Demersal Trawl Fishery Research Articles

Performance and impact on the seabed of an existing- and an experimental-otterboard: Comparison between model testing and full-scale sea trials

A new door has been designed to reduce hydrodynamic drag coefficient and increase spread of door commonly used in the Mediterranean commercial demersal trawl fisheries. Flume tank testing and engineering sea trials provide data which allow us to illustrate the performance and impact on the seabed of an existing door and a new door design. In the flume tank, each model was tested over a range of attack angles and for a limited range of otterboard heels. Curves of spreading-, drag- and down-force coefficients have been calculated. In the case of sea trials in order to extract the hydrodynamic coefficients an analysis has been applied and a mathematical model was used to calculate attack angle functions. From analysing the differences between engineering sea trials and flume tank tests we have deduced some conclusions about additional ground contact forces on sea trials that affect the performance of the doors. Moreover, a comparison between reaction forces of the flume tank and the estimation of reaction forces at sea has been given. Finally, this study allowed us to notice important differences between traditional and experimental otterboards.

Investigating the consequences of Marine Protected Areas for the South African deep-water hake (Merluccius paradoxus) resource

AbstractEdwards, C. T. T., Rademeyer, R. A., Butterworth, D. S., and Plagányi, É. E. 2009. Investigating the consequences of Marine Protected Areas for the South African deep-water hake (Merluccius paradoxus) resource. – ICES Journal of Marine Science, 66: 72–81. Protected areas are often touted as important management tools to mitigate the uncertainty inherent in marine ecosystems, and thereby improve the long-term prospects for sustainable resource use. However, although they certainly play an important role in conservation, their usefulness in improving fishery yields is contentious. We present a simulation model that explores spatial closure options, and apply it to the demersal hake trawl fishery off South Africa. The model is based on the age-structured approach used for current assessments, representing the dynamics of the deep-water hake Merluccius paradoxus within a zonally disaggregated spatial system. Fitting the model to two zones, which demarcate a potential closed area from the remaining fished area, we investigate the consequences that such a protected area could have for the fishery. Our model suggests that area closures would have a negligible benefit for the fishery, regardless of the level of hake movement between areas. This is likely the result of the model's simplicity, and we suggest additional factors that should be considered to quantify the impact of Marine Protected Areas on the fishery more reliably.

Comparison of diamond- and square-mesh codends in the hake (<i>Merluccius merluccius</i> L. 1758) trawl fishery of the Adriatic Sea (central Mediterranean)

A traditional codend (40 mm diamond mesh) and an experimental codend (40 mm square mesh) made of 2.6 mm diameter knotless PA netting were tested on a commercial trawl net on a sandy-muddy bottom (~70 m depth) of the Adriatic sea in order to assess the size selectivity of European hake ( Merluccius merluccius ) and the reduction of the discards in a demersal multi-species trawl fishery. The catch was sorted as target species and bycatch and the selectivity with respect to European hake was investigated using the covered codend method. The square-mesh codend allowed a reduction of about 37% of the fraction discarded at sea. The mean catch obtained with the experimental codend was lower than that obtained with the traditional codend (about 14 kg per haul) but the short-term economic losses were low (~12%). The square-mesh codend was also more selective than the traditional codend, giving the juveniles of European hake a good chance of escape: the mean selection length (L 50 ) was 7.60 cm in DM and 12.98 cm in SM. Moreover, the mesh configuration did not affect the gear’s performance. Thus, the square-mesh codend could be an easy, inexpensive and useful solution for a more sustainable management of the resources in Adriatic multi-species fishery.

Effect of fishing tactics on the standardization of cardinalfish (Epigonus crassicaudus) catch rates in the demersal multispecies fishery off central Chile

We analyzed the allocation of effective fishing effort and the standardization of cardinalfish (Epigonus crassicaudus) catch rates in the multispecies demersal trawl fishery off central Chile. The period analyzed covered from 1997 to 2004 and included detailed information about fishing hauls. Each haul that contained cardinalfish was assigned into a particular fishing tactic (cluster) by using multivariate analysis of their catch composition. The catch rate standardization was carried out by generalized linear models (GLM). Three fishing tactics were discovered: the first directed effort at cardinalfish, the second at common hake (Merluccius gayi gayi), and the third at Patagonian grenadier (Macruronus magellanicus). Fishing tactic was used as an explanatory variable in the proposed GLM. The fishing tactic effect was one of the most important factors in explaining the variance in the GLM. These results are discussed in the context of how the assignation of a fishing tactic allows unbiased abundance indices to be obtained in this kind of multispecies demersal fishery.

Selectivity of a 50-mm diamond mesh knotless polyethylene codend for commercially important fish species in the Aegean Sea

Summary This research investigated effects of changing the diamond mesh size on codend selectivity in Mediterranean fisheries. The selectivity of a typical 50-mm diamond knotless polyethylene (PE) codend used in the Turkish fishery in the Aegean Sea was measured for commercially important species, in particular hake (Merluccius merluccius), horse mackerel (Trachurus trachurus), anglerfish (Lophius piscatorius) and John Dory (Zeus faber). Fishing trials were carried out on the commercial trawler ‘Hapuloglu’ between 9 and 12 December 2006 using a modified trawl net. Selectivity data were collected by the covered codend method and analysed by means of a logistic equation (Maximum Likelihood Method). The mean selectivity curve was estimated from individual hauls, taking between-haul variations into account. Mean mesh size of the codend was 49.4 mm as measured by digital calliper. Mean values for 50% retention length of hake and horse mackerel were estimated to be 11.4 and 15.6 cm total length; corresponding selection ranges were 4.1 and 5.5 cm respectively. The 50-mm diamond mesh codend showed adequate selectivity compared to the minimum landing size (MLS) for horse mackerel, while for hake it selected specimens in a size range far lower than the MLS. No selectivity values could be determined for anglerfish or John Dory. To design a more selective codend for the Turkish demersal trawl fishery, not only mesh size regulations but also other codend characteristics and netting material properties must be urgently considered.

A bio-economic indicators suite for the appraisal of the demersal trawl fishery in the Southern Adriatic Sea (Central Mediterranean)

The state of demersal fishery in the Southern Adriatic Sea (GFCM-GSA 18, Central Mediterranean), years 1996–2003, from a biological, social and economic point of view was analysed using 47 indicators: 22 biological indicators obtained from fishery-independent data through yearly experimental bottom trawl surveys (“Medits” Programme), and 25 socio-economic indicators estimated from fishery-dependent data, available from the monitoring system of the Italian Institute for Economic Research on Fisheries and Aquaculture (IREPA). Biological indicators were applied for “single-species” ( Eledone cirrhosa, E. moschata, Illex coindetti, Merluccius merluccius, Mullus barbatus, Nephrops norvegicus, Parapenaeus longirostris, Raja clavata, Zeus faber) and for “multi-species” analysis. Economic indicators describing economic performance, productivity, costs and prices, and the overall economic sustainability of fishery were estimated. Social indicators and a general indicator summarising social sustainability were also considered. Indicators’ values were displayed using the Traffic Light system. Both fishery-independent and fishery-dependent indicators highlighted a progressive decline of the trawl fishery system in the GSA 18. This decline was mainly related to the ongoing depletion of the traditional fishery target species (mostly long-living, late-maturing species) partially replaced by the increase of traditional accessory species (generally short-living species), as well as to the reduction of productivity and increasing costs. The whole procedure was proposed as a contribution to the identification and applicability of bio-economic indicators for fishery management purposes.

Size selection by diamond- and square-mesh codends in multi-species Mediterranean demersal trawl fisheries

Mediterranean demersal trawl fisheries traditionally operate using small diamond-shape meshes in the codend, which tend to retain almost all animals. We investigated the effect of mesh configuration on the size selectivity of nine species (the scaldfish Arnoglossus laterna, the broad-tail shortfin squid Illex coindettii, the European hake Merluccius merluccius, the red mullet Mullus barbatus, the Norway lobster Nephrops norvegicus, the common pandora Pagellus erythrinus, the deepwater rose shrimp Parapenaeus longirostris, the Mediterranean horse mackerel Trachurus mediterraneus and the poor-cod Trisopterus minutus capelanus) commonly captured in the Mediterranean demersal trawls. Two codends having meshes with the same opening (ca. 38 mm) but different mesh configuration (DM38: diamond-mesh and SM38: square-mesh configuration) were fished daily and alternately on the same trawl. They demonstrated a substantial improvement in selectivity with square-mesh. Selectivity was measured using the covered codend technique, the cover being supported by circular hoops. The results were analysed taking into account the between-haul variation in selectivity. Two other important external variables were identified: the trawling depth and the codend catch, which for some species influenced between-haul variation, but there was no evidence of a coherent effect on selectivity parameters. The selectivity of 40 mm diamond-mesh codend has been reported to be rather poor because a large proportion of the codend catch is immature and smaller than the minimum landing size (MLS) or first maturity size. With the exception of the flatfish ( A. laterna), the effect of a change of mesh configuration from diamond- to square-mesh on size selectivity positively affected the retention length at 50% ( L 50). However, square-meshes were found to be unsuitable for flat and/or deep-bodied fish as these escape more readily from diamond-meshes. The Council Regulation (EC) No. 1967/2006, concerning management measures for the sustainable exploitation of fishery resources in the Mediterranean, establishes the MLS of marine organisms. However, the increase in L 50 with square-mesh would not avoid some of the existing contradictions in allowing the use of codend mesh which leads to lower L 50 than the MLS.

Selectivity of flexible size-sorting grid in Mediterranean multispecies trawl fishery

The demersal multispecies trawl fishery in the western Mediterranean Sea has a poor selectivity; large numbers of juvenile fish are caught when using the legislated 40-mm diamond mesh cod end. The selectivity of a flexible sorting grid with 20-mm bar spacing (BS20) installed in front the conventional trawl cod end was investigated. The standard covered cod end method was used. Data was adequate for analyzing the selectivity of European hake Merluccius merluccius, poor cod Trisopterus minutus, greater forkbeard Phycis blennoides and Norway lobster Nephrops norvegicus. The selectivity of the 20-mm sorting grid was compared with the performance of 40-mm diamond (DM40) and square mesh (SM40) cod ends. The effect of a guiding funnel on the performance of the grid (BS20-f) was also explored. Finally, the potential changes in yield per recruit (Y/R) and biomass per recruit (B/R) after implementing a sorting grid were explored. An overall improvement in the 50% selection length (L50) with all four species was substantial when comparing the BS20 to the DM40. Higher improvement in L50s was achieved when the grid was equipped with the guiding funnel. When comparing the performance of the BS20-f to the SM40, there was no marked difference in L50s. The Y/R and B/R, however, are substantially higher with BS20-f for poor cod and about the same for European hake. BS20 compared to SM40 achieved lower Y/R and B/R for all four species; the smallest difference was observed for Norway lobster. Further development is required if the sorting grid is to be introduced into commercial fisheries.

A global assessment of the impact of fisheries-related mortality on shy and white-capped albatrosses: Conservation implications

Hundreds of thousands of seabirds are killed each year as a result of interacting with longline and trawl fishing operations, and the severity of the impact varies regionally. Shy and white-capped albatrosses, Thalassarche cauta and Thalassarche steadi respectively, are phenotypically similar species known to be incidentally killed by fishing operations. The magnitude of this mortality has not previously been assessed across their range. Here we examine recent effort and bycatch rates in fisheries known to incidentally kill these species and qualitatively evaluate the level of impact of each fishery. Results indicate that over 8500 of these albatrosses may be killed annually, although the reliability of this estimate is low due to the paucity of comprehensive observer data in most fisheries. Of the estimated deaths of all seabird species in the fisheries assessed, trawl and longline fisheries killed birds in approximately equal proportions, but when the mortality levels of shy-type albatrosses were examined, trawl fisheries were responsible for 75% of all deaths. Data suggest most of these birds were killed in South African, Namibian and New Zealand demersal trawl fisheries and the South Africa pelagic longline fishery. Because most adult shy albatrosses are comparatively sedentary and rarely found outside Australian waters, it is primarily juvenile shy albatrosses that regularly encounter fishing fleets known to kill large numbers of albatrosses. In contrast, throughout most of their range juvenile and adult white-capped albatrosses are exposed to fisheries that collectively kill many thousands of these albatrosses each year. These data emphasise the urgent need for robust assessments of the impact of bycatch at a species and population level, and the urgent implementation of effective mitigation measures.

Fishing practice, gear design, and the ecosystem approach—three case studies demonstrating the effect of management strategy on gear selectivity and discards

Abstract Graham, N., Ferro, R. S. T., Karp, W. A., and MacMullen, P. 2007. Fishing practice, gear design, and the ecosystem approach—three case studies demonstrating the effect of management strategy on gear selectivity and discards. – ICES Journal of Marine Science, 64: 744–750. A basic tenet of the ecosystem approach to fisheries management is that harvesting is conducted with minimal impact on juvenile fish, non-target species, and marine habitats. A range of technical modifications of fishing gears aimed at improving their selective properties is available to help achieve these goals, but their effectiveness varies. Through three case studies, we describe how management controls influence fishing behaviour and the adoption of more selective gear, and demonstrate how conservation goals can be discouraged or encouraged by the strategy. In Norway, limits set on the maximum quantity of sublegal fish that may be retained on board, in combination with a ban on discarding, resulted in substantial area closures in the Barents Sea. Therefore, to gain access, fishers developed technical modifications to enhance gear selectivity. In both shrimp and demersal trawl fisheries, the modifications are now being used by virtually the whole fleet. To reduce cod mortality in the North Sea, mesh sizes were increased and effort restrictions introduced, but the measures also affected other fleets, notably those targeting Nephrops: fishers for that species reduced their mesh size to prevent loss of target species and to avoid effort restrictions. Although management measures may have resulted in reduced fishing mortality on cod, they placed additional pressure on other stocks by encouraging vessels to switch gears, and it is likely that discard rates have increased. In the eastern Bering Sea fishery for walleye pollock, the adoption of more-selective fishing gears was encouraged by regulations requiring fisheries to be curtailed when bycatch rates of prohibited species are exceeded, leading to underutilization of the target species through premature closures. Fishers now act cooperatively by providing real-time data on bycatch hot spots, allowing tactical fishing decisions to be taken to avoid such areas.

Bycatch and discarding in the South African demersal trawl fishery

Observers aboard commercial trawlers collected data on the total catch composition of 614 and 479 hauls made by vessels operating off the south and west coasts of South Africa, respectively. On the south coast, four fishing areas were identified on the basis of target species and fishing depth. On the west coast, hauls were separated into those targeting hake Merluccius spp. in four depth ranges (0–300, 301–400, 401–500, and >500 m) and those targeting monkfish Lophius vomerinus. For each area, the catch composition was calculated and the species assemblages were investigated using cluster analysis and multi-dimensional scaling. Finally, for each coast, the weight of fish discarded annually was estimated. On the south coast, although hake dominated, between 21% and 47% of the catch was not hake, depending on the fishing area. In comparison, hake dominated west coast catches, the proportion of hake increasing with depth. For each fishery investigated, approximately 90% of the catch was processed and landed. However, estimates of annual discards indicate that the south and west coast fisheries may annually discard 9000 or 10,000 t and 17,000 or 25,000 t, of undersized and unutilizable fish and offal, respectively, depending on the estimation method used. When developing strategies to limit or enhance utilization of bycatch, cognisance should be taken of the differences in catch composition between the south and west coasts and of the importance of bycatch revenue to south coast fishing companies.

Managing South Africa's trawl bycatch

Abstract Walmsley, S. A., Leslie, R. W., and Sauer, W. H. H. 2007. Managing South Africa's trawl bycatch. – ICES Journal of Marine Science, 64: 405–412. Over the past few decades, it has become widely recognized that the management strategies of world fisheries must ensure sustainability of bycatch as well as of target species. South Africa implemented a pilot observer programme from 1995 to 2000 to collect data on the levels and patterns of bycatch and discards in the demersal trawl fishery. Here, the results of that programme are used in conjunction with information on bycatch value and compliance to assess the issues and problems regarding bycatch and discarding in the fishery. In general, bycatch components can be placed into one of three categories (discarded bycatch, retained bycatch, and processing waste), each of which present different management problems and require different management approaches. The results were used to formulate a bycatch management plan for the demersal trawl fishery in South Africa. Given the need to continue monitoring bycatch, the performance of the pilot observer programme is reviewed, and the levels of sampling effort required for a national programme are discussed.

Sustainable use of flatfish resources: Addressing the credibility crisis in mixed fisheries management

Many flatfish species are caught in mixed demersal trawl fisheries and managed by Total Allowable Catch (TAC). Despite decades of fisheries management, several major stocks are severely depleted. Using the Common Fisheries Policy (CFP) as an example, the failure of mixed-fisheries management is analysed by focussing on: the management system; the role of science; the role of managers and politicians; the response of fisheries to management. Failure of the CFP management could be ascribed to: incorrect management advice owing to bias in stock assessments; the tendency of politicians to set the TAC well above the recommended level; and non-compliance of the fisheries with the management regulations. We conclude that TAC management, although apparently successful in some single-species fisheries, inevitably leads to unsustainable exploitation of stocks caught in mixed demersal fisheries as it promotes discarding of over-quota catch and misreporting of catches, thereby corrupting the basis of the scientific advice and increasing the risk of stock collapse. This failure in mixed demersal fisheries has resulted in the loss of credibility of both scientists and managers, and has undermined the support of fishermen for management regulations. An approach is developed to convert the TAC system into a system that controls the total allowable effort (TAE). The approach takes account of the differences in catch efficiency between fleets as well as seasonal changes in the distribution of the target species and can also be applied in the recovery plans for rebuilding specific components of the demersal fish community, such as plaice, cod and hake.

The use of a square mesh codend and sorting grids to reduce catches of young fish and improve sustainability in a multispecies bottom trawl fishery in the MediterraneanEl uso de copo de malla cuadrada y rejillas separadas para reducir las capturas de pece

In order to improve the capacity of bottom trawl fishing gears to reduce catches of young fish and discards in a highly exploited demersal trawl fisheries in the Mediterranean, the size-selection performance of a 36-mm square-mesh codend and two sorting grids with 20 and 15 mm bar spacing was assessed. Alternate hauls were used to assess the selectivity of 36-mm square-mesh codend. Selectivity of sorting grids was assessed using a double codend in which fish that escaped through the grid were captured in the lower codend while other fish were guided into the upper codend. The mean selection length (L50) for European hake was 18.5 cm and that for the Atlantic horse mackerel was 14.0 cm with the 36-mm square-mesh codend . These values are close to their current minimum landing sizes (20 cm for hake and 12 cm for horse mackerel). The sorting grid with 20-mm bar spacing showed L50 value of 13.3 cm for hake, suggesting that a larger grid-spacing would be needed to obtain sufficient sorting performance. Similarly, for Atlantic mackerel the estimated L50 of 14.3 cm indicates that larger grid spacing is required to attain an L50 that would be close to the current MLS (18 cm). For Atlantic horse mackerel and red mullet, the L50 obtained with the sorting grid with 20-mm bar spacing was close to the MLS of these species (the MLS of red mullet is 11 cm). The selectivity of the sorting grid with 15-mm bar spacing was generally very poor. Size-selection performance of sorting grids was assessed for the first time in the local fisheries. In order to effectively improve size-selection, seasonal and depth-dependent differences between target and by-catch species must be taken into account, which is a good indicator of the difficulty of implementing a single mesh size or grid spacing in the Mediterranean demersal trawl fishery.

Size selection of cod by rigid grids—Is anything gained compared to diamond mesh codends only?

Size selective grids were introduced into the Barents Sea demersal trawl fishery in 1997. It was believed at the time that in comparison to standard diamond mesh codends, grids had better selective properties, i.e. narrower selection range. Few studies have directly compared the selective properties of the combined grid and codend with that of the codend only. In this study we describe two experiments carried out in 2002 and 2003, where we directly compared the properties of codend selectivity for cod ( Gadus morhua) with the combined selectivity of a Sort-V grid and a codend. In 2002 we used small-meshed covers, whereas the twin trawl method was used in 2003. In both experiments we compared the selectivity of a 135 mm codend only with that of a trawl fitted with a 135 mm codend and a 55 mm grid, which is the mandatory configuration. In 2003 we also estimated the selectivity of a 155 mm diamond mesh codend, having an L 50 similar to that of the grid and 135 mm codend combination. The results presented no evidence that the grid and mesh combination had sharper size selection than codend meshes. The introduction of the mandatory use of grid in the fishery in 1997 therefore only increased L 50, and corresponded to a mesh size increase of around 20 mm. Mean selection length of the grid was inversely related to catch rates in the 2002 experiment when large catches were taken. Grid selection appeared less affected by seasonal variations in degree of stomach fullness or condition than mesh selectivity. Other situations where grid selection might perform better than mesh selection are discussed.

Optimising codend configuration in a multispecies demersal trawl fishery

Abstract The relative efficiencies and selectivities of conventional and modified codends were examined in a demersal trawl fishery targeting several species, including eastern king prawns, Penaeus plebejus (Hess), whiting, Sillago spp. and cephalopods. The modifications to codends included: (i) reducing the circumference; (ii) increasing the mesh size in conventional diamond‐mesh designs; and (iii) orientating meshes on the bar so that they were square shaped. The codends were tested against a fine‐meshed control in paired comparisons onboard three commercial trawlers. The conventional codend comprised 41‐mm diamond‐shaped mesh attached to an anterior extension section at a ratio of 150 to 100 meshes and was demonstrated to be non‐selective for the targeted species. Reducing codend circumference to 100 meshes and increasing the size of mesh to 45 mm both improved selection for eastern king prawns, but the lateral mesh openings were estimated to be insufficient to allow juveniles of the other key species to escape. By contrast, codends made from 35‐ and 41‐mm mesh hung on the bar improved the size selection for eastern king prawns and selected stout whiting, Sillago robusta (Stead) (the smallest commercial‐sized fish) across narrow selection ranges and at 50% sizes of retention (L50s) that were closely correlated to the transverse morphology of fish and the maximum mesh opening. With the exception of a reduction in catches of octopus, Octopus spp., by the 41‐square codend, there were no other impacts on commercial catches by the square‐mesh designs. It was concluded that diamond‐mesh codends are inappropriate for use throughout this multispecies fishery and that a modified design comprising at least 35‐mm mesh hung on the bar is required to minimise the fishing mortality of unwanted sizes of the key target species. The results are discussed in terms of the utility of these types of modifications for closely regulating selection in penaeid‐shrimp trawls.

Distribution, population structure and biological aspects of Squalus spp. (Chondrichthyes:Squaliformes) from New South Wales and adjacent Australian waters

Four species of Squalus dogsharks, S. megalops, S. cf. mitsukurii, Squalus sp. B and Squalus sp. F inhabit shelf and upper-slope depths off New South Wales and adjacent Australian states. During fishery surveys between 1976 and 2001, distributional, size composition and reproductive data were collected for these species. Adult size classes dominated catches and, for S. megalops, heavily biased sex ratios were observed. Although no female data were available for the rarely caught Squalus sp. B, reproduction in the other three species was found to be continuous with no evidence of seasonality. Fecundity was 1–3 embryos for S. megalops, 1–5 for Squalus sp. F and 4–10 for S. cf. mitsukurii. All species are commercially exploited, contributing to the mixed species demersal trawl fishery off New South Wales. Stocks of some species are greatly depleted on the main trawling grounds, but the overall distributions of all species include large areas of lightly exploited habitat.

Exploring the effects of fishing on fish assemblages using Abundance Biomass Comparison (ABC) curves

Abstract The possible effect of fishing on dominance patterns in the South African south coast demersal trawl fishery is assessed using Abundance Biomass Comparison (ABC) curves for the period 1986–2003. The ABC method compares the ranked distribution of abundance among species against the similar distribution of biomass among species. The temporal pattern in the ABC curves and the W-statistic for two depth groups (<100 m and 101–200 m), and for the whole area combined, shows a gradient of change in the demersal assemblages from neutral (W ≥ 0) towards negative (W < 0), suggesting a disturbed or stressed condition. This corresponds to the onset of longline fishing effort in 1994, still ongoing in 2003, superimposed upon declining trawl effort in the same region. The ABC method shows promise as a guide for assessing the effects of fishing on fish communities, being based on established r- and k-selection theory. More modelling and comparative work is needed to establish acceptable ranges for the W-statistic, and their application in an ecosystem approach to fisheries management.

Commercial testing of a sorting grid to reduce catches of juvenile hake (Merluccius merluccius) in the western Mediterranean demersal trawl fishery

Mediterranean demersal fisheries have experienced an ongoing decline in catches over the past 20 years as a result of excessive increases in effort caused both by growth in trawler engine power and by rapid technological advances in fish finding and fishing technology. This has led to an overexploitation of these resources. An increasing share of the catches consists of immature individuals. This study was undertaken to test a sorting grid with a bar spacing of 20 mm as a means of excluding juveniles in the commercial hake (Merluccius merluccius) fishery in the Catalan Sea, western Mediterranean. The grid was placed in the extension section of the gear 5 m in front of a cod-end. Divided cod-end design was used to collect the escapees and target species. The mean selection length (L50) of the ten hauls was 14.2 ± 0.7 cm SE, with a selection range of 7. 3c m± 0.4 SE. The biomass of hake under L50 that escaped through the grid represented 50.1% ± 6.7 SE of the total hake biomass. These results are promising and indicate that a sorting grid can be used in excluding young hake. This was a first step toward implementation of sorting grids in commercial trawl gears as means of avoiding unwanted catches of small individuals in the hake fisheries in the Mediterranean Sea. Further trials are required to improve the sorting efficiency of the grid.

A modelling approach for assessing short-term catch losses as a consequence of a mesh size increase

Abstract Short-term effects of an increase in mesh size of a square mesh window codend were modelled with a stochastic size-selective simulation model using the 1999 fisheries data from the southern Baltic Sea (SD 25). Vessel-type-dependent selectivity estimates and catch per unit of effort (CPUE) data from Baltic cod ( Gadus morhua ) demersal trawl fishery were utilized to estimate the catch losses. When the window mesh size is increased by 15 mm (from 105 to 120 mm), overall catch loss of marketable size of fish during the first month would be 36–49%. Due to the marked effect of vessel type on overall codend selectivity, the decrease in marketable catch for side trawlers would be 42–60% and for stern trawlers 31–45%. With a 38-cm minimum landing size, discarding would decrease by 66–71%. In case fishermen would like to compensate the loss in the marketable catch by increasing their fishing effort, they would have to increase the effort by 55–90%. A 62–123% increase in fishing effort would be required to compensate the decrease in the marketable catch if both the window mesh size (from 105 to 120 mm) and the minimum landing size (from 38 to 40 cm) are increased. Fishermen would likely not be able to increase effort to such an extent. Instead, they might try to circumvent the regulations by intentionally decreasing the selectivity of their gear, i.e. by gear manipulation.