Conventional Trawl Research Articles

Bottom Trawling and Multi‐Marker eDNA Metabarcoding Surveys Reveal Highly Diverse Vertebrate and Crustacean Communities: A Case Study in an Urbanized Subtropical Estuary

ABSTRACTEstuarine habitats serve as critical feeding and nursery grounds for many aquatic species and support fisheries. However, monitoring these complex ecosystems using conventional trawling methods is destructive, costly, and labor‐intensive. This study compared trawling and a multi‐marker environmental DNA (eDNA) metabarcoding approach to monitor marine vertebrate and crustacean communities in an estuarine environment in subtropical Hong Kong. We analyzed 16 bottom trawl samples and the eDNA from 32 two‐liter water samples using primer sets specific to fishes and mammals (MiFish‐U, 12S‐V5, and Berry‐Fish) and decapod crustaceans (MiDeca). We found that the eDNA approach detected more pelagic and demersal fishes (237 vs. 106 in trawling) and elasmobranchs (6 vs. 3) than trawling. The eDNA approach was also more effective than trawling in detecting threatened vertebrates (16 vs. 4), including the Indo‐Pacific Finless Porpoise and the critically endangered Large Yellow Croaker. Among the detected fish at species level, 70 species were detected by both approaches, 32 species were detected by trawling only, and 142 species were detected by the eDNA approach only. Regarding crustaceans, the eDNA approach detected slightly fewer decapods (61 vs. 77) and stomatopods (5 vs. 8) than trawl surveys. However, the eDNA approach could be enhanced through the development of suitable decapod‐specific primers and the expansion of the local reference database. In addition, multivariate analyses of the eDNA data revealed spatial patterns of fish and crustacean assemblages that might be associated with sediment loading, oxygen, and nutrient levels. Furthermore, there was a positive correlation between eDNA read counts and trawl catch, but their correlation coefficient was low. We conclude that eDNA metabarcoding can provide high‐resolution detection of species, composition, and unravel community–environment relationships in estuarine ecosystems. Overall, integrating the non‐destructive eDNA approach can complement the conventional trawling method for better‐informed sustainable fishery management and conservation.

Commercial krill fishing within a foraging supergroup of fin whales in the Southern Ocean.

Commercial krill fishing within a foraging supergroup of fin whales in the Southern Ocean.

Improving the performance of a generic fisheye as a secondary bycatch reduction device in a south-eastern Australian penaeid-trawl fishery

The utility of a generic device (termed a ‘fisheye’) and its simple modification for cumulatively reducing bycatch in penaeid trawls was assessed during two experiments in a south-eastern Australian fishery. In the first experiment, conventional trawls (with no fisheye) were compared to simultaneously towed, identical trawls installed with a typical fisheye (termed ‘mesh fisheye’) comprising an open triangular frame terminating in a semi-elliptical vertical opening (350 × 150 mm) in the top of the trawl extension at 1.1 m anterior to the codend drawstring. Trawls with the mesh fisheye maintained catches of penaeids (mostly school prawns, Metapenaeus macleayi) but retained significantly less total bycatch (by 39 %) and fewer unwanted southern herring, Herklotsichthys castelnaui (by 60 %) and tailor, Pomatomus saltatrix (by 52 %). During the second experiment, trawls with the mesh fisheye were simultaneously compared to those with a modified design comprising a solid panel of aluminium in the lower half of the triangular frame (termed the ‘solid fisheye’). The solid fisheye excluded significantly more bycatch (by 52 %) and small southern herring (by 47 %), and also the numbers of several other species, including mulloway (by 38–41 %) without affecting penaeid catches. The improved performance of the solid fisheye was attributed to its greater anterior displacement of water and posterior area of lower perceived flow for fish. The results and simplicity of the fisheye modification support not only its regional use, but also testing in other national and international fisheries.

Environmental DNA captures elasmobranch diversity in a temperate marine ecosystem

AbstractMany sharks, skates, and rays (elasmobranchs) are highly threatened by the activities of commercial fisheries, and a clear understanding of their distributions, diversity, and abundance can guide protective measures. However, surveying and monitoring elasmobranch species can be highly invasive or resource‐intensive, and utilization of non‐invasive environmental DNA‐based methods may overcome these problems. Here, we studied spatial and seasonal variation in the elasmobranch community of the Western English Channel using environmental DNA (eDNA) collected from surface and bottom waters periodically over an annual cycle (2017–2018). In total we recovered 13 elasmobranch species within eDNA samples, and the number of transformed eDNA reads was positively associated with species (hourly) catch data resolved from 105‐year time series trawl data (1914–2018). These results demonstrate the ability of eDNA to detect and semi‐quantitatively reflect the prevalence of historically dominant and rare elasmobranch species in this region. Notably, eDNA recorded a greater number of species per sampling event than a conventional trawl survey in the same area over the same sampling years (2017–2018). Several threatened species were recovered within the eDNA, including undulate ray, porbeagle shark, and thresher shark. Using eDNA, we found differences in elasmobranch communities among sampling stations and between seasons, but not between sampling depths. Collectively, our results suggest that non‐invasive eDNA‐based methods can be used to study the spatial and seasonal changes in the diversity and abundance of whole elasmobranch communities within temperate shelf habitats. Given the threatened status of many elasmobranchs in human‐impacted marine environments, eDNA analysis is poised to provide key information on their diversity and distributions to inform conservation‐focused monitoring and management.

Validating a narrow codend cover and improving selectivity in south-eastern Australian fish trawls targeting eastern school whiting, Sillago flindersi

As part of attempts to improve selection among south-eastern Australian benthic fish trawls, an experiment was done to assess: (1) for confounding effects of a narrower-than-recommended hooped cover over a conventional trawl codend; and then (2) the associated size selection of eastern school whiting, Sillago flindersi; and (3) if this could be improved by inserting a large panel of smaller square-shaped mesh. The narrow cover was required to avoid contacting the seabed and did not affect trawl wing-end spread or catching efficiency. Despite a stretched mesh opening (SMO) of 96 mm and much larger than all S. flindersi (11–26 cm total length; TL), the conventional codend had a low 50% size at retention (L50) of 16.8 cm TL (selection range of 5.9 cm), which was close to the minimum marketable length. The capture of small S. flindersi by the large-meshed codend was attributed to an excessive increase in circumference (from 100 to 200 meshes) and a legally permitted ‘catch-separating’ rope to restrict fractional mesh openings to <~21%. Replacing ~1/4 of the 96-mm SMO netting in the codend and anterior extension with a panel of square-shaped mesh (~42-mm SMO hung on the bar) marginally (p = 0.065) improved size selection for S. flindersi (by allowing some fish <~15 cm TL to escape). Increasing the size of mesh in the panel to ~47 mm hung on the bar might allow more smaller S. flindersi to escape. But using a smaller mesh size throughout the entire codend, concurrent with other gear or spatio-temporal controls when targeting S. flindersi might be a more coherent management option than retroactively handicapping the existing large-mesh codends.

Flatfish survivors have tales to tell: Cold seawater and reduced deployment duration contribute to the survival of European plaice (Pleuronectes platessa) discarded by Belgian beam trawlers

The ‘high survival’ exemption to the European Landing Obligation regulation intensified the relevance of quantifying the survival of commonly discarded European plaice (Pleuronectes platessa) within the discard-intensive Belgian beam-trawl fishery, together with the search for ways to maximize discard survival. In this study we worked together with vessel owners and the crews of three Belgian beam trawlers representing the small (≤221 kW) engine power, coastal (n = 1 vessel), ‘Eurocutter’ (1), and large (>221 kW) (1) fleet segments. During 12 commercial trips which included 99 beam-trawl deployments in 2014 and 2015, three scientific observers assessed the survival of 3849 plaice aboard the vessel (by checking for the presence of reflex responses and bleeding injury; on-board survival assessment) and counted how many survived among a sub-sample (n = 489 plaice) monitored for at least 9 days in captivity (post-release survival). These fish were monitored alongside 160 captive controls. Only three control fish died (<2%) during daily monitoring. Survival of plaice discarded from conventional trawls of Belgian beam-trawl vessels representing the three fleet segments was estimated to range between 41–58 %, 11–28 %, 2–4 % (95 % confidence interval; Kaplan-Meier models) for trips of the coastal (≤221 kW), Eurocutter (≤221 kW) and >221 kW vessel, respectively. More challenging operational fishing conditions of >221 kW compared to ≤221 kW vessels such as ∼150 min gear deployment and ∼37 min sorting durations meant that down to only 20 % of undersized plaice were still alive when landed on deck. Such immediate mortality was hardly observed among ≤221 kW vessels. These effects were cumulative to the effect of elevated seawater temperatures of >14 °C during summer, because despite warm waters, immediate mortality among ≤221 kW vessels was low compared to >221 kW vessel-trips. Out of those fish that were still alive after sorting at the point of discarding and monitored for 9 days, on average 23 % (0%–100 %, 95 % CI) survived. Post-release survival was associated with stress and injury sustained during the capture and sorting process (proxied by the reflex impairment and injury index) and seawater temperature. Survival rates can be improved by choosing when to fish, and restricting gear deployments to practicable minimum durations. Without further adaptations to current fishing practices, justifying a high survival exemption for discarded plaice in the Belgian beam-trawl fishery will remain a challenge in compliance with the Landing Obligation.

Much ado about nothing: An example of how failed incentives thwarted the implementation of the EU landing obligation

With the introduction of the landing obligation in the EU common fisheries policy, there has been several initiatives to reduce unwanted catches of quota-regulated species. In this study, we present a flexible sorting grid as a potential solution for the problem of bycatch of saithe in the pelagic herring trawl fishery in the Skagerrak. The development of the grid was initiated by the industry and finalised through an industry-science collaboration project. The selectivity of the grid was evaluated through the use of an underwater camera system during conventional trawling with an industrial pelagic herring trawler. The results showed that the grid reduced the bycatch by more than 90%, from approximately 5% to 0.5% of the total catch. Given the scale of the pelagic fishery, this reduction could decrease unwanted bycatch of saithe by up to 1000 tonnes per year in the Skagerrak alone. These results were communicated to national industry and management representatives and to responsible EU management bodies and advisory councils. The work conducted within the project also drew interest at the yearly negotiations between EU and Norway. In their agreement for 2018, the EU and Norway agreed to establish a working group on technical measures in the Skagerrak tasked with reviewing selectivity of pelagic trawls as part of the terms of reference. However, in the agreed record of the delegations, the question of pelagic gear selectivity in the Skagerrak was a non-issue, although they concluded that the demersal bycatch can be substantial, that a sorting grid can reduce this bycatch and a sorting grid remains mandatory for Norwegian vessels in the Skagerrak. We argue that this result can be seen as a missed opportunity for the EU to reduce unaccounted bycatch in the Skagerrak and ensure better implementation of the landing obligation.

Use of the SmeltCam as an Efficient Fish Sampling Alternative Within the San Francisco Estuary

Resource managers often rely on long-term monitoring surveys to detect trends in biological data. However, no survey gear is 100% efficient, and many sources of bias can be responsible for detecting or not detecting biological trends. The SmeltCam is an imaging apparatus developed as a potential sampling alternative to long-term trawling gear surveys within the San Francisco Estuary, California, to reduce handling stress on sensitive species like the Delta Smelt (Hypomesus transpacificus). Although believed to be a reliable alternative to closed cod-end trawling surveys, no formal test of sampling efficiency has been implemented using the SmeltCam. We used a paired deployment of the SmeltCam and a conventional closed cod-end trawl within the Napa River and San Pablo Bay, a Bayesian binomial N-mixture model, and data simulations to determine the sampling efficiency of both deployed gear types to capture a Delta Smelt surrogate (Northern Anchovy, Engraulis mordax) and to test potential bias in our modeling framework. We found that retention efficiency—a component of detection efficiency that estimates the probability a fish is retained by the gear, conditional on gear contact—was slightly higher using the SmeltCam (mean = 0.58) than the conventional trawl (mean = 0.47, Probability SmeltCam retention efficiency &gt; trawl retention efficiency = 94%). We also found turbidity did not affect the SmeltCam’s retention efficiency, although total fish density during an individual tow improved the trawl’s retention efficiency. Simulations also showed the binomial model was accurate when model assumptions were met. Collectively, our results suggest the SmeltCam to be a reliable alternative to sampling with conventional trawling gear, but future tests are needed to confirm whether the SmeltCam is as reliable when applied to taxa other than Northern Anchovy over a greater range of conditions.

Diel vertical movements determine spatial interactions between cod, pelagic fish and krill on an Arctic shelf bank

Predator-prey spatial interactions are essential to understand ecosystem processes like predation rates and trophic interaction strength. In marine systems, such spatial interactions are highly dynamic and difficult to observe, as predators, prey and resources are mobile and responsive to each other, and also since shifting vertical light gradients strongly affect the space use of visual predators and their prey. We visited a bank area in the northern Barents Sea with cold bottom waters (~1°C), and combined conventional trawl and acoustic sampling with broadband hydroacoustics to obtain long-range, fine-scale observations of interactions between cod, their planktivore prey (capelin and polar cod) and krill. We caught cod in demersal trawl hauls but could not detect them with the vessel acoustics. However, broadband acoustics mounted on a submersible probe allowed us to track individual cod, revealing that they remained mostly within 10 m of the bottom throughout the diel cycle. In the morning, cod lifted slightly from the seabed indicating feeding activity, which corresponded with more fresh prey in cod stomachs in the morning. During daylight, krill pushed towards the bottom, sharing habitat with cod, while the planktivores aggregated in pelagic schools at the cost of lost feeding opportunities, overlapping with their krill prey only during twilight hours. The diel light cycle was an important driver of the spatial movements and aggregations, and krill appear to hide from capelin among the cod near bottom, while cod take advantage of descending pelagic fish after dawn to feed with a minimum of effort.

Refining a simple anterior fish excluder (SAFE) for penaeid trawls

Following recent international efforts to assess anterior modifications to trawls as a means for precluding unwanted catches, a promising device termed the ‘simple anterior fish excluder’ (SAFE) was modified for an Australian penaeid-trawl fishery. We tested a conventional penaeid trawl against those rigged with two SAFEs; each comprising a plastic banner located between the otter boards but orientated as either single- (original design) or double-concave configurations. Compared to the conventional trawl, both SAFEs caught significantly fewer absolute and standardised (ha–1 trawled) numbers of the most abundant non-target species, southern herring (Herklotsichthys castelnaui), although mean reductions were greater in the trawls with the double-concave (by 59 and 50%) than single-concave (44 and 36%) SAFE. Neither absolute nor standardised catches of the targeted school prawns (Metapenaeus macleayi) were affected. The results reiterate the generic utility of anterior modifications for reducing bycatch and maintaining function across different geometries. Configuring the SAFE as a double concave did partially improve its efficiency, but it also complicated fishing operations. Therefore, we conclude that owing to its simplicity, the single-concave design might be a preferable configuration for penaeid-trawl fisheries—pending assessments of other refinements.

Refining a Nordmøre-grid bycatch reduction device for the Spencer Gulf penaeid-trawl fishery.

Incremental refinements were made to a generic Nordmøre-grid to minimise bycatches of blue swimmer crabs Portunus armatus and giant cuttlefish Sepia apama, while maintaining catches of western king prawns Melicertus latisulcatus in the Spencer Gulf penaeid-trawl fishery. These refinements involved varying bar spaces, escape-exit areas and guiding-panel lengths, and were compared against a conventional trawl. Catches of teleosts and M. latisulcatus largely remained unaffected by the changes. Maximum reductions in P. armatus and S. apama bycatches (both ~90%) were achieved with a Nordmøre-grid comprising 38-mm bar spaces, 0.81- or 1.05-m2 escape exits and a 2.7-m guiding panel. Catching fewer P. armatus should reduce abrasion and crushing of M. latisulcatus in the codend and so increase the value of this targeted species. While noting some unresolved operational concerns, these results demonstrate the potential improvements in selectivity in this fishery using a Nordmøre-grid, primarily by mechanical separation.

T45 side panels improve penaeid-trawl selection

The utility of fitting panels of mesh turned 45° (‘T45’ or ‘square-shaped’ mesh) in a generic penaeid trawl body was investigated for improving selectivity. Compared to a conventional trawl comprising 43-mm diamond-shaped mesh throughout the body and a codend made from 27-mm T45 mesh, replacing just the side panels (18% of the total trawl area) with those made from 32- and 35-mm T45 mesh generally improved size-selection for school prawns, Metapenaeus macleayi—which was the dominant species—and especially in the trawl with 35-mm T45 mesh. This trawl also had a trend of improved size selection for the much less abundant eastern king prawns, Penaeus plebejus. Commercial catches of both penaeids were not negatively affected by the modifications. No teleosts were significantly affected by T45 side panels—although the predicted mean numbers of species with individuals smaller than the 32- and 35-mm T45 meshes were slightly lower. We conclude that retroactively inserting T45 side panels with an appropriate mesh size into penaeid-trawl bodies could be a simple and effective strategy for improving size selection; either as an adjunct to T45 codends, or as an alternative modification.

Design and test of a grid to reduce bycatch in the longfin inshore squid (Doryteuthis pealeii ) trawl fishery

A species separation grid was tested for a squid trawl to reduce finfish bycatch in the Nantucket Sound longfin inshore squid (Doryteuthis pealeii) fishery in southern New England, USA. The experimental trawl with a grid significantly reduces bycatch of summer flounder (Paralichthys dentatus) (76.4%, p < .001), black sea bass (Centropristis striata) (71.7%, p = .001), smooth dogfish (Mustelus canis) (86.0%, p < .001), and total bycatch (69.2%, p < .001) when compared to a conventional trawl. The catch rate of scup (Stenotomus chrysops) is 40.2% less than in the experimental trawl, but this difference is not statistically significant (p = .258). However, the experimental trawl also reduces targeted squid capture by 47.5% (p < .001), which is commercially unacceptable. Length analysis indicates no size effect on the retention for squid between the trawl with a grid (experimental) and the one without a grid (control), but the experimental trawl significantly reduces larger scup (>27 cm FL) and larger black sea bass (>37 cm TL), and all summer flounder size-classes. Therefore, this grid design may not be a suitable bycatch reduction device for the Nantucket Sound squid trawl fishery, and further work is needed to understand squid behavior within a trawl to develop a successful bycatch reduction strategy for the New England longfin inshore squid fishery.

EVALUACIÓN EXPERIMENTAL BIOECONÓMICA DE CAMBIOS EN LA TECNOLOGÍA DE CAPTURA DE CAMARÓN CON REDES DE ARRASTRE EN AGUAS SOMERAS DEL PACÍFICO COLOMBIANO

With active participation of the fishing industry, the biological and economic effects of technological changes in shrimp trawling fishing on the Colombian Pacific coast were evaluated to reduce bycatch and fuel consumption. Two boats were used simultaneously in experimental fishing surveys that compared the performance of a trawl net prototype and the conventional trawl net used by the fleet, quantifying in both cases the effects of the turtle excluder device (DET) and two fishing grounds (central-north and centralsouth). The performance fishing variables used to test the effect of changes in fishing technology on the target catch (CO) and the bycatch [incidental catch (CI) and discard (D)], were the relative fish or shellfish abundance, individual sizes, net revenues and fuel consumption by boat. Effects of fishing grounds, trawl net type, presence of device and the interaction between these factors, were evaluated using a three way ANOVA. The effect of new technologies on lengths of organisms from the catch components (CO, CI and D), was evaluated by comparison of the length frequency distributions using the Chi-square test and by modeling size selectivity of the trawl net prototype. Differences in fuel consumption between types of trawl nets of each boat were evaluated by t- test. Totals of CO, CI and D were 1436, 12163 and 15428 kg, respectively, with a global proportion between target catch and bycatch of 1:19. The bycatch was composed by 217 taxa, of which 32 families, mainly fishes, made up the CI and 52 were part of discard (fish, crustaceans and mollusks). The CO showed greater abundance on the central-north fishing ground than on the central-south one, whereas a contrary tendency showed the CI and D, in response to resources availability provided by oceanographic conditions and habitat. The prototype trawl net reduced in 34.8% the CO, 16.3% the CI and 28.7% the D (45% of bycatch), being significant the reductions of CO and D; whereas the DET reduced significantly the CI and D, without affecting the CO mainly in the conventional trawl net. Lengths frequency distributions of CO, CI and D differed between trawl net types, with a clear tendency to that the prototype trawl net selected greater lengths of shrimps and fishes. The use of the trawl net prototype implied a non significant reduction of the net revenues by haul (23.4%), whereas the use of the DET represented a significant reduction in net revenues of 77 %, mainly in the conventional trawl net. The prototype trawl net with prototype trawl doors reduced the fuel consumption in 36%, whereas the same net with conventional trawl doors reduced 23 %, which represents a saving in variable costs per fishing trip between $ 10`125 000 and $ 6`468 750 (Colombian pesos in 2005). The adoption of new fishing technologies by the industrial shrimp fleet from the Colombian Pacific has scientific evidence that it demonstrates the reduction of byctach without putting at risk the economic benefits derived from a selective fishing

Increasing lateral mesh openings in penaeid trawls to improve selection and reduce drag

In an attempt to improve selection and engineering efficiency in an Australian penaeid fishery, the mesh size and side taper of a generic trawl design were reduced (from 41 to 35mm) and increased (from 1N3B to 1N5B), respectively and, within the revised design, the independent and combined utility of (i) increasing the frame-line hanging ratio, and (ii) replacing the diamond-orientated wing/side panel with square-orientated mesh were investigated. Compared to the 41-mm mesh conventional trawl, all of four smaller-meshed designs (comprising either loose or tight hanging ratios, and with diamond- or square-mesh wing/side panels) had lower drag (9–12%) and required less fuel (up to 4–12%). Some of the small-meshed trawls caught fewer unwanted fish—attributed to the steeper side panels allowing individuals to swim forward and escape. Within the small-mesh trawls, there were incremental improvements in size selectivity for the targeted school prawns, Metapenaeus macleayi associated with both modifications. But the square-mesh wing/side panels were by far the most effective, reducing the catches of sub-commercial school prawns by up to 72%. The results support simple, retrospectively fitted alterations to existing penaeid-trawl bodies to improve selection and efficiency.

Measuring and assessing the physical impact of beam trawling

Abstract Beam trawling causes physical disruption of the seabed through contact of the gear components with the sediment and the resuspension of sediment into the water column in the turbulent wake of the gear. To be able to measure and quantify these impacts is important so that gears of reduced impact can be developed. Here we assess the physical impact of both a conventional 4 m tickler-chain beam trawl and a “Delmeco” electric pulse beam trawl. We measure the changes in seabed bathymetry following the passage of these gears using a Kongsberg EM2040 multi-beam echosounder and use a LISST 100X particle size analyser to measure the concentration and particle size distribution of the sediment mobilized into the water column. We also estimate the penetration of the gears into the seabed using numerical models for the mechanical interaction between gears and seabed. Our results indicate that the seabed bathymetry changes between ∼1 and 2 cm and that it is further increased by higher trawling frequencies. Furthermore, our results suggest that the alteration following the passage of the conventional trawl is greater than that following the pulse trawl passage. There was no difference in the quantity of sediment mobilized in the wake of these two gears; however, the numerical model introduced in this study predicted that the tickler-chain trawl penetrates the seabed more deeply than the pulse gear. Hence, greater alteration to the seabed bathymetry by the tickler-chain beam trawling is likely to be a result of its greater penetration. The complimentary insights of the different techniques highlight the advantage of investigating multiple effects such as sediment penetration and resuspension simultaneously and using both field trials and numerical modelling approaches.

The effect of mesh orientation on netting drag and its application to innovative prawn trawl design

Prawn fisheries around the world comprise fuel intensive enterprises currently stressed financially by rising diesel costs. An avenue for relieving the situation is to improve the energy efficiency of trawling by raising the productivity of fishing per litre of fuel consumed. This paper presents work to develop a new prawn trawl design that leads to reduced trawl system drag. The trawl has a ‘double-tongue’ format, which refers to extensions forward of the upper and lower panels to form two additional towing points for the trawl. For this design concept, named ‘W’ trawl, drag generated in the trawl is largely directed to the centreline tongues and transferred forward to the trawler through a connected sled and towing wire. The associated reduction of drag-transfer to the wings makes the trawl substantially easier to spread and results in smaller otter boards being required and subsequently reduced overall drag of the trawl system. The study determined the effect on frame-line tensions of implementing T0 (diamond) and T45 (square) mesh in the main body and side sections of trawl models of conventional and ‘W’ configuration, with the aim to establish an optimal combination of mesh orientation for the principle parts of the ‘W’ trawl. The objective was to achieve minimum netting drag and beneficial strain transfer within the trawl such that maximum trawling performance (catch per unit of fuel) might be obtained in the field. T45 mesh in the side sections of the trawl was found to exhibit a progressively lower drag compared to T0 mesh as the flow speed increased, but the extent of drag reduction was not of practical significance. The ‘W’ trawl showed a capacity of redirecting 59% of the total netting drag to the centre line tongues when T45 netting was implemented in the body section, and only 40% when T0 orientation was used. However, the introduction of bracing ropes (at E=0.71) along the upper and lower centrelines of the T0 version of the “W” trawl improved the drag transfer to the tongues from 40% to 50% of the total drag. Overall, the most practical and economic configuration of the model ‘W’ designs tested produced an estimated drag reduction of 8.3%±0.6%, compared to the conventional trawl. It is expected that drag saving benefits in practice will be more substantial as the tested trawl models were not completely representative of practical commercial gear in that they had minimum twine area to make the experiment most sensitive to the drag-effect of mesh orientation.

Benthic Debris Accumulation in Bathyal Grounds in the Antalya Bay, Eastern Mediterranean

Sunulan bu calisma Ocak-Mayis 2012 tarihleri arasinda Antalya Korfezinde (dogu Akdeniz) yurutulmustur. Geleneksel bir dip trolune sahip olan arastirma gemisi kullanilarak 200 ile 800 m arasindaki derinliklerde orneklemeler yapildi. Batial zonun bentiginde bulunan buyuk boyutlu deniz kirleticilerin materyal, bolluk ve dagilimlari incelendi. 32 trol cekimi suresince toplami 220 kg olan 920 parca kirletici toplandi. Copun yogunluk miktarlari tum alan icerisinde 18,5-2.186,0 kg/km ve 1152.762,0 parca/km arasinda degismektedir. Derinlik katmanlari ile coplerin herhangi bir gurubunun dagilimi arasinda anlamli bir fark bulunmamistir (P<0,05). Adet olarak elde edilen toplam copun plastik, metal, cam ve diger guruplara yuzdesel olarak dagilimlari sirasiyla, %81,1, %2,2, % 3,9 ve %12,8’dir. Adet olarak plastik en yogun malzeme gurubunu olusturmaktadir.

Selection of the mesh size of control codend in analysis of size selectivity of multi-codends beam trawls

In comparative fishing method,one of important experimental methods in research on size selectivity of trawls,the control codend mesh size is crucial and a possible upper limit is generally suggested to be half the mesh size of the test codend. However,the suggestion is unavailable when the body size of target specie of trawl is small enough,especially in shrimp or prawn trawl fisheries. In order to discuss how to choose a rational mesh size of control codend,experiments,in which test codends with different mesh sizes (20,30,35 and 40 mm) were rigged in a same multi-codends beam trawl and fished simultaneously,were carried out in Lvsi Fishing Ground. The catch frequency of Parapenaeposis hardwickii,Pseudosciaena polyactis and Collichthys lucida retained by codends were fitted to a selectivity analysis model in which the selectivity curves of different codends were regarded as geometrical similar under two contrary hypotheses (hypothesis 1:the 20 mm mesh size codend is regarded as test codend and selective for catch species and,hypotheses 2:as control codend and non-selective for species). The result demonstrated that the goodness of model fit and the selective parameters of different test codends were not affected significantly by the two contrary hypotheses. Taking the difference in relative fishing intensity and statistical method of analyzing size selection between multi-conends trawl and conventional trawls into account,authors suggest that the assumption of non-selectivity of 20 mm mesh size control codend is acceptable in analysis of size selectivity in the experiemts,which means that the general upper limit of control codend mesh size could be broken.

Can we increase haddock yield within the constraints of the Magnuson–Stevens Act?

Georges Bank haddock is a recently recovered fish stock in the New England groundfish fishery. Due to federal constraints under the Magnuson–Steven Act, however, this stock cannot be optimally exploited due to the bycatch of other critical species in the New England groundfishery such as cod and yellowtail flounder which are overfished. The Ruhle trawl and Separator trawl are examples of recent advances in gear technology that have been shown to significantly increase haddock to bycatch ratios. This study models the groundfish fishery through a mixed-stock yield model which incorporates technological interactions. We also develop a socio-economic model that quantifies the amount of employment and producer surplus associated with three trawl types. Our results explore policy situations regarding the use of the new trawls. By bridging the biological and socio-economic models, we are able to view the fishery as a system that more accurately represents stakeholder views. Our model shows that each trawl, when used exclusively, produces different optimum strategies and therefore an optimum management strategy would most likely include a combination of trawl types. Our results also support the logic of using modified trawls for haddock fishing trips in which bycatch is strictly regulated (“B days”) as the Ruhle trawl is able to maintain 80% of catches caught by a conventional trawl while reducing bycatch up to over 60%. This paper is a first step towards an aid for policy makers to examine fishery gear trade-offs and the resulting biological and socio-economic consequences of different management actions within the constraints of the Magnuson–Stevens Act.