Nephrops Trawl Research Articles

Survivability of discarded Norway lobster in the bottom trawl fishery of the Bay of Biscay

Abstract In the context of the landing obligation set by the new Common Fisheries Policy (CFP), Norway lobster Nephrops norvegicus was identified as a species likely to have high survival rate when discarded in the bottom trawl fishery of the Bay of Biscay. Previous studies in this area reported a survival rate between 30% and 51%, but the experiments were done on a limited monitoring period and the seasonal variations were not investigated. This study was designed to obtain a reliable value for survival rate after a 14-day monitoring period in onshore tanks allowing considering delayed mortality. The study also tested the effect on the survival rate of using a discarding chute system, a sorting device that was made mandatory on the 1st of January 2017 for Nephrops trawlers in the Bay of Biscay. This device, which enables fishermen to discard undersized Nephrops back to the sea while sorting, led to an increased average survival rate (51.2%) compared with the standard sorting practice (36.9%). The impact of biological, environmental and fishing operation related variables on survival from the first day of captivity to the end of the monitoring period was examined using a generalized linear model. The results of the GLM indicate that injuries, season and duration of the air exposure, significantly influence the survival from the 1st day of captivity to the end of the monitoring period. The survival rate was higher for non-injured Nephrops as well as for Nephrops that have undergone short air exposure, in summer and autumn.

Efficacy of selective devices in reducing discards in the Nephrops trawl fishery in the Bay of Biscay

Abstract The Nephrops fishery in the Bay of Biscay is an important commercial fishery which generates large amounts of discards owing to the use of small mesh trawls. To reduce discards, French trawlers were equipped with a variety of selective devices, from 2005 onwards. This study examines their efficacy using data from the French on-board observer programme, 2003–2010. Generalized linear models were built for catches, discards, and landings of Nephrops and hake, controlling for the other factors which drive the variability in these variables. A dorsal square-mesh panel meant to let small hake escape did not affect hake catch, but was found to decrease Nephrops catches and discards. Among the devices intended to reduce Nephrops discards, the flexible grid was the most efficient, as it decreased catches and discards in large proportions while increasing landings but this result was supported by a small number of observations; a larger mesh size in the codend (80 mm instead of 70) slightly decreased Nephrops discards; and a ventral square-mesh panel was not found to affect catch or discards of either species. The design of the on-board observer programme was meant to estimate discard amounts, which limited their utilization to investigate factors for discarding.

Investigating the recent decline in gadoid stocks in the west of Scotland shelf ecosystem using a foodweb model

Abstract Abundance and biomass of cod, haddock, and whiting in the waters off of the west coast of Scotland (wcoS) have undergone large changes in recent years, most notably a recent decline. These three species contribute a considerable part of Scottish demersal landings from this area and as such it is important to understand why these stocks are behaving the way they are. A number of explanations for the decline have been proposed, including: seal predation, pressure from Nephrops trawls, and fishing pressure more generally. We used an ecosystem model of the wcoS continental shelf (<200 m depth) to investigate whether these proposed explanations for declining gadoid stocks are feasible. Results suggest that the rise in the grey seal population over recent years has not led to the decline in gadoid stocks; there is insufficient bycatch by the Nephrops fleet to have a large impact on gadoid stocks; however, fishing, as a key driver of the west of Scotland shelf ecosystem, has impacted stocks and by decreasing fishing levels to maximum sustainable yield cod biomass may increase slightly though not returning to previous levels. Although this means we are little further forward in understanding the cause of recent gadoid declines in the area, the development of this model has enabled us to further our knowledge and understanding of aspects of trophic structure and the impacts of fishing on the wcoS.

Integrated environmental assessment of fisheries management: Swedish Nephrops trawl fisheries evaluated using a life cycle approach

Fisheries management needs to broaden its perspective to achieve sustainable resource use. Life cycle assessment (LCA) is an ISO standardized method to evaluate the environmental impacts of products using a broad and systematic approach. In this study, the outcome of a management regime promoting species-selective trawling in Swedish Nephrops trawl fisheries was studied using LCA methodology by quantifying the impacts per kilogram of landing using two different fishing methods. Demersal trawling has previously been found to be both energy intensive and destructive in terms of seafloor impact and discards. It is demonstrated that species-selective trawling fulfils management objectives, although with tradeoffs in terms of fuel consumption and associated GHG emissions. To prioritize between impacts, one must be aware of and quantify these potential tradeoffs. LCA could be an important tool for defining sustainable seafood production as it can visualize a broad range of impacts and facilitate integrated, transparent decision making in the seafood industry. It is also concluded that, with current LCA methodology, use of total discarded mass could increasingly be distinguished from potential impact by applying two new concepts: primary production requirements and threatened species affected.

The Impact of Subsidies on the Ecological Sustainability and Future Profits from North Sea Fisheries

BackgroundThis study examines the impact of subsidies on the profitability and ecological stability of the North Sea fisheries over the past 20 years. It shows the negative impact that subsidies can have on both the biomass of important fish species and the possible profit from fisheries. The study includes subsidies in an ecosystem model of the North Sea and examines the possible effects of eliminating fishery subsidies.Methodology/Principal FindingsHindcast analysis between 1991 and 2003 indicates that subsidies reduced the profitability of the fishery even though gross revenue might have been high for specific fisheries sectors. Simulations seeking to maximise the total revenue between 2004 and 2010 suggest that this can be achieved by increasing the effort of Nephrops trawlers, beam trawlers, and the pelagic trawl-and-seine fleet, while reducing the effort of demersal trawlers. Simulations show that ecological stability can be realised by reducing the effort of the beam trawlers, Nephrops trawlers, pelagic- and demersal trawl-and-seine fleets. This analysis also shows that when subsidies are included, effort will always be higher for all fleets, because it effectively reduces the cost of fishing.Conclusions/SignificanceThe study found that while removing subsidies might reduce the total catch and revenue, it increases the overall profitability of the fishery and the total biomass of commercially important species. For example, cod, haddock, herring and plaice biomass increased over the simulation when optimising for profit, and when optimising for ecological stability, the biomass for cod, plaice and sole also increased. When subsidies are eliminated, the study shows that rather than forcing those involved in the fishery into the red, fisheries become more profitable, despite a decrease in total revenue due to a loss of subsidies from the government.

The effect of forced mesh opening in the upper panel of a Nephrops trawl on size selection of Nephrops, haddock and whiting

In an attempt to reduce catches of juvenile fish in the Icelandic Nephrops norvegicus fisheries, the mesh sizes in the upper part of a commercial Nephrops trawl were increased and the netting hung to force wide mesh opening. Two different designs, each with 135 mm nominal mesh upper panel were compared to an unmodified commercial trawl in June 2009. Design A had a 23.2 m long panel and design B had a 16.1 m long panel, extending respectively to 9.7 and 15.0 m from the cod-line. The panels in both designs were one third narrower and longer than the matching lower panel. With design A, about half of Nephrops, haddock ( Melanogrammus aeglefinus) and whiting ( Merlangius merlangus) catches were lost, while with design B, the losses were restricted to smaller specimen of fish and Nephrops, i.e. a marked size selection was observed. These trials demonstrate the potential for size selection by a simple manipulation of a Nephrops trawl to reduce discarding.

The selectivity of the Swedish grid and 120 mm square mesh panels in the Scottish Nephrops trawl fishery

Two sets of trials were conducted, on the west and east coasts of Scotland, to estimate the selectivity of an 80 mm diamond mesh codend fitted with (i) a 35 mm Swedish grid, (ii) a 120 mm square mesh panel (SMP) at 6–9 m from the codline and (iii) a 120 mm SMP at 12–15 m from the codline. The main results can be summarised as follows: • the Swedish grid gear retained all Nephrops < 40 mm carapace length and between 75 and 90% of Nephrops ≥ 41 mm; • the SMP gears retained between 70 and 88% of Nephrops below about 37 mm and all Nephrops ≥ 40 mm; • the grid gear retained fewer whitefish than the SMP gears. No cod > 34 cm, haddock > 35 cm, whiting > 38 cm and hake > 41 cm were retained by the grid gear; • and the position of the SMP had no significant effect on the retention of Nephrops, cod, haddock, or whiting.

Smooth age length keys: Observations and implications for data collection on North Sea haddock

Age at length keys (ALKs), which give the probability of age given length, are a fundamental component of many age-based fish stock assessment methods. Usually, ALKs are compiled from readings of otoliths or scales taken from length-stratified sub-samples of fishery landings or research vessel trawls catches. The assessment process is data intensive when there are numerous fleets assessment sub-regions to be sampled over an annual cycle, making the collection and analysis of material costly and time consuming. Hence, the data are almost always sparse, often with voids that require in-filling before use in assessments. Though robust statistical procedures for automatically in-filling data voids have been developed, they have not been widely adopted, and procedures often remain manual. Here we use Generalized Linear Models to derive the probability of age given length from sparse data gathered during the International Bottom Trawl Survey and Scottish commercial sampling program. The keys are used to test statistically for differences between ALKs from different sampling regions, differing gear geometries, different sampling programs and fish at different life stages. The results of the comparisons suggest that ALKs from the commercial sampling program are not, in general, comparable to those generated by the IBTS program. We also found that ALKs from Nephrops trawls are significantly different from those generated by other trawls over the same sampling region. The tests also suggest that age at length distributions differ not only between but also within IBTS roundfish areas, in part due to differences between ALKs for mature and immature fish. These differences are an important factor when considering a reduction in the resolution of sampling areas and when combining data from countries with differing fleet compositions. They also raise important questions about present protocol for collection of otolith data from IBTS survey trawls.

Interactions between fishing strategies of Nephrops trawlers in the Bay of Biscay and Norway lobster diel activity patterns

Abstract Norway lobster, Nephrops norvegicus (L.), in the Bay of Biscay exhibited diel activity patterns with more individuals outside their burrows at dawn and dusk, increasing catchability at these times. Data from an on board observer programme on Nephrops trawlers between 2002 and 2005 were used to assess variability in catchability in commercial catches. Catch numbers per haul varied spatially and between months, but no signal for diel variations was found. Fishing strategies developed by the Nephrops trawlers had several components. On a seasonal level, they started around sunrise. On a haul level, haul duration decreased from haul to haul, with the longest hauls taking place at the time of the highest catchability. By‐catch of hake, Merluccius merluccius (L.), increased more than proportionally with haul duration.

A cost–benefit analysis of improving trawl selectivity in the case of discards: The Nephrops norvegicus fishery in the Bay of Biscay

The Nephrops trawl fishery is one of the most important fisheries in the Bay of Biscay. The fishery management essentially relies on conservation measures, a total allowed catch (TAC) for Nephrops together with a minimum landing size (MLS) and minimum trawl mesh size (70 mm stretched mesh). These measures have failed to prevent high discard levels of many species that characterize the fishery. Nephrops trawlers thus discard about half of their Nephrops catches in numbers (a third in weight) of which only 30% survive. Nephrops discards mainly occur in younger Nephrops age groups below the MLS. This is a waste for the already overexploited Nephrops stock as well as for the fleet. Based on a bio-economic simulation model, the paper analyses the consequences of several scenarios of improving Nephrops selectivity. The potential impacts of these scenarios on Nephrops biomass, landings, discards and economic indicators (e.g. rent) are examined and a cost–benefit analysis of each scenario is carried out. We show that a better exploitation pattern would benefit fisheries that have high level of discards. Reducing non-commercial Nephrops discards leads to positive net present values of rent with better value realized from the production potential and limited short-term losses for the fishing units. By taking account of the economic dynamics of increasing effort however, we show that selectivity measures are insufficient. Regulating access to the fishery is also required to ensure the sustainability of the fishery and a better exploitation of the production potential.

Species-selective Nephrops trawling: Swedish grid experiments

This paper presents a summary of all catch-comparison gear tests performed between 2002 and 2006 with Nordmøre-type sorting grids in the Swedish Nephrops trawl fishery in the Skagerrak and Kattegat. The overall aim was to minimise fish by-catch while maintaining catch rates for the target species, i.e. Nephrops. The results showed that a trawl fitted with a grid and a square-mesh cod-end, caught less small and marketable cod, haddock, whiting, hake and plaice than did a standard commercial trawl. For cod larger than 35 cm, the catch reduction was close to 100%. For marketable Nephrops no difference in catch amounts was detected. Results of three modified grid variants are also reported here. None of these showed convincing results in terms of better species-selectivity or superior handling characteristics than the standard grid. The results on fish by-catch reduction indicate that the Swedish grid may be a useful tool to reduce non-target mortality and to decouple Nephrops from cod and other demersal fish species under existing and future recovery plans.

Occurrence of pelagic stingray Pteroplatytrygon violacea (Bonaparte, 1832) in the North Sea

A mature male pelagic stingray Pteroplatytrygon violacea (99 cm total length, 33·5 cm disc length and 42 cm disc width; 2·5 kg) was caught by commercial Nephrops trawl in the North Sea (at c. 54º49′ N; 0º57′ W) on 14 November 2005. Records of P. violacea in the north‐east Atlantic are reviewed, with only two vagrant specimens known from the North Sea, representing the most northerly records of this species in the Atlantic Ocean.

The potential for new Nephrops trawl designs to positively effect North Sea stocks of cod, haddock and whiting

Within commercial fisheries, particularly mixed fisheries, both target and non-target species are often discarded. Discarding represents a potentially significant loss to the productivity of fish stocks; it can have damaging ecological consequences, and is a potential cause of the failure of recovery plans. The Nephrops fishery in the North Sea is classified as a mixed fishery. Nephrops trawls are constructed with smaller meshes than trawls used to target whitefish; consequently, the bycatch of juvenile fish can be substantial. Several new Nephrops trawl designs have been tested in the North Sea. The data from these trials are used to investigate the potential impact of their implementation on cod, haddock and whiting stocks in the North Sea (including the Kattegat and Skagerrak). The model examines five trawl designs, and also the scenarios of a cessation of discarding in all North Sea fisheries and in just the Nephrops fishery. The model is deterministic, and evaluates the relative differences between scenarios assuming all other variables remain constant. If discarding of cod, haddock and whiting in the North Sea fisheries were eliminated, stocks would increase by 41%, 14% and 29%, respectively, within 10 years. Eliminating discarding in the Nephrops fishery alone would increase stocks by 2%, 1% and 13%, respectively, reflecting the relative proportion of catches of these species in the Nephrops fishery. For cod and haddock, the introduction of the Nephrops trawl with a grid with a square-mesh codend was the only scenario in which a notable increase in stock number was observed. This trawl design facilitates the escape of fish of all ages/sizes from the trawl, effectively making the Nephrops fishery a single-species fishery. For whiting, stock numbers and landings increased under all scenarios, but forecasted landings were lower than if current discard patterns continued in all except the no-discards scenario. The dependency of the results on the validity of the assumptions and on the accuracy of the input data is discussed.

Gear technology in Nephrops trawl fisheries

Nets with a small mesh size are required to catch Nephrops norvegicus, consequently large quantities of small whitefish are also caught, and much of this bycatch is undersized and is discarded dead. The main bycatch species are whiting (Merlangius merlangus), haddock (Melanogrammus aeglefinus) and cod (Gadus morhua). Here we summarize the known behavioural reactions of these species towards conventional trawls and review the results of using different trawl modifications to increase selectivity of Nephrops trawls. The trawl modifications are categorised as separator grids, separator and guiding panels, square-mesh panels, capture avoidance designs and codend modification. Finally, the extent to which these developments have been legislated for is discussed including the conditions under which new gear regulations have been introduced.

Separation of fish from Nephrops norvegicus into a two-tier cod-end using a selection grid

The legally sized fish by-catch is an important economic component of many Nephrops fisheries. However, the small mesh size typically used by Nephrops trawlers results in many undersized fish being discarded. A selection grid was inserted in a two-tier cod-end in an attempt to separate the target species, Nephrops, from the fish by-catch and to retain the by-catch in the upper cod-end. Good separation would allow fish size selection to be improved by using a larger mesh size in the upper cod-end. The separation properties of grids with two different bar spacings and two lower gaps were investigated. Nephrops, haddock and dab were caught in reasonable numbers. Most small Nephrops passed into the lower cod-end, with the proportion passing into the upper cod-end increasing to between ∼40 and ∼70% at carapace lengths of 50 mm. The grids with the larger bar spacing allowed more Nephrops into the lower cod-end, but there was no significant difference in separation between grids for haddock or dab. The proportion of haddock passing into the upper cod-end increased from ∼60% for 10 cm haddock to ∼85% for haddock of 20 cm or more. The proportion of dab passing into the upper cod-end increased from ∼30% for 10 cm dab to ∼75% for dab of 20 cm or more.

Selective properties of the cutaway trawl and several other commercial trawls used in the Farne Deeps North Sea Nephrops fishery

Discarding of roundfish species in many European Nephrops fisheries remains at relatively high levels, despite the existence of mandatory gear-based technical measures specifically aimed at mitigating this phenomenon. This work reports on a new design of Nephrops trawl (the cutaway trawl), tested in the Farne Deeps fishery of the North Sea, and which reduced by-catches of whiting by 50% (across the length range), without any loss of Nephrops. The cutaway trawl could therefore potentially be used to reduce discarding of this species in certain fisheries. Several comparisons are made between the cutaway trawl and three designs of commercial trawl in existing use in Nephrops fisheries. Comparisons are made from both a biological and economic standpoint. Vessel size (length and engine power) did not affect the selectivity parameters obtained with the cutaway trawl, when the sea trials were repeated on vessels ranging from 9.9 m LOA (150 hp) to 21 m LOA (650 hp).

Seals and fisheries in the Clyde Sea area (Scotland): traditional knowledge informs science

Results obtained from circulating a questionnaire on seal/fisheries interaction around two groups of stakeholders (trawlermen and creel fishers) from the Scottish Clyde fishing fleet are presented. An overall return rate of 30% was achieved representing 664 man-years of traditional knowledge. Nowadays, most commercial fishing in the Clyde Sea area targets Norway lobsters ( Nephrops norvegicus), using either towed (nephrops trawl) or static (baited creel) gear. Nephrops trawling generates a whitefish by-catch. All those respondents fishing within the confines of the Clyde Sea area reported experiencing seals interfering with their fishing activities. Both grey seals ( Halichoerus grypus) and common, or harbour, seals ( Phoca vitulina) occur locally but most respondents did not distinguish between them. The majority of trawlermen reported seals taking fishes from nets and damaging fishes. Only 9% of trawlermen, however, noted gear damage in spite of 91% reporting capturing seals (occasionally or rarely) in their gear. Seals caught in trawls were mostly retrieved dead (exact incidence unknown). Sixty-three percent of creelers who responded reported both damage to fishes and interference with creels. Seals may break into, or smash, creels to steal the bait and in so doing liberate catches. Contact with seals was not thought to taint fishing gear. Seals seemed to target cod ( Gadus morhua) and hake ( Merluccius merluccius) in trawls and seemed to select the larger individuals. Creel fishers highlighted damage to the bellies of fishes whilst trawlermen stressed damage to the tails. As much as 10% of the whitefish caught by trawlermen was unsaleable due to seal damage. Damage caused by seals was readily distinguishable from damage caused by other agents. ‘Rogue’ individual seals were not solely to blame. Typically, 9–12 seals (even sometimes as many as 25) might be encountered by any one fisher, especially when gear was being hauled. Half the trawlermen who participated regarded seals as being either a ‘considerable’ or ‘moderate’ threat to their livelihoods but 45% regarded them as a ‘minor’ or ‘non’-problem. Grey seal populations have undoubtedly increased considerably locally over the last 40 years. Eighty-six percent of trawlermen were in favour of some sort of seal cull (cf. 14% against), with some 45% of trawlermen in favour of halving seal numbers locally. Over 60% of creelers who responded were looking for even more stringent cut-backs in seal numbers, i.e. of more than 70%.

Discard composition of the Nephrops fishery in the Clyde Sea area, Scotland

Demersal fishing gear such as otter-trawls generate large amounts of unwanted by-catch. The Norway lobster ( Nephrops norvegicus) is the most important shellfish resource in UK waters and although the fishing effort has increased considerably over past decades the ecosystem effects of this fishery have yet to be evaluated. This study provides the first assessment of the catch and discard composition from Nephrops trawls in the Clyde Sea area with particular emphasis on invertebrate discards. Nephrops constituted only between 14 and 23% of the total catch (volume); other invertebrates and fish accounted for the remainder of the catch. On an average, 9 kg of discards were produced per kilogram of Nephrops. The catch composition differed markedly between samples from the north and south Clyde Sea areas. Trawls from the south yielded a significantly higher biomass of Nephrops (30% cf. 4% in the north) and fish discards (55% cf. 36% in the north) whereas catches from the north contained more invertebrates (60% cf. 15% in the south). Crustaceans and echinoderms accounted for up to 83 and 73% of the discards, respectively. Samples from the north also contained a greater variety of invertebrate species (93 taxa cf. 51 taxa in the south). The differences between the two study areas are likely to be a reflection of differing bathymetries, hydrographic conditions and ground types in each area.

An analysis of discards from the French trawler fleet in the Celtic Sea

Discards of the French trawler fleet operating in the Celtic Sea in 1997 were studied. Twenty-six trips, representing 0.8% of the total fishing effort, were sampled. This fleet consists of three métiers, benthic trawlers, Gadoid trawlers and Nephrops trawlers. The fleet discarded an estimated 30 000 tons of animals in 1997, while landing 63 000 tons. The total quantity discarded did not differ among métiers, but the species composition of discards did. Benthic trawlers discarded mainly by-catch species, whereas Gadoid and Nephrops trawlers discarded primarily their target species. Whiting, megrim, Nephrops and hake were discarded in larger numbers than landed. Hence discards should be taken into account in catch-based assessments. However, discards were found to be highly variable between trips and between years. In addition, no reliable auxiliary variable could be found when various factors were investigated to explain the quantities of commercial species discarded. The only explanatory factor valid for any species was the smallest size of that species in the catch. Hence there is a need for regular sampling of discards. Copyright 2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights reserved .

Utilisation of invertebrates discarded from the Nephrops fishery by variously selective benthic scavengers in the west of Scotland

Demersal trawl fisheries generate large quantities of discards which temporarily increase the amount of carrion available to benthic communities and lead to a faster energetic turn- over. This study examines the availability of discarded material to the benthos, assesses consumption times of different items and identifies scavengers attracted to those invertebrates most frequently dis- carded from Clyde Sea Nephrops trawlers. In field and laboratory trials, heavy-shelled dead whelks (Buccinum undatum, Neptunea antiqua) sank faster than softer-bodied species like cephalopods (Allotheuthis subulata, Rossia macrosoma) or echinoderms (Ophiura ophiura, Asterias rubens), mak- ing most discards available to the benthos (at ca. -60 m CD (chart datum)) within minutes after dis- carding. SCUBA and time-lapse camera observations in the Clyde Sea and Loch Sween indicated bait utilisation times between 24 and 48 h. Fast-moving animals like brachyuran crabs were the first to arrive at discard bait piles whose composition mimicked typical discards from the Clyde Sea Nephrops fishery. Bimonthly deployments of traps baited with invertebrate discards in the north of the Clyde Sea showed that A. rubens, followed by Pagurus bernhardus, Liocarcinus depurator and whelks, were the most abundant megafaunal scavengers. Fine-meshed funnel traps deployed inside those creels yielded up to 2819 amphipods per trap, with Scopelocheirus hopei and Orchomene nanus accounting for most of the catch. Together with whelks, A. rubens and Carcinus maenas, O. nanus showed a clear preference for crustacean bait. By contrast, Pagurus bernhardus was more attracted to A. rubens and, in 1 trial, to O. ophiura bait. Traps deployed in the south of the Clyde Sea yielded generally lower numbers and species diversity in the catch, with Nephrops being the most abundant megafaunal scavenger. It showed a preference for L. depurator and conspecific bait. While the results show that a range of epibenthic species readily utilise invertebrates discarded from Clyde Sea Nephrops trawlers, it is unknown to what extent discards subsidise benthic communities as infor- mation on the ecological energetics of the species involved locally is currently lacking.