from 80 mm mesh, as 20mm mesh would pick up large quantities of benthos causing backwash. The bags were attached in such a way as to cover the gap between the fishing line and the ground gear. These two parts of the net are connected by toggle chains (see Figure 1). The bags themselves attach to the fishing line and hang back behind the ground gear as shown in Figure 2.
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The experimental design involved the two vessels towing in parallel and in a straight line, with one boat slightly ahead of the other. For additional safety reasons the vessels stayed at least 500m apart. Both vessels attempted to shot the trawls to ensure that the gear was stable on the bottom at the same time but this was not always possible. This was mostly due to the difficulties in deploying the GOV trawl. The leading vessel was alternated to reduce bias that may be caused through disturbance of the fish, as recommended by ICES (ICES 2006b,c). The tow distance was standardized to 2 nautical miles, with Cefas Endeavour towing at 4 knots and the smaller-engined Our Lass II towing at its normal operational speed of around 3 knots. Tows were conduced between 7am and 7pm and there was at least an hour between each pair of comparative tows. Consecutive tows were never on the same geographical position as the one immediately before. The aim of the first set of comparative tows in January 2008 was to sample a number of different grounds with different catch compositions and different densities of cod and other commercial species. Three areas were identified which could be sampled in the somewhat inclement conditions during January. These were Skate Hole, the Yorkshire coast between Hartlepool and Filey, and parts of The Wash near Skegness. The comparative tows in September 2008 concentrated on obtaining as many additional samples as possible with reasonable numbers of cod to allow for comparison of the catch rates and determination of the effects of time of year on the escape probability. It was considered this might best be achieved by remaining on a single site, Baymans Hole, a traditional fishing ground for the whitefish fleet in September.
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Figure 1. Photograph of attachment of fishing line to ground gear by means of toggle chain on the RV Cefas Endeavour GOV trawl.
Figure 2. Diagram of ground gear collecting bags attached to the ground gear of the GOV trawl. Sampling Location, time, sea condition, and gear parameters were recorded for each haul and sampling of catches was restricted to species of commercial importance. All fish of a species, or a representative sub sample if the catch was large, were sampled for length composition after each haul.
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On the Our Lass II, the catches were sorted by the crew into retained and discarded portions and measured by an observer. On the research vessel, catches from each of the escape bags and from the GOV were worked up separately. Commercial species were weighed by species, and all other organisms were weighed and recorded as benthos. One otolith per 1-cm length group per station was collected for cod during both surveys. For logistic and operational reasons not all stations could be completed by both vessels. Therefore, for the purposes of comparison between vessels only those stations fished by both vessels were used. To examine the rate of escape below the net, all Cefas Endeavour tows were included where all four codends (main net and three escape bags) contained valid samples (i.e. no damage to or fouling of the codend). Data analysis The relative selectivity of the commercial and GOV trawls was evaluated by comparing the length compositions of cod in the two gears. The standardised catch-rates (nos. of cod per 2 nautical miles) of the two gears were examined on a station by station basis to determine the relative magnitude of the catches. Catch rates of the two gears were plotted against each other to determine if they varied in direct proportion across a wide range of cod densities and conditions even if the catch rates differed in absolute terms due to differences in gear design. Robust regressions were fitted to the data. Escapement of cod under the ground gear of the GOV gear was examined using a generalized additive model (GAM) Individuals, retained in the main gear were ascribed a response of 1, those caught in the escape bags a response of 0. The response was then modelled as a function of the time of year of sampling (factor as only two time periods were available), the length of an individual (bi-cubic spline smoother), and the total number of individuals in the catch (bi-cubic spline smoother). The general from of the additive predictor is thus:
)()()(_ catchlength
Sep
Jantimetime XfXfXpredictoradditive
where a is the intercept, b is the slope of each of the seasonal effects and f is the local regression function describing the effects of length and catch. The additive predictor is then linked to the scale of the response using a logit link-function. A logit link function was employed to relate the additive predictor to the scale of the resposnse. It was unclear whether each individual should be evaluated as an independent observation, or whether the individual observation was at the level of the sample, because there were interactions between individuals. In this case we have chosen to look at individuals as independent samples, but include the total catch of cod as an explanatory variable. Statistically speaking this is less than ideal, but the indications from the data were that when caught on their own cod acted as individuals, but when caught in greater numbers they interacted. A single over-dispersion parameter could therefore not adequately represent the true underlying error structure. Including total catch as an additional dependent variable improves
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model performance and reduces process error, but does not alow effective comparison of deviance components in the form of the AIC nor are the resulting p-values reliable. Results Comparative tows Comparative tows were carried out at 26 stations in January and 14 in September. All were completed successfully by both vessels, and 4713 cod were caught in total. The number of cod, haddock, whiting and plaice caught by each vessel is listed in Table 1, with the Cefas Endeavour catches split between those found in the codend and those retained in the escape bags. Our Lass II and Cefas Endeavour caught similar numbers of cod in January 2008. In September, Our Lass II caught around 12 times more cod than Endeavour (Table 1a, Figure 3). The overall length range of cod caught was broadly similar between the two vessels. However, the length frequency of cod taken by Our Lass II contained a much higher proportion of cod of length 250–450 mm than recorded by Cefas Endeavour in either the main codend or the escape bags. In fact the Our Lass II length frequencies from September indicate greater similarities with the hard-ground catches from January provided by the FV Jubilee Quest suggesting that different components of the population were sampled by the two gears. The Our Lass II caught more haddock and plaice than Endeavour, but fewer whiting (Table 1b-d). However the differences in catch rate of these species between the two gears did not vary so much between January and September as was the case for cod. In January 2008, the catch rates of cod taken by Our Lass II and Cefas Endeavour were not only of similar magnitude, but tended to vary in proportion to each other (Figures 3 and 4). Although there was a positive correlation of cod catches between the two vessels in September 2008, the differences in catch rates at each station varied widely (Figure 4).
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Table 1. Results of comparative tows between RV Cefas Endeavour and FV Our Lass II during January and September 2008. (a) COD January 2008 September 2008 Total Our Lass II (a) Nos. caught 384 3593 3977
(b) Nos in main codend 336 274 610 (c) Nos. in escape bags 83 43
Cefas Endeavour
Proportion escaped (c/(b+c)) 0.2 0.14 Catch ratio of Endeavour to Our
Lass II (b/a) 0.88 0.08
(b) HADDOCK January 2008 September 2008 Total Our Lass II (a) Nos. caught 1906 3459 5635
(b) Nos in main codend 1258 1919 3177 (c) Nos. in escape bags 87 9
Cefas Endeavour
Proportion escaped (c/(b+c)) 0.06 0.00 Catch ratio of Endeavour to Our
Lass II (b/a) 0.66 0.55
(c) WHITING January 2008 September 2008 Total Our Lass II (a) Nos. caught 31297 17222 48519
(b) Nos in main codend 33759 25759 59518 (c) Nos. in escape bags 218 78
Cefas Endeavour
Proportion escaped (c/(b+c)) 0.06 0.00 Catch ratio of Endeavour to Our
Lass II (b/a) 1.08 1.50
(d) PLAICE January 2008 September 2008 Total Our Lass II (a) Nos. caught 1712 619 2331
(b) Nos in main codend 559 125 684 (c) Nos. in escape bags 198 34
Cefas Endeavour
Proportion escaped (c/(b+c)) 0.26 0.21 Catch ratio of Endeavour to Our
CEND size = 1 f ishOUR size = 11 f ishCEND size = 598 fish
Figure 3. Cod length frequency, numbers per station and distribution of catches for Our Lass II and Cefas Endeavour in 2008. The Cefas Endeavour catches are divided into those retained by the standard gear (yellow), and those caught by the escape bags (red). The distribution map shows catch rates only those cod retained in the standard GOV gear and in the commercial gear.
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Figure 4. Correlation between Cefas Endeavour and Our Lass II cod catches by station. Lines are robust regressions of the estimated relationship for each season. The significant difference in the slopes reflects the seasonal difference in the commercial catch rates. The fit is poorer in September (red) than in January (blue). Escape rates underneath the GOV Some 17% of the cod caught over all tows by Cefas Endeavour were retained in the escape bags, with the escape rate decreasing from 20% in January to 14% in September. Escape rates of plaice were of similar magnitude to those of cod, but escape rates of haddock and whiting were very low (Table 1b-d). The GAM model indicated there were effects of season, length and total catch numbers on the proportion of the catch that was retained (i.e. did not escape through the gaps between the ground gear and fishing line). Retention rates increased with fish length although the relationship was poorly defined for the largest and smallest fish (Figure 5). Retention rates also increased with the size of catch up to about 35 fish, after which samples became too scarce in the analysis to be certain of any effect. This suggests that if cod encounter the net as individuals rather than in a group, they are more likely to detect the gap between the footrope and fishing line and escape underneath the net. Behaviour patterns of cod may differ when they are in a group, reducing the probability of individual fish detecting the gap, and resulting in more of the fish entering the GOV. The shape of the spline effects shown for the combined January and September data in Figure 5 remained the same when the model was fitted independently to the data for the two periods. This gives greater confidence in the validity of the results than suggested by the GAM confidence limits shown in Figure 5.
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Figure 5. Results of GAM model examining escape rates of cod with effects of season (top left (CEND 15X/08 = September; CEND1X/08 = January), length of the individual (top right) and the total sample catch of cod in number (bottom left). Higher values indicate a greater probability of retention in the main gear, lower values a greater chance of escape underneath the ground gear. Dashed lines give GAM estimates of the uncertainty around the trend line. Applying results to a stock assessment model Method The current stock assessment for North Sea cod carried out by the ICES Working Group on the Assessment of Demersal Stocks in the North Sea and Skagerrak (WGNSSK) uses a version of the ADAPT model (B-Adapt; ICES 2009). This is a survey-tuned catch-at-age model that for North Sea cod assumes that the fishery removals at age (landings and discards) are known up to 1997, but that after 1997 only the relative age compositions from fishery sampling are known. It is assumed that there could be other forms of removals (e.g. misreporting or additional natural mortality) not accounted for after 1997. The B-Adapt model provides estimates of total removals from the stock (in excess of the assumed fixed annual values of natural mortality) from 1998 onwards that result in the model estimates of abundance
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following the temporal trends from the IBTS survey indices as closely as possible. Any unaccounted removals are assumed to have the same age composition as the reported landings and discards. The ratio of B-Adapt estimates of total removals weight to the total reported landings and estimates of discards in each year is termed the “catch multiplier”. A critical assumption of the method is that the IBTS survey indices are linearly proportional to the true population abundance. This assumption could be violated if escapement rates under the GOV net increase at low stock sizes because there are more instances where cod encounter the net individually rather than in groups. The current North Sea cod assessment uses two IBTS tuning indices. One is the combined quarter 1 International Bottom Trawl Survey (IBTS) index and the other is the combined quarter 3 IBTS index. Due to the complexity of applying the results from the GAM analysis on these combined data sets, and the bias that may occur as a result, it was decided to carry out model runs including only the Cefas IBTS survey series, using the same B-Adapt model settings as used by WGNSSK (ICES, 2009). This was considered sufficient to evaluate the potential extent that density and length-dependent variations in escapement rate (as shown by the GAM fit) could impact on the outcome of an assessment. Note that this is a purely exploratory exercise to gauge the possible magnitude and implications of catchability trends induced by density and length-dependent escapement of cod under the GOV ground gear, and does not imply any future changes to the ICES assessment. The first B-Adapt run used the standard UK (E+W) tuning index as supplied to ICES (Table 2). This had no modifications and was taken directly from the R code used for computing the standard survey indices. For the second run, the same R code was used for computing the indices but with the GAM analysis applied to the data to adjust station catch rates for probability of escapement according to observed catch numbers (Table 3). The relationship was truncated at 35 fish per tow (and maintained at the same value for catches > 35 fish) to reduce any additional uncertainty in the results due to the poor fit of the GAM beyond this catch level (Figure 5). The number of tows with such high catch rates of cod is typically very low, and a high fraction of hauls have only small numbers of cod (Figure 6). The majority of cod taken on the surveys are less than 60cm long (Figure 6). The revised indices in Table 3 are between 4% and 24% higher than the original indices in Table 2, with the revisions generally highest for the younger age classes which are likely to dominate stations with high catch rates. The adjustments generally increased over time until the early-mid 2000s then declined, with the most marked decline observed for the younger ages (Figure 7). This must reflect trends in the frequency distribution of catch rates per tow, which will reflect trends in recruitment and mortality.
Theme session L - Bringing collaborative science – industry research data into stock assessment and fishery management: evaluating progress and future options.
Table 2. UK (Cefas) IBTS quarter 3 standard tuning index for cod. (First column is the survey effort by year – set to 1.0 to show the catch rates are standardised; subsequent columns are stratified mean catch rates in numbers of fish, for ages 0 to 5. Only ages 0 – 4 are used in the assessment) North Sea/Skagerrak/Eastern Channel Cod, Tuning data for extended survey. Updated 29 Apr 09 101 IBTS_UK_Q3_old 1992 2008 1 1 0.5 0.75 0 4 1 2.605354652 3.727114357 0.241162003 0.070695149 0.054588439 0.012080054 1 0.579081398 1.129615908 0.99052244 0.125137751 0.024173337 0.024852454 1 3.226820525 4.0815701 0.471312447 0.233428355 0.028295078 0.007552606 1 0.040039615 1.721070468 2.019148481 0.18359897 0.084485407 0.002473927 1 6.28162229 1.02041445 1.1000992 0.2601794 0.02912119 0.03036624 1 0.007454854 6.483818567 0.449465701 0.086530843 0.040150229 0.002361476 1 0.596165712 0.179040445 2.122869001 0.125028577 0.012645984 0.010284509 1 0.144660263 0.55742758 0.084031724 0.359557189 0.019730411 0.009445902 1 0.06050048 1.448220447 0.299624094 0.022934778 0.04835246 0 1 0.039754473 0.265960609 0.804083612 0.049127952 0.002829655 0.006986675 1 0.02829163 1.20785906 0.22200979 0.19329003 0.0254226 0 1 0.033559706 0.205715073 0.272021561 0.065351315 0.049084061 0.005310163 1 0.037309519 0.527205275 0.186275971 0.049733028 0.011864866 0.01469452 1 0.35427363 0.297897125 0.277402652 0.050246327 0.010042702 0.002361476 1 0.108576031 1.494534255 0.259935369 0.081292291 0.009035211 0 1 0.925510426 0.640403088 0.937798912 0.055883373 0.034317561 0.024643497 1 0.013582343 1.441168644 0.23134457 0.184985803 0.019619202 0.02147585
Figure 6. Frequency distribution of cod catches per tow and total length compositions of cod during Cefas IBTS Q3 surveys of the North Sea during 1993-2003 and 2004-2008.
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Figure 7. Adjustments to annual Cefas IBTS Q3 indices for age groups 0 – 4 to correct for catch-dependent rates of escapement under the GOV net as shown in Fig. 5.
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Results The adjustment to the Cefas IBTS Q3 survey indices to account for escapement under the net resulted in a small increase in the B-Adapt catch multiplier estimates (Figure 8a). The difference in the catch multipliers was slightly larger from 2004 – 2006 coinciding with the period with the largest upward revision to the IBTS indices (Figure 7). Since B-Adapt will adjust the catch multipliers to try and achieve constant catchability in the survey indices as far as possible, any trend in survey catchability (e.g. due to trends in escapement rates) may be transferred into the catch multiplier. Hence, the B-Adapt estimates of spawning stock biomass and fishing mortality are hardly changed between the two runs (Figures 8b-c). The differences in catch multipliers between the two runs are however very small compared with the overall trend in the multipliers over time, indicating that any trend in GOV catchability characteristics due to trends in escapement are likely to have been small. Discussion The GOV gear and the North Sea IBTS survey in general has been a point of contention between scientists and fishermen for some time. Cod fishery management is currently a major issue for the industry, and fishermen have pointed to the ground gear of the GOV as one of the major confounding issues in the different perceptions of industry and scientists on the state of the stock. The FSP study reported here has provided an important scientific insight into the interaction between fish and trawl gear, underlining the value of cooperation between industry and scientists in such work. A number of significant conclusions can be drawn helping us to understand the difference in perceptions of the state of the stocks between industry and scientists. Comparative catch rates A prevailing view of the fishing industry was that the difference in catch rates between commercial gears and the GOV was attributable to the difference in the habitats sampled. This latest study, however, indicated that when the GOV and a commercial cod trawl were fished side by side on softer ground, catch rates of cod were comparable during winter, but in autumn the GOV catch rates were much lower than commercial catch rates (<10%) and the average length of the cod caught was greater. Reduced catch rates of cod in the GOV trawl during autumn were not associated with greater escapement underneath the ground gear due to improved visibility. Contrary to this expectation, escapement decreased by roughly 6% rather than increasing. The very different catch rates of cod in the GOV and commercial trawl in September was not reflected in the catch rates of other species. The differences in catch rates of haddock, whiting and plaice in the two gears showed a much smaller seasonal effect than observed for cod. One possible explanation for this may be that cod may react differently to seasonal changes in the visibility of the gear. During winter, when the water is generally turbid, there may be less marked differences between trawl designs in how cod react to sediment plumes from the doors and sweeps or to visible trawl components such as the bridles. During the brighter and less turbid summer months the sediment cloud thrown up by the doors and other gear components will be much
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more discernible to visual predators such as cod, and the trawl gear itself will be more visible. Consequently, the longer bridles on the commercial gear and the steeper towing angle may increase the effective size of the net compared with the GOV as water clarity improves. (a) Catch multipliers
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ort
ality
original index
new index
Figure 8. Comparison of (a) catch multipliers, (b) spawning stock biomass (SSB) and (c) fishing mortality from single fleet B-Adapt runs using the original UK IBTS Q3 survey indices and the new series adjusted for escapement..
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The strong positive correlation between the catch rates of cod in the GOV and the commercial net in January suggests that the two types of net may provide similar trends in abundance of cod at this time of year on soft ground. In contrast, the ratio of the catches of the two nets at each station varied very widely in September, and many more tows than the 14 achieved would have been needed to establish if the two gears could provide similar trends in abundance at this time. Higher catch rates using the commercial gear could increase the accuracy of the assessments, because better indications of year-class strength would be possible, particularly for older, scarcer ages. However, the length frequency information indicates that the main difference in selectivity between the gears occurred between 250 and 450 mm fish length, a range indicative of younger more abundant ages. Ground gear design Fisheries scientists at ICES have argued that the set up of the ground gear on the IBTS surveys is unlikely to be a major factor in the catchability characteristics of the net. This was also shown to be the case by the FSP project which showed escapement rates of 14-20% on average through the gaps between the footrope and the fishing line. The escapement rate was too small to explain the large differences in catch rate between the GOV and the commercial trawl in September 2008. The tendency for the escapement rate to be inversely correlated with overall catch numbers and to be less pronounced for larger cod means that the catchability of the GOV will not be stationary over time for each age class, a key assumption in the ICES stock assessment. However, the present study indicated that any trends in catchability since 1992 due to density-dependent escapement rates could have been relatively small, and adjusting the survey indices for this had only a minor effect on the results of some trial runs using the ICES assessment model for North Sea cod. Comparison with other studies A Scottish Industry Science Partnership project was carried out on the commercial twin rigger MFV Russa Taign in May 2008 to compare the catch rates of a 36/47 GOV trawl (rigged with ground gear “B” as used in the Scottish IBTS surveys in the northern part of the North Sea), and the vessel’s own Seaway Nets whitefish hopper trawl (Reid et al 2009). Over the 66 hauls carried out, there was little or no difference in bulk or length frequency of cod and haddock caught. For other species the GOV appeared slightly better (hake and lemon sole), while the commercial net was better for plaice, megrim, saithe and anglers. For most species the catch rates in the two nets were strongly correlated. However, for cod and whiting, there was a relatively poor relationship between the haul by haul catches in the two nets. In both species, but particularly for cod, there were hauls where large catches were found in one net, but not in the other (this was similar to the FSP results for cod in September 2008). Reid et al (2009) thought this could be due to small-scale patchiness (given the short 30-min tows) or to aggregation behaviour causing the fish to be herded by only one of the nets. The use of a twin rig arrangement in the Reid et al SISP study had some advantages in reducing variability due to patchiness of the fish but also placed some constraints on the deployment of the trawls, for example causing some reduction in the spread of
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the nets, having to use a compromise towing speed (3.5 knots), and fitting a 20mm blinder to both codends. The behaviour of cod interacting with the twin-rig may also have been different to the behaviour if encountering each net when towed on its own. The FSP study using Our Lass II and Cefas Endeavour deployed the GOV and commercial nets using the normal procedures for each, but introduced additional variables such as the greater distance between the nets and different size and power of the vessels. The SIPS study noted that the more complex rigging of the GOV, for example the use of the kite, led to frequent problems in deployment. In addition the GOV was regularly damaged during tows, particularly in the wing and belly meshes. In contrast the commercial net required little maintenance and was relatively easy to deploy. Conclusions The results of this FSP project indicate that only through cooperation between fisheries scientists and the fishing industry do we have the knowledge and expertise to understand the processes involved in the interaction between fish and fishers. For example, neither scientists nor industry suspected that the escape rate of cod, and other species, would be lower during the summer when the water is clearer than during the winter, so all participants learned something from the study. Important findings from the study are:
In January the GOV catch rates for cod on soft ground were >80% of those recorded for the commercial trawl, with 20% of the fish escaping below the GOV ground gear, and the catch rates of cod in the two types of net were strongly correlated. In September the GOV catch rates were just ~8% of those fish caught by the commercial gear, and the difference between the gears varied widely between tows. However the rate of escape below the GOV net decreased to about 14%.
The percentage of cod escaping below the GOV ground gear could vary from 15 to 25% during the UK IBTS Q3 survey, depending on abundance on the ground. This could potentially cause the catchability characteristics of the GOV to drift over time in response to changes in the density distribution of cod in the survey area. However, adjustments to IBTS indices using the relationship between catch rate and escapement recorded in the FSP experiment resulted in only small changes to the ICES assessment.
The fishing industry partners in the FSP project expressed the following opinions on the results of the study:
The differences in the cod catches by the two vessels in September 2008 prove beyond doubt that the gear and set-up used by the Cefas Endeavour does not provide a true reflection of the absolute biomass of cod during September.
The sweep length and the trawl doors are not compatible and overspread the gear so that the sweep angle is too shallow.
However, industry members also acknowledge that despite a much lower catch rate than might be expected from commercial vessels, in relative terms
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the trend line (percentage increase and decrease) in cod stock size would be similar to trends in commercial catch rates.
Remaining issues are the small correction for GOV escape rate with increasing catch rates and past changes in gear geometry caused by a change of research vessel and gear deployed which could be significant.
The results underline the importance of continuing industry and scientific cooperation in understanding the interaction between GOV and fish on the ground.
Acknowledgements This work was carried out in cooperation with the skippers and crew of the FV Our Lass II (skipper James Locker). Cefas sincerely thanks the skippers and their crews for their contributions to the programme, in particular Arnold Locker who not only helped to arrange the program but also participated on board the Cefas Endeavour during the surveys. Thanks are also due to Fred Normandale and Joe Horwood for their individual inputs to this research initiative, especially related to their detailed knowledge of the issues important to fisheries and the management of stocks in the North Sea. Dave Reid and Rob Kynoch of the FRS Marine Laboratory, Aberdeen, are thanked for the provision of gear plans and expertise on the use of escape bags, as well as for sharing their knowledge of the behaviour of fish in relation to fishing gear. We also acknowledge with thanks the contributions made by the Cefas staff in particular Dave Peach and Simon Armstrong, in sampling and entering and collating data, Jose De Oliveira, for help with the initial B-Adapt runs and Mike Armstrong for his significant editorial input.
References ICES 2006a. Manual for the International Bottom Trawl Surveys, Revision VII. ICES. 2006b. Report of the ICES International Bottom Trawl Survey Working Group
(IBTSWG). ICES CM 2006/RMC:03 ICES 2006c. Report of the Study Group on Survey Trawl Standardisation (SGSTS).
ICES CM 2006/FTC:05 ICES. 2009. Report of the ICES Working Group on the Assessment of Demersal
Stocks in the North Sea and Skaggerak (WGNSSK). ICES 2009/ACOM:10. Kupschus, S., Locker, A., Peach, D., Armstrong, S. and Harley, B. 2009. Project 24X:
North Sea GOV gear trials continuation. Final Report, 2007-08 Fisheries Science Partnership. http://www.cefas.co.uk/data/fisheries-science-partnership-(fsp).aspx
Reid, D.G., Kynoch, R.J., Penny, I., Summerbell, K. and Edridge, A. 2009. Catching
Performance of the GOV Survey trawl – SISP report 01/09. http://www.marlab.ac.uk/FRS.Web/Uploads/Documents/SISP0109.pdf
