Fisheries Science Partnership: 2008/09 Provisional Final Report Programme 8: Bass gillnet selectivity Prepared by: Andy Revill, Oliver Wade, René Holst, Jon Ashworth, and Nigel Stead Cefas, Lowestoft The MFV Rachel S (GY 305) skippered by Nigel Stead docking in Lowestoft (October 2008) January 2009 1
Summary This FSP programme (No. 8) investigated the selective characteristics of gillnets over a range of mesh sizes in relation to the European seabass (Dicentrarchus labrax). Accurate information on the relationship between gillnet mesh size and fish selectivity is important if managers are to set appropriate minimum fish landing sizes (MLS) and net mesh sizes. Such knowledge allows managers to set regulations that can help to minimise wasteful or unsustainable fishing practices, for example by reducing the likelihood of undersized fish being caught in the gillnets. Currently, such information is sparse for the European seabass (herein referred to as bass), particularly in relation to the North Sea gillnet fisheries. This study provides new data on bass selectivity over a range of gillnet mesh sizes, so providing the basis for informed decision-making. The European seabass (Dicentrarchus labrax) of the Grimsby registered fishing vessel Rachel S (cover plate). Three replicate trials were conducted and all three produced consistent and comparable results. The analysis of catch data indicated that bass selectivity peaked at 41 44 cm in gillnets with a mesh size of 90 cm. This peak increased with larger mesh sizes, reaching 54 58 cm with 120 mm gillnets. Figure 1. Skipper Nigel Stead at the wheel of the MFV Rachel S during the bass selectivity trials To quality control the trials, the selectivity of Dover sole (Solea solea) was also determined over the same range of gillnet mesh sizes. The results were compared with previous estimates from Danish trials (1999) and were comparable (i.e. within 1 2 cm). The investigation was carried out by comparing the catches of bass in gillnets of mesh size 90, 100, 108 and 120 mm deployed simultaneously on fishing grounds in the southern North Sea. The work was carried out during October and November 2008, with the aid of skipper Nigel Stead (Figure 1) and crew 2
Table of Contents Summary...2 Table of Contents...3 The Fisheries Science Partnership...3 Introduction...4 The European seabass...4 The English Channel and North Sea fisheries...4 The basic theory of gillnet selectivity and fish entrapment...5 The modal length optimum selectivity...5 Small fish (less than modal length)...5 Big fish (greater than modal length).5 The effect of increasing mesh size...5 Method...6 Experimental design...6 Statistical data analysis and modelling7 Choosing a uni- or bi-modal model..7 SELECT method...8 Fryer method...8 SELECT GLMM method...8 Selectivity parameters K 1 and K 2...9 Quality control comparison...9 Results...11 Sea trials...11 Estimation of selectivity parameters (bass)...11 Estimation of selectivity parameters (sole) for quality control / comparative purposes...11 Examples of how the results can be used in management (for guidance purposes only)...15 Conclusions...16 References...16 Appendix 1...17 Detailed Operational Plan agreed before the trials...17 The Fisheries Science Partnership The Fisheries Science Partnership (FSP) is a Defra-funded collaborative programme of scientific research between the UK fishing industry and scientists. Since it was established in 2003, the programme has undertaken numerous projects, such as investigations into fishing gear selectivity, the examination of spatial patterns and catch compositions, and time-series of relative abundance of commercial species. A full description of the development, aims and reports of the FSP programme can be found at the Cefas website detailed below. www.cefas.co.uk 3
Introduction The European seabass Bass (Figure 2) are found in the eastern Atlantic Ocean, the North Sea, the Mediterranean and the Black Sea. Young bass inhabit estuaries and brackish waters, where they live in schools, eventually migrating and dispersing into smaller groups in deeper offshore water as they grow and age. Typically feeding on small fish, sandeels, crustaceans and molluscs, bass can grow up to a metre in length and weigh >10 kg. Bass are caught in trawl, gillnet and recreational fisheries and are caught throughout the water column depending upon where the fish are grouped at a particular time. Figure 2. Some of the European seabass (Dicentrarchus labrax) caught during the trials The English Channel and North Sea fisheries Landings of bass by UK and other EU fleets from the English Channel and North Sea increased to historically high levels recently following a series of strong year classes entering the fishery, from 1989 to the early 2000s. Stock assessments carried out by Cefas (e.g. Pawson et al., 2007) suggest that bass stocks (including those in the North Sea) are at high levels of abundance. The International Council for the Exploration of the Seas (ICES) days that these stocks can withstand current exploitation rates, but that fishing effort should not increase (ICES, 2004). In the UK, recreational fishers and, more recently, some inshore commercial fishers have proposed changes to the existing technical regulations on bass fishing (MLS and gillnet mesh size). These stakeholders cite a declining availability of large bass as justification for this proposal. Government recently carried out a consultation as a basis for raising the national bass MLS and introducing complementary mesh size regulations for the gillnet fisheries. Eventually the decision was made not to increase the MLS, but to evaluate alternative management approaches and to review the MLS after a few years. The issue of identifying both an appropriate MLS and the relevant associated gillnet mesh sizes is therefore of importance to fishery managers. Until now, reliable estimates for these parameters have been sparse; particularly in relation to the North Sea bass gillnet fisheries. By providing new data on gillnet selectivity; this work helps managers to make informed decisions on bass selectivity across a broad mesh size range of interest. 4
The basic theory of gillnet selectivity and fish entrapment The modal length optimum selectivity As a general rule, gillnets entangle fish of medium length, while the smaller and larger fish avoid capture. The exact length range of fish that become entangled depends largely on gillnet geometry (i.e. mesh size, hanging ratio, etc) and the species of fish (i.e. fish behaviour, fish shape, etc). Gillnet selectivity curves are generated for specific species, such as bass in this case, and are generally bell -shaped (Figure 3). A gillnet will be most effective at catching fish of a length corresponding to the peak of the bellshaped curve (i.e. the apex of the selectivity curve), a size often termed the modal length. Gillnet selectivity curves are assessed on a relative scale (i.e. relative to other lengths), as usually there is no information on the absolute number of fish in the population that encounter the net. likely that it will become entangled in the gillnet. The effect of increasing mesh size The usual effect of increasing mesh size would be to shift the entire bell-shaped selectivity curve to the right (accompanied with a proportionate increase in the modal length and spread). The implications of this would be that more large fish and fewer small fish would be caught in gillnets with large mesh. Small fish (less than modal length) Bass smaller than the optimum (modal) length will increasingly escape entanglement in a gillnet because they can more readily pass through the meshes. The smaller the bass, the greater the proportion that will avoid entanglement, so the relative retention rate decreases as bass length decreases below the modal length. Figure 3. A typical bell-shaped gillnet selectivity curve Big fish (greater than modal length) Bass larger than the optimum (modal) length are also less likely to become entangled, because the gillnet will not readily entrap their larger body parts. The larger the bass, the less 5
Method Experimental design The gillnet selectivity trials were carried out on fishing grounds in the southern North Sea (Figure 4) aboard the FV Rachael S (GY 305), a wooden-hulled netter of 20 m reg. length with a 172 kw engine. Table 1. The relative fishing powers of the gillnets used during the trials (subsequently compensated for in the SELECT GLMM analysis using an offset) Mesh size Trips 1 and 2 Hanging ratio No. of meshes deep Relative fishing power 90 mm 0.5 40 1.00 100 mm 0.5 40 1.10 108 mm 0.5 30 0.90 120 mm 0.5 30 1.00 Mesh size Hanging ratio Trip 3 No. of meshes deep Relative fishing power 90 mm 0.5 50 1.25 100 mm 0.5 40 1.11 108 mm 0.5 30 0.90 120 mm 0.5 30 1.00 Figure 4. Location of the bass gillnet selectivity trials in the southern North Sea In all, 24 gillnets (bottom-setting type) were newly made for the study and differed from each other in terms of their mesh size. Four mesh sizes were used, six gillnets of each mesh (i.e. 6 90 mm, 6 100 mm, 6 108 mm, and 6 120 mm; all full stretched mesh length). Each gillnet was 107 m long, but the fishing height varied between the mesh sizes, thus giving rise to a non-uniformity of fishing surface areas (Table 1). This particular variance was subsequently negated by including an appropriate offset in the SELECT GLMM analysis (see Methods section). The gillnets were constructed with colourcoded markings to allow the crew and scientist to identify easily the different mesh sizes during fishing operations (Figure 5). All 24 gillnets were substituted with brand new replacements after the second trip in order to eliminate any potential variance in fishing performance attributable to net damage. All gillnets used throughout the trials were constructed from the same monofilament material with the same twine diameter (0.40 mm). The hanging ratios of the nets were identical in all cases (0.5), as was the rigging of the flotation and lead-lines. For each deployment (haul), the 24 nets were shot in close proximity to each other and allowed to fish for the same soak time. Twice per day, all 24 gillnets were retrieved after typical soak times of ~10 h (mean 10.3 h, SD 1.6 h) (Figure 6). 6
activity was abandoned because it proved impractical and the results are therefore not considered further. Figure 5. Some of the new gillnets with colour coded staples to facilitate identification of the different mesh sizes On retrieval, the catches were removed and each net was cleaned and redeployed. This process was repeated for up to five days (weather depending), then the vessel returned to port to land its catch. Three netting trips were completed during October and November 2008, so providing repetition of the trials to assist in the overall evaluation of results. Figure 6. Hauling the gillnets All fish caught in the gillnets were measured to the nearest cm below, by a Cefas scientist on board. Every fish was identified to species level, and no subsampling of the catch was necessary throughout the trials. During the trials some effort was made to obtain additional catch data from surface driftnets of varying mesh size. However, this Statistical data analysis and modelling Choosing a uni- or bi-modal model An initial analysis of the catch data was undertaken to determine whether a uni- or bimodal model provided the best representation of the selectivity (catch) process for bass in the gillnets. A uni-modal model is bell-shaped with a single peak, and indicates that fish size is the primary factor influencing the selectivity process. Bi-modal models are more complex and are a mixture of two uni-modal shapes. They may have one or two peaks and result when two or more mechanisms influence the selectivity process in gillnets (e.g. some fish species may be caught in relation to their size, but some may be entrapped by teeth, fins, etc). Generally, one peak in a bi-modal model is more dominant because it represents the primary mechanism influencing the capture process. As a general rule, it is preferable to use the more simple type of model (i.e. uni-modal) unless the more complex bi-modal model provides a better representation of the selective process (known as a better fit ). An initial analysis of the pooled catch data (i.e. all hauls pooled for each trip) from these trials indicated that a bi-modal curve provided a marginally better fit (than a uni-modal model) for one of the three trips but did not improve the fit for the other two trips. The analysis of pooled-haul data is useful as a guide for model fit, but it can produce 7
results that are artificially too accurate because it can ignore the variation inherent in the underlying individual hauls. In addition, a bi-modal model may provide a good fit on pooled-haul data through suppression of the underlying variation associated with the individual uni-modal hauls in their non-pooled form. Therefore, further analysis was necessary and was undertaken by fitting both uni- and bimodal models to the catch data for each individual haul. This individual-haul analysis is more accurate than using pooled-haul data because full account of the underlying variation in the catch data is accounted for at a haul level. With this individual-haul analysis, the bimodal model did not provide any fit for 56% of the hauls. For the remaining 44% of the hauls, the bi-modal model did not provide any significantly improved fit over the uni-modal model. The bi-modal model did not provide any fit for any of the hauls from trip 2, and only for one haul from trip 3. For the reasons above, the uni-modal model was selected as the model of choice for analysis. SELECT method The gillnet variation of the SELECT method, as described by Millar and Holst (1997), is widely accepted as the method of choice for generating selectivity parameters for an individual haul. The method is an established technique for gillnet selectivity analysis, and uses advanced analytical techniques such as maximising probability using the loglikelihood function. Having selected the uni-modal model as the model of choice to represent bass selectivity, the SELECT method was used to determine the selectivity parameters for each haul. This analysis was undertaken, and it produced a good fit to the data for 75% of the hauls analysed, whereas the remaining 25% did not produce any fit. Fryer method Typically at this stage, the 25% of hauls (not producing a fit) would be discarded and the analysis would continue, based solely on the remaining 75% of hauls. Traditionally, the Fryer method (Fryer, 1991) would then be used subsequently to estimate the average trip selectivity parameters from the individual selectivity curves from each haul obtained during a single trip (e.g. see Madsen et al. 1999). In the context of this work, the Fryer method has some inherent potential bias because it would utilise only those hauls that produced a good fit (i.e. 75% of hauls in this case). The approach disregards data from those hauls without a good fit (25% of hauls in this case), so could be potentially biased as a result of this haul-selection. SELECT GLMM method In order to eliminate the potential bias described previously and to make full use of all the data from all hauls, an alternative approach was developed for this work in conjunction with the statistician René Holst (DTU-Aqua, Hirtshals, Denmark). This alternative approach incorporated an analysis of the individual hauls using the 8
SELECT method within the framework of a generalised linear mixed model (GLMM). An offset was included within the GLMM framework to nullify the effect of the different fishing powers (fishing areas) of gillnets of differing mesh size. This SELECT GLMM method incorporates all data from all hauls while taking appropriate account of individual haul variability. The SELECT GLMM analyses were performed on the datasets from all hauls within each trip, so providing three replicates to help facilitate a useful end comparison. The use of GLMM as a tool to obtain mean estimates from multiple repeat analyses has become more accessible in recent years because of the enhanced availability of suitable analytical software (see Holst and Revill, 2009). In this work, a GLMM approach was possible because the models fitted to the individual hauls for bass were uni-modal in structure. Uni-modal models can readily be reduced to log-linear models (see Millar and Holst, 1997), so can adequately be handled within the framework of a GLMM analysis. Bi-modal models cannot be reduced to loglinear form so cannot be handled using the GLMM approach. This was, however, not an issue in this work because the uni-modal was the preferred model. The SELECT GLMM analyses produced two parameters (K 1 and K 2 ) which can be used to describe the selective process for the fish species of interest. These are described in more detail in the following section. Selectivity parameters K 1 and K 2 For any species caught in sufficient quantity in gillnet selectivity experiments, the selectivity parameters (K 1 and K 2 ) can be determined by statistical analysis. The parameters provide useful information on the selective properties of gillnets in relation to the species of interest. The K 1 parameter can be multiplied directly by gillnet mesh size to give the maximum point of retention in the selectivity curve for a given species, across the range of mesh sizes used in the experiments. The length of fish at this peak retention point is often referred to as the modal length of a gillnet selectivity curve. The square root of the K 2 parameter can be multiplied by the gillnet mesh size to give the spread (width) of the bell-shaped gillnet selectivity curve for that species. The spread of the selectivity curve corresponds directly to the standard deviation (SD), so ±2 SD about the modal length will incorporate 95% of the area under that selectivity curve. The K 2 parameter and the associated spread can be used to indicate what proportion of fish at varying lengths above and below the modal length will be retained by the gillnets of various mesh size. Quality control comparison To provide quality control for our selectivity experiments, a species was selected whose selectivity parameters were already known from previous experimentation so could be used to provide a comparison. Dover sole was chosen for this purpose, because it was abundant on the fishing grounds and Danish workers had previously undertaken 9
a substantial gillnet selectivity study on it (Madsen et al., 1999). An analysis for sole was undertaken that followed the same method undertaken for bass (described previously). This resulted in the generation of the selectivity parameters K 1 and K 2 for sole. 10
Results Sea trials The fishing vessel Rachel S (Figure 7) operated out of Lowestoft (UK) to the fishing grounds in the southern North Sea. The gillnets were deployed on rough / hard ground on the edge of the sand banks in areas known locally as Gabbard Banks, Four Mile Knoll, Enterprise Shoal, The Falls and The Galloper. Three successful trips were completed, giving rise to a total of 23 hauls, with each haul supplying a set of catch data from the 24 gillnets. For the third trip, all gillnets were replaced with brand new nets, because some had become damaged during the preceding two trips. A range of species was caught throughout the sea trials and the raw catch data are detailed in Table 2. Estimation of selectivity parameters (bass) The selectivity parameters for bass were consistent and closely replicated between all three trips (Table 3, Figure 8). The modal length (i.e. peak) of the bass selectivity curves was estimated to be 41 44 cm from the gillnet with the mesh size of 90 mm (Table 4, Figure 8). This modal length for bass selectivity increased with the larger mesh size, as would be expected, reaching 54 58 cm for the gillnets with a mesh size of 120 mm. The spread of the selectivity curves ranged from 3 to 6 cm (Table 4, Figure 8). Figure 7. The MFV Rachel S (GY 305) returning to Lowestoft after a fishing trip Estimation of selectivity parameters (sole) for quality control / comparative purposes The selectivity parameters derived for sole were closely replicated between all three trips. The modal (peak) length of sole was estimated to be 28 29 cm from the gillnet with a mesh size of 90 mm (Table 4) in the three trips. This increased to 37 38 cm for the gillnets with a mesh size of 120 mm. The spread of the selectivity curves ranged from 4 to 7 cm. The selectivity parameters for sole obtained in this study were highly comparable (within 1 2 cm) with those previously determined in Danish trials (Madsen et al., 1999). 11
Table 2. Raw catch data from the 24 bottom set gillnets from the three trips Trip 1 Trip 2 Trip 3 Total Date of trip 6 12 th October 2008 21 26 th October 2008 5 9 th November 2008 Number of hauls during trip 10 8 5 23 Species caught (Latin name) (Common name) Number of fish caught Dicentrarchus labrax European seabass 325 26 37 388 Solea solea Sole (Dover Sole) 60 71 64 195 Trisopterus luscus Bib 47 96 51 194 Scophthalmus rhombus Brill 1 1 2 Gadus morhua Cod 84 75 60 219 Limanda limanda Dab 69 55 13 137 Scyliorhinus stellaris Nurse hound 3 3 Galeorhinus galeus Tope 2 2 Eutrigla gurnardus Grey gurnard 1 1 Aspitrigla cuculus Red gurnard 1 1 Melanogrammus aeglefinus Haddock 1 1 Clupea harengus Herring 48 8 36 92 Trachurus trachurus Horse mackerel 19 3 9 31 Microstomus kitt Lemon sole 1 1 Scyliorhinus canicula Lesser spotted dogfish 449 546 956 1951 Scomber scombrus Mackerel 48 5 8 61 Pleuronectes platessa European plaice 14 9 3 26 Trisopterus minutus Poor cod 2 2 Mustelus asterias Starry smooth hound 274 136 121 531 Alosa fallax Twaite shad 2 2 Raja clavata Thornback ray 3 12 6 21 Trigla lucerna Tub gurnard 14 4 13 31 Scophthalmus maximus Turbot 1 1 Merlangius merlangus Whiting 661 524 182 1367 Table 3. Selectivity curve parameters derived from the SELECT GLMM analysis of the catch data Species Trip Selectivity curve parameters K 1 (SE) K 2 (SE) 1 0.48385 (0.00649) 0.00275 (0.00021) 2 0.46464 (0.00857) 0.00132 (0.00022) Bass 3 0.45200 (0.00498) 0.00101 (0.00011) 1 0.30922 (0.01424) 0.00337 (0.00063) 2 0.31396 (0.00984) 0.00209 (0.00024) Sole 3 0.31802 (0.01625) 0.00253 (0.00042) (SE) = standard error of the estimated parameter 12
Table 4. Modal length and spread of the selectivity curves in relation to gillnet mesh size Species Trip Gillnet mesh size (mm) 90 100 108 120 Modal length * (cm) Bass Sole Bass Sole 1 44 48 52 58 2 42 46 50 56 3 41 45 49 54 1 28 31 33 37 2 28 31 34 38 3 29 32 34 38 Spread** of selectivity curve (cm) 1 5 5 6 6 2 3 4 4 4 3 3 3 3 4 1 5 6 6 7 2 4 5 5 5 3 5 5 5 6 * Modal length = (K 1 mesh size) ** Spread = (K (1/2) 2 mesh size) Table 5. Comparison of sole selectivity parameters between studies Sole (Solea solea) Point of maximum retention in the selectivity curves obtained during the study Gillnet mesh size Madsen et al. (1999) Present study 90 mm 30 cm 28 29 cm 100 mm 33 cm 31 32 cm 108 mm 35 cm 33 34cm 120 mm 39 cm 37 38cm 13
Figure 8. Selectivity curves derived for bass from the catch data (NB: Peak heights are varied because this reflects the relative fishing powers of the nets used in the experiments) 14
Examples of how the results can be used in management (for guidance purposes only) With the aid of examples, we describe here how the results obtained from this work may potentially be used as a tool in fisheries management. (a) Question: Which are the best results to use from the three sea trials? Answer: Although there is little variation between the trials, a pragmatic approach would be to use average values from all three sea trials. This gives us selectivity parameters K 1 and K 2 (below) which can be used to answer many typical management questions. We also convert K 2 into average spread to simplify our analyses here: K 1 = 0.46683 (used to locate modal length) K 2 = 0.001693 (used to give spread of selection curve) Average spread of bass selectivity curves ( 90 mm K 2 ) + ( 120 mm 2 ) 2 = K = ( 3.7 + 4.9) 2 = 4. 3 = 4.3 cm (b) Question: What is the modal length of bass in a gillnet of 95 mm? Answer: (d) Question: What mesh size of gillnet is the optimum (modal length) for catching bass of 55 cm? Answer: Modal peak K 1 = 55 cm 0.46683 = 118 mm (e) Question: What is the largest mesh size of gillnet that will not catch any bass of 30 cm and below? Answer: (Fish length + *(3 average spread)) K 1 = (30 cm + (3 4.3 cm)) 0.46683 = 92 mm NB: *(3 average spread) equates to (3 SD) (f) Question: What mesh size of gillnet will give a relative selection rate of 0.1 (10%) for bass of 40 cm in the gillnet? Answer: (40 cm bass + (average spread (cm) (-2 ln(relative retention rate))) K 1 = (40 + (4.3 (-2 ln(0.1))) 0.46683 = (40 + (4.3 (-2-2.30259)) 0.46683 = (40 + (4.3 (4.605)) 0.46683 = (40 + (4.3 2.146) 0.46683 = (40 + 9.2) 0.46683 = 49.2 0.46683 =105 mm Mesh size K 1 = 95 mm 0.46683 = 44 cm (c) Question: What is the spread of a bass selectivity curve in a gillnet of 95 mm mesh? Answer: Mesh size (mm) K = 95 mm 0.001693 = 95 mm 0.041146 = 3.9 cm 2 15
Conclusions This work successfully determined the gillnet selectivity parameters for bass over a useful range of mesh sizes. The results were closely replicated between the three trips undertaken. The gillnet selectivity parameters obtained here can be used to provide some management advice on bass selectivity for gillnets over a useful range of mesh sizes. The optimal selection for bass (modal length) was 41 44 cm in the gillnets with a mesh size of 90 cm. This selectivity peak increased with the larger mesh sizes, reaching 54 58 cm with the 120 mm gillnets. The selectivity parameters obtained for Dover sole closely replicated estimates provided by a previous Danish study undertaken in 1999. The two sets of results were within 1 2 cm, so providing some quality control / assurance for the output from this work. All bass catch data gathered throughout the trials were incorporated into the selectivity parameter estimates by utilising a SELECT GLMM modelling approach. This is a useful advance in the methods available for the statistical analysis of gillnet selectivity data. References Fryer, R. 1991. A model of the between-haul variation in selectivity. ICES Journal of Marine Science 48: 281 290. Holst, R., and Revill A. A simple statistical method for catch comparison studies. 2009. Fisheries Research 95: 254 259. models. ICES Journal of Marine Science 54: 471 477. Pawson, M. G., Kupschus, S., and Pickett, G. D. 2007. The status of sea bass (Dicentrarchus labrax) stocks around the England and Wales, derived using a separable catch-at-age model, and implications for fisheries management. ICES Journal of Marine Science 64: 346 356. Acknowledgements Acknowledgements are made to: The crew and skipper of the Rachel S for their enthusiasm and support throughout these trials. Kurt L. Christensen. Danish Consulate / Meridian Fish Sales Ltd, Grimsby. The MFA in London and Lowestoft for their support. The manufacturers of and riggers of the gillnets used during these trials (A and R netting Leiston, Suffolk, and Advanced Netting, Clacton-on-Sea) Andy Payne, Mike Smith, David Maxwell and Jane Medler (Cefas) for their contribution to and helpful support of this work Professor Russel Millar of the University of Auckland (New Zealand) for the provision of the SELECT routine in R code, used in the statistical analysis The FSP team (Defra, Cefas, NFFO) for their support of this programme This work was funded by the UK (Defra) Fisheries Science Partnership programme 2008/09 and by the Defra funded R and D project MF1002. ICES. 2004. Report of the ICES Advisory Committee on Fishery Management and Advisory Committee on Ecosystems, 2004. ICES Advice. Volume 1, Number 2. 1544 pp. Madsen, N., Holst, R., Wileman, D., and Moth- Poulsen, T. 1999. Size selectivity of sole gill-nets fished in the North Sea Fisheries Research. Fisheries Research 44: 59 73. Millar, R. B., and Holst, R. 1997. Estimation of gill-net and hook selectivity using log-linear 16
Appendix 1 Detailed Operational Plan agreed before the trials 1. Vessel: Rachael S 2. Skipper: Nigel Stead 3. Agent: Meridian (Fish Sales) Limited 4. Port of operations: Grimsby 5. Period and location of Survey: 3 trips (each one up to a maximum of six days per trip). Total survey time not to exceed 18 days. To be conducted during the period between 1 st October and 30 th November 2008. Vessel can fish within ICES area IV c (within British fisheries limits only) for the period of the survey. Provisional agreed sailing dates are (0800) 6 th October, 19 th October and 28 th October. These dates are not fixed, for instance if bad weather delays operations. The vessels will sail to and from Lowestoft for the duration of the trials, subject to operational and safety considerations. 6. Quota: Vessel will be off quota and the relevant dispensation from European Council Regulation 850/98 will be issued. This is to be carried onboard by CEFAS representative scientist all times onboard during the survey. The dispensation will only be valid if the terms of the dispensation are met in full. CEFAS will make arrangements with Defra for the dispensation to be issued. 7. Primary aim of the survey: To target bass with gillnets and record catches using a range of mesh sizes. These will include the industry standard mesh size, and meshes both above and below that industry standard mesh size. Obtaining the data from the gillnets over the range of mesh sizes must take precedence over all other fishing considerations, other than those concerning safety and the well being of the crew. 8. Why do this survey? The data obtained will demonstrate the length ranges of bass caught by a range of mesh sizes. This is very useful information as it produces firm evidence on which size of fish are caught in relation to mesh size. The trials will also ascertain if there are difference size ranges caught in surface drift nets compared to bottom set nets of the same mesh size. 9. Gillnet plan and deployment regimen. A plan of the gillnets and the fleets must be drawn up prior to sailing. This must detail exactly how many gillnets are in each fleet and what are the mesh sizes of the gillnets in each fleet. This must be provided to all participants before departure. It is extremely important that catches can be specifically related back to a particular gillnet mesh size at all times. An agreed protocol for the deployment and retrieval of the gillnets must be agreed prior to departure. Cefas will provide the gillnets required for the trials prior to departure which will be delivered to Lowestoft (BFP Fish Sales) (01502 564 390). The skipper and crew will rig the nets and mark the gear so that different mesh sizes can be recognised easily once hauled aboard. The skipper will decide exactly when and where to deploy the bottom set gillnets and drift nets so as to maximise the catches of bass across the full length range. The skipper shall take as much care as possible to avoid cetacean (whales, dolphins and porpoises etc) entanglement at all times. 10. What data needs to be recorded? It is particularly important in selectivity studies to obtain good quality data with as little data raised as possible. a) Haul details Date / Time / Position of nets (Markers buoys lat and long) / Length of net / No of nets in fleet / Specific mesh sizes of nets in fleet and how many of each mesh size in each fleet and in what order / Depth range / ICES rectangle / Time of shooting / Time of hauling / Soak time / Any other useful comments such as tidal flow, weather etc. b) Catch All catch needs to be recorded (not benthos) / Species / Length / How the animal is caught on net (i.e. by teeth / gilled /? any other way) /Whether discarded / retained / What gillnet (mesh size) the catch originates from /Any other useful comments 11. Crew and skipper. The crew are to be available to assist the CEFAS scientist onboard whenever requested / required. The skipper will retain the usual authority and responsibilities and has the overriding responsibility to ensure safety of the vessel and all persons aboard. 12. Cruise reports. On completion of each of the three trips a short summary of the cruise, including successes and failures, points of interest should be complied by the CEFAS scientist. The skipper / owner should sign to agree with the contents of the cruise report. 13. Scientist sea going qualifications. The onboard CEFAS scientist must bring all necessary sea going qualifications with him/her during the trips. I agree to adhere to this detailed operational plan on behalf of CEFAS..Date...Skipper Rachael S Date 17