Predicting optimal combinations of by‐catch reduction devices in trawl gears: A meta‐analytical approach

Global efforts to reduce unwanted catches have led to the development of a vast array of by‐catch reduction devices (BRDs), in particular for mixed trawl fisheries. Some of these BRDs could likely benefit from being combined. However, the number of possible combinations would be prohibitive to be tested experimentally. Therefore, in this study we propose a meta‐analytical approach that combines the data available on BRDs tested independently in a fishery and predict the theoretical selectivity of all possible combinations of those devices. This allows to identify promising BRD combinations, worth experimental investigation and flexible trawl configurations, where the selectivity can be substantially modified by adding or removing one BRD, thus aiding fishermen in adapting to high variability in catch composition and quota availability. To illustrate the approach, we used BRDs developed for the well‐studied Nephrops (Nephrops norvegicus, Nephropidae) directed mixed trawl fishery in the Skagerrak and Kattegat seas. We predicted the selectivity of 100 BRD combinations for Nephrops, cod (Gadus morhua, Gadidae) and haddock (Melanogrammus aeglefinus, Gadidae), compared them in terms of absolute selectivity and performance under realistic catch scenarios, from both single‐ and multispecies perspectives, and identified 15 BRD combinations that could be worth future experimental investigation. The meta‐analytical approach makes best use of existing knowledge and leads to new insights about the potential for improvement and flexibility in trawl selectivity. This could benefit a variety of mixed trawl fisheries and help developing a new generation of more flexible gears, with multiple BRDs integrated in their structure.     

Selective haddock (Melanogrammus aeglefinus) trawling: Avoiding cod (Gadus morhua) bycatch

The critical condition of the North Sea cod stocks has resulted in restrictions on not only cod, but also haddock and other species that are caught together with cod. Thus full exploitation of the haddock stock is unachievable unless cod can be excluded from the haddock catch. We designed a selective trawl based on the behavioral differences between haddock and cod as they enter a trawl, i.e., cod stay close to the seabed whereas haddock rise above it. The trawl's fishing line is raised ～60 cm above the seabed to allow cod to escape beneath the trawl while haddock are retained. To collect the escapees, three sampling bags were attached beneath the raised fishing line. The selective haddock trawl reduced the total catch of cod by 55% during the day and 82% at night, and 99% of the marketable haddock was caught during the day and 89% at night. Cod escape rates were highly length dependent: smaller cod escaped the trawl in greater numbers than did larger individuals. Whiting, saithe, lemon sole, and plaice were included in the analysis.     

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.     

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.     

The growth of larval cod and haddock in the Irish Sea: a model with temperature,prey size and turbulence forcing

We applied a physiological individual‐based model for the foraging and growth of cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) larvae, using observed temperature and prey fields data from the Irish Sea, collected during the 2006 spawning season. We used the model to estimate larval growth and survival and explore the different productivities of the cod and haddock stocks encountered in the Irish Sea. The larvae of both species showed similar responses to changes in environmental conditions (temperature, wind, prey availability, daylight hours) and better survival was predicted in the western Irish Sea, covering the spawning ground for haddock and about half of that for cod. Larval growth was predicted to be mostly prey‐limited, but exploration of stock recruitment data suggests that other factors are important to ensure successful recruitment. We suggest that the presence of a cyclonic gyre in the western Irish Sea, influencing the retention and/or dispersal of larvae from their spawning grounds, and the increasing abundance of clupeids adding predatory pressure on the eggs and larvae; both may play a key role. These two processes deserve more attention if we want to understand the mechanisms behind the recruitment of cod and haddock in the Irish Sea. For the ecosystem‐based management approach, there is a need to achieve a greater understanding of the interactions between species on the scale a fish stock is managed, and to work toward integrated fisheries management in particular when considering the effects of advection from spawning grounds and prey–predator reversal on the recovery of depleted stocks.     

Co‐viability of a Mediterranean demersal fishery affected by the discard ban under alternative management scenarios

The joint viability, or co‐viability, of a Mediterranean Sea mixed demersal fishery was examined by applying a bioeconomic fisheries model to the main seven target stocks of the fishery under biological, social and economic constraints. The stocks of interest were hake, black‐bellied angler, deep‐water rose shrimp, red mullet, blue and red shrimp, Norway lobster and blue whiting, all of which were assessed as overexploited. Their combined production corresponds to 34% by volume and 52% by value of the landings of the demersal fishery studied. Simulation scenarios based on effort reduction, changes in the selectivity pattern and implementing a fisheries restricted area were compared over the simulation horizon of 2015–2030. The latter two scenarios were also examined in terms of full or partial compliance. The effect of a discard ban on the economy of the fleet was also analysed. The results show that the fishery is not co‐viable because some stocks are not viable in the projection period, but social and economic viability was ensured in all cases. As is often the case for mixed fisheries, a unique management measure is unlikely to improve the situation of all stocks simultaneously, but all the management measures tested are better than the status quo.     

Generic harvest control rules for European fisheries

In European fisheries, most stocks are overfished and many are below safe biological limits, resulting in a call from the European Commission for new long‐term fisheries management plans. Here, we propose a set of intuitive harvest control rules that are economically sound, compliant with international fishery agreements, based on relevant international experiences, supportive of ecosystem‐based fisheries management and compatible with the biology of the fish stocks. The rules are based on the concept of maximum sustainable yield (MSY), with a precautionary target biomass that is 30% larger than that which produces MSY and with annual catches of 91%MSY. Allowable catches decline steeply when stocks fall below MSY levels and are set to zero when stocks fall below half of MSY levels. We show that the proposed rules could have prevented the collapse of the North Sea herring in the 1970s and that they can deal with strong cyclic variations in recruitment such as known for blue whiting. Compared to the current system, these rules would lead to higher long‐term catches from larger stocks at lower cost and with less adverse environmental impact.     

Net selectivity studies in the Northern Ireland Nephrops fishery

The effect on catch composition of a large mesh lifter net fitted to a trawl cod-end was investigated in the Northern Ireland Nephrops fishery in the north west Irish Sea (ICES area Vila). Two experiments were conducted using chartered commercial fishing vessels. In the first experiment, parallel hauls showed there to be no significant difference in the Ncphrops size distribution caught by nets with and without a lifter net. The result of the second experiment, in which the cod-end was fitted with a small mesh cover to retain escaping Nephrops, did not suggest that the cod-end meshes were blinded by the lifter. Field data are contrasted with those from theoretical studies of mesh obstruction.     

The fishery for Nephrops norvegicus (Linnaeus, 1758) in the central Adriatic Sea (Italy): Preliminary observations comparing bottom trawl and baited creels

The Norway lobster, Nephrops norvegicus, is mainly targeted by bottom trawlers whose selectivity (mesh size = 40 mm stretch) with respect to this species is poor producing large quantities of discards both in terms of demersal fish species and undersized Nephrops. The Pomo pit (central Adriatic Sea) is a very important nursery area for European hake, Merluccius merluccius, but is also the main trawling ground for the Adriatic fleet exploiting Nephrops. The use of baited creels may constitute an alternative to bottom trawling. This paper summarises experimental data collected from the western Pomo pit in order to compare bottom trawl and baited creel catches with the aim of evaluating the ecological consequences of creeling. Because of high scavenger activity in the area, the effect of this on the creel fishery was also investigated.A large fraction of the trawl catch was composed of juvenile undersized individuals of commercial species, the same was not true for creels. Composition of Nephrops catches differed markedly between creels and the bottom trawl. Scavenger activity was very high. The results indicate that ecologically the use of baited creels appears to be a valid alternative to bottom trawling but, unfortunately, may not comprise an economically viable solution in the investigated area, which is 30 nautical miles offshore.     

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.     

Proximity of Pacific cod to the sea floor: Using archival tags to estimate fish availability to research bottom trawls

The percentage of Pacific cod available to Alaskan bottom trawl surveys was estimated from the proximity of tagged cod to the seafloor. Archival tags recorded time and depth data at 15- or 30-min intervals. The distance of a cod above the bottom was computed by subtracting tag depth from bottom depth, estimated as the maximum depth recorded during each 24-h day. These estimates of bottom depth are based on the assumption that cod approach the seafloor at least once a day, and do not undergo migrations up or down bottom gradients. To ensure that migrations over variable bottom gradients did not confound our estimates of bottom depth, we narrowed our analysis to 11 tagged cod that were recaptured in areas of flat bathymetry, and only analyzed data recorded within one month prior to recapture (N = 29,462 depth recordings). Pacific cod maintained short distances above the seafloor that often differed between day and night. Over 95% of the daytime tag recordings occurred within 10 m of the seafloor. Average effective headrope heights for survey bottom trawls currently used in the eastern Bering Sea and Alaska Gulf/Aleutian Islands groundfish surveys are approximately 2.5 and 7 m, respectively. In the absence of any behavior responses to an approaching trawl, we would expect 47.3% of the cod within the water column are available to the trawl used on the eastern Bering Sea survey and 91.6% are available to the trawl used on the Gulf/Aleutian Islands surveys. This study demonstrated that Pacific cod are highly demersal, and current values of trawl survey catchability (Q) used in current stock assessments are consistent with estimates of cod availability to the trawl gear.     

Definition, dynamics and stability of métiers in the Irish otter trawl fleet

The Irish otter trawl fleet operates in a complex multi-species multi-gear fishery, spanning a wide geographic area, and involving around 275 trawlers. Factorial and clustering methods were applied to 2003 fishing trip data to define thirty-three métiers. Definitions were based on six trip characteristics taken from logbooks, namely: fishing gear, mesh size, vessel length, species composition, area, and month. Métiers exploiting demersal species or species groups are characterised by single vessel bottom otter trawls, typically with mesh sizes of 70 mm or more, operating year round. This includes nine Nephrops dominated métiers highlighting the importance of this species to the fleet. Many demersal métiers are characterised by groups of species, such as mixed whitefish or slope species. Métiers exploiting pelagic species are often focussed on single species, and are typically seasonal, mid-water trawling (often paired) with mesh sizes less than 70 mm. Pelagic métiers account for the majority of landings by over an order of magnitude in several cases. Demersal métiers account for the majority of fishing trips and effort, (primarily Nephrops métiers), and vessels (primarily mixed species métiers). The new métier definitions were found to be appropriate remained relevant despite declining fleet landings and effort between 2003 and 2006. Species compositions within these métiers have generally remained similar to the proportions defined in 2003. These robust métier definitions present opportunities to improve fisheries sampling, assessment and management. Although métiers pose complexity challenge for such applications they can be used the building blocks for appropriate management units.     

Spatial patterns of whiting abundance in Scottish waters and relationships with environmental variables

This paper uses objective classification methods, a combination of principal components analysis (PCA) and cluster analysis, applied to long-term average data, to define areas of similar seasonal patterns of whiting abundance in Scottish waters, based on fishery data on landings and effort (by month and by ICES rectangle). A geographic information system (GIS) is used to qualitatively describe the relationships of these spatial patterns of whiting abundance with (a) trawl survey catch rates by age-class, and (b) environmental factors. The results show that the spatial patterns of whiting abundance are related to age, as well as to depth and to spatial patterns of sea surface temperature (SST) in winter. The area (within the North Sea) of highest whiting abundance, and the largest seasonal change in whiting abundance, corresponds to the area of highest survey catch rates for older (>2 years old) whiting in winter, a particular spatial pattern of SST in winter and water depths of 100–200 m. This spatial pattern of SST may indicate an effect of the inflow of the North Atlantic water. This study provides a guide for selecting study areas for future quantitative analysis and the methods proposed may also provide a useful management tool.     

Present state and perspectives for the southern Brazil shelf demersal fisheries

The demersal fish stocks in southern Brazil were assessed from landings and catchper-unit effort data trends between 1975 and 1994, available information on the life history patterns, and population dynamics of the most important species. The fishing gears, mostly otter and pair bottom trawls in the 1970s, diversified towards double-rig trawls for fish and shrimp and bottom gill nets in the mid 1980s, and bottom longlines in the upper slope in the early 1990s. There were also some less successful attempts with traps for fish and crabs. The demersal fisheries are more intensive in winter when migratory species move northward, associated with the seasonal displacement of the western boundary of the Subtropical Convergence. Landings between 1975 and 1994 were mostly of sciaenid fishes (>70%) and elasmobranchs (10%) and oscillated around 59 000 t. There was not a decrease in total landings but a shift from more to less exploited stocks. Since 1989, 4% of the landings came from depleted stocks of Pagrus pagrus (L.), Netuma barba (Lacépède), Pogonias cromis (L.) and Rhinobatos horkelli (Muller & Henle); 37% from overexploited stocks: Micropogonias furnieri (Desmarest), Paralichthys patagonicus Jordan, Squatina guggenheim Marini, and Galeorhinus galeus (L.); 24% from heavily exploited stocks of Umbrina canosai Berg, and Macrodon ancylodon (Bloch & Schneider); and 35% of Cynoscion guatucupa (Cuvier) and small stocks with unclear status. The demersal fisheries are virtually unmanaged and future landings are expected to decrease if fishing pressure remains high. Effective management will require the participation of all three countries in the region because most of the stocks fished in southern Brazil are shared with Uruguay and Argentina.     

Results of a short study on by-catches and discards in pelagic fisheries in Scotland (UK)

Observers were placed on pelagic vessels in the Scottish fisheries for mackerel (Scomber scombrus), herring (Clupea harengus), “maatje” herring (herring caught just before their first spawning) and argentines (Argentina silus) to monitor by-catch composition and discarding practices. A total of 67 days was spent at sea, 11 on the argentine fleet, 28 on the herring fleet, 12 on the “maatje” herring fleet and 16 on the mackerel fleet. The level of by-catch generally ranged from <1% to around 2.5% of the total catch. The argentine fishery took a significant proportion of blue whiting (Micromesistius poutassou) (approx. 10% of the overall catch) but this was landed and sold in the market. The greatest range of by-catch species was found in the argentine fishery, including 11 species of fish and one species of squid. Discarding rates in the fisheries varied, with herring and argentine fisheries showing no discards, the mackerel fishery a discard rate of around 4% and the “maatje” herring fishery a discard rate of around 11%. By-catches included small numbers of gannets (Morus bassanus) but no marine mammals. If the sampled trips were representative, results indicate that marine mammal by-catch events typically occur during less than one in 20 hauls in the fisheries studied. A larger scale study is needed to confirm this. Cetaceans were sighted in the vicinity of the fishing boats during 4% of observed fishing activities.     

Effect of trawling with traditional and ‘T90’ trawl codends on fish size and on different quality parameters of cod <Emphasis Type="Italic">Gadus morhua</Emphasis> and haddock <Emphasis Type="Italic">Melanogrammus aeglefinus</Emphasis>

The effect of trawling on fish size and on different quality parameters of cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) was evaluated after conducting 16 valid hauls using two trawls in a double rig fitted with a traditional and a novel ‘T90’ codend, respectively. The total catch volume during the fishing period was 47.6 metric tons, with an average catch per codend of 1.5 (range 0.5–2.9) tons. The mean haul duration was 5 h. The catch was assessed according to fish size, mortality, external damage, initial white muscle pH and development of rigor mortis. Fillet quality (colour, blood spots, gaping) was assessed after 1 week of freeze-storage. Our results showed there was no difference between the two types of nets in terms of catch volume, but significantly slightly bigger fish were caught with T90 than with the traditional trawl net (p < 0.05). Haddock caught with the traditional trawl net had more external injuries related to the trawl gear than haddock caught with the T90 gear (p < 0.05). The gaping frequency for cod caught with the traditional trawl net tended to be higher than cod caught with the T90 gear, but the difference was not significant (p = 0.07). No other differences in fish quality between fish caught in the trawl nets were observed.     

Wasted fishery resources: discarded by-catch in the USA

Fishery by‐catch, especially discarded by‐catch, is a serious problem in the world's oceans. Not only are the stocks of discarded species affected, but entire trophic webs and habitats may be disrupted at the ecosystem level. This paper reviews discarding in the marine fisheries of the USA; however, the type, diversity and regulatory mechanisms of the fisheries are similar to developed fisheries and management programmes throughout the world. We have compiled current estimates of discarded by‐catch for each major marine fishery in the USA using estimates from existing literature, both published and unpublished. We did not re‐estimate discards or discard rates from raw data, nor did we include data on protected species (turtles, mammals and birds) and so this study covers discarded by‐catch of finfish and fishable invertebrates. For some fisheries, additional calculations were required to transform number data into weight data, and typically length and weight composition data were used. Specific data for each fishery are referenced in Harrington et al. (Wasted Resources: Bycatch and discards in US Fisheries, Oceana, Washington, DC, 2005). Overall, our compiled estimates are that 1.06 million tonnes of fish were discarded and 3.7 million tonnes of fish were landed in USA marine fisheries in 2002. This amounts to a nationwide discard to landings ratio of 0.28, amongst the highest in the world. Regionally, the southeast had the largest discard to landings ratio (0.59), followed closely by the highly migratory species fisheries (0.52) and the northeast fisheries (0.49). The Alaskan and west coast fisheries had the lowest ratios (0.12 and 0.15 respectively). Shrimp fisheries in the southeast were the major contributors to the high discard rate in that region, with discard ratios of 4.56 (Gulf of Mexico) and 2.95 (South Atlantic). By‐catch and discarding is a major component of the impact of fisheries on marine ecosystems. There have been substantial efforts to reduce by‐catch in some fisheries, but broadly based programmes covering all fisheries are needed within the USA and around the world. In response to international agreements to improve fishery management, by‐catch and discard reduction must become a regular part of fishery management planning.     

Using commercial landings data to identify environmental correlates with distributions of fish stocks

We examined the efficacy of using commercial landings data to identify potential environmental correlates with fish distributions. Historical landings data for two commercially important species, Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus), were used along with historical conductivity, temperature, and depth (CTD) data to infer monthly mean spatial distributions of catch per unit effort (CPUE), temperature, salinity, density, and stratification over Georges Bank. Relationships between CPUE and these environmental variables plus bottom sediment type and bottom depth were examined on seasonal, annual, and interannual time scales. Empirical analysis suggests that both cod and haddock are found preferentially in water temperatures of approximately 5°C in winter/spring, and as high as 10–11°C during late fall. Both species are also found preferentially over coarse sand and gravel as opposed to fine sand, and in water depths between 60 and 70 m. These preferences appear to vary seasonally. The above results are consistent with findings of previous investigators using semi‐annual research trawl survey data, and suggest that commercial landings data, despite their known errors and biases, can be used effectively to infer associations between fish and their environment.     

Choosing best practices for managing impacts of trawl fishing on seabed habitats and biota

Bottom trawling accounts for almost one quarter of global fish landings but may also have significant and unwanted impacts on seabed habitats and biota. Management measures and voluntary industry actions can reduce these impacts, helping to meet sustainability objectives for fisheries, conservation and environmental management. These include changes in gear design and operation of trawls, spatial controls, impact quotas and effort controls. We review nine different measures and actions and use published studies and a simple conceptual model to evaluate and compare their performance. The risks and benefits of these management measures depend on the extent to which the fishery is already achieving management objectives for target stocks and the characteristics of the management system that is already in place. We offer guidance on identifying best practices for trawl‐fisheries management and show that best practices and their likelihood of reducing trawling impacts depend on local, national and regional management objectives and priorities, societal values and resources for implementation. There is no universal best practice, and multiple management measures and industry actions are required to meet sustainability objectives and improve trade‐offs between food production and environmental protection.