Otter Trawl Research Articles

Population structure of the bigtooth corvina Isopisthus parvipinnis from the Southwest Atlantic Ocean as determined by whole-body morphology

Geometric morphometrics analysis provides evidence from the adaptive character of fish to the aquatic environment resulting from an evolutionary process. Thus, phenotypic variations among individuals are primarily due to heritability of genetic factors modulated by the surrounding environment. Analysis of body morphometry can provide important information on fish population structure, an essential prerequisite for conservation requirements and sustainable fisheries management. The “bigtooth corvina”, Isopisthus parvipinnis, is an euryhaline sciaenid fish widely distributed in the shallow and warm waters from the western Atlantic Ocean. It is often caught as bycatch of shrimp fisheries in Brazil. Although there is some basic ecological information about the species, knowledge about the stock structure is, at present, very limited. This work aims to investigate if the different environments of the South-Southeastern Brazilian coastal waters (São Paulo, Paraná, and Santa Catarina states) could affect the body shape of I. parvipinnis, allowing to define different population units through phenotypic geometric morphometrics. Juvenile fish, ranging between 66 and 139 mm of total length, were captured from September 2018 to May 2019 by research vessels and artisanal fleets using an otter trawl. Fish were photographed with 13 anatomical landmarks, and truss network analyses were performed using tpsUtil and tpsDig softwares. Data were analyzed through uni and multivariate statistical tests. Results showed significant differences among locations for all morphometric distances individually. Moreover, reclassification matrix presented a relatively high overall rate (79%). The hereby data suggest that I. parvipinnis in the study area is not a single and homogeneous stock, supporting the existence of a metapopulation structure. However, it is highly recommended to use complementary information, namely other natural tags, to corroborate these findings.

Environmental Implications of Discarding Fish in Northern Spanish Coastal Bottom Otter Trawl Fisheries

Overfishing contributes to the depletion of fish stocks. Among fishing practices, discarding fish is a waste of valuable resources and results in lost income that is associated with its effects on future stock growth. This practice depends on several factors, although trawl fisheries account for the highest quantity of discards. In this work, the effects of a specific section of the Galician (Northwestern Spain) bottom trawl fishery were evaluated by using the ecological footprint, focusing on the contribution of the rate of discards. We selected fishing grounds and carbon footprints as the categories for the analysis. The results showed that 11.6 global hectares (gha) per metric ton of catch were required for an average fishing trip, ranging between 2.2 and 27.1 gha per metric ton. Discards represented 55% of this value, while the carbon footprint only represented 11%. Therefore, measures are needed to reduce the significant effects of discards, especially in these fisheries. The findings indicate that the discard ban promoted by the Common Fisheries Policy could reduce the ecological footprint of these fisheries by 3.4 gha per metric ton of catch landed on average.

Including ecosystem descriptors in current fishery data collection programmes to advance towards a holistic monitoring: Seabird abundance attending demersal trawlers

To develop an ecosystem-based approach to fisheries management a holistic perspective is necessary that goes beyond target species management to preserve ecosystem functioning and, therefore, secure future food availability. To achieve these objectives, current fishery data collection programmes should widen their objectives to include the collection of ecosystem descriptors to effectively take advantage of funding and human resources in relation to fisheries monitoring already in place. From an ecological perspective, fishing discards are food subsidies unnaturally available that can profoundly impact the life history traits and population dynamics of seabirds, as well as community structure. In 2015, we took advantage of the Data Collection Framework (DCF) programme, monitoring the Basque trawling fleet, to monitor seabird abundance associated with trawlers as an additional task to be performed by the observers. The main objectives were (1) to develop a standard protocol from an interdisciplinary expert committee, (2) to obtain baseline information of seabird association with trawlers to be able to track changes and (3) to understand seabird abundance in relation to discard facilitation processes based on environmental and fishing parameters. Based on the developed standard protocol, more than 21 species of seabirds were recorded in 241 fishing hauls in the Bay of Biscay (ICES areas 8abd). This biogeographic area is an important migratory flyway and wintering area, where maximum number of seabirds attending trawlers were recorded between December and April (2015–2019). Based on Generalised Linear Models, seabird abundance was higher in multiple situations: harsher environmental conditions, less available discards, shallower areas, increased period of discard availability, higher number of trawlers simultaneously operating in the same fishing ground, in addition to the months of February and April. The core discarding ground was located in the outer French shelf between the latitudinal range of 44.5°N and 46°N, especially for otter trawl fishing. Our approach puts into value the seabird counts that the observers of the DCF can perform systematically, collecting relevant information on the effect of trawling on other biodiversity components such as seabirds. This information will be critical to respond to the application of the reform of the Common Fisheries Policy regarding the effect of the Landing Obligation that seeks fishing sustainability.

Shape optimization approach for cambered otter board using neural network and multi-objective genetic algorithm

The shape optimization approach of the cambered otter board has been performed by the integration of the neural network model and the multi-objective genetic algorithm (MOGA). Because the excellent performance of an otter board is expressed by great lift and less drag force, in this study the lift and drag coefficients were chosen as objective functions to obtain the optimal otter board. The Bézier curve represented the cambered otter board as a simple structure with five control points resulting in the six coordinates, which were adopted as the design variables. The hydrodynamic characteristics of twenty-five otter board models were calculated in a two-dimension computational fluid dynamics (CFD) analysis at an attack angle of 20°. The implicit fitness function in the MOGA algorithm was then obtained by the backpropagation neural network model based on the estimated results of CFD calculation. A set of thirty optimal otter board models were extracted in the optimal solutions of the MOGA, and two optimal models were selected to verify the feasibility of the approach by hydrodynamic and visualization experiments with a comparative hyper-lift trawl door (HLTD). The model 1 showed greatest lift-to-drag ratio before the attack angle of 30° as a high lift-to-drag ratio otter board, and the model 2 showed a large lift coefficient and lift-to-drag ratio than the HLTD before the attack angle of 25° as a large lift force otter board. Through the flow distribution around the model 2, it is observed that the flow separation on the suction side is prevented as a result of less drag owing to the modified shape. In summary, the shape optimization approach is efficient in designing optimal otter board to satisfy supposed needs in otter trawling.

Use of social network analysis to examine preferential co-occurrences in Atlantic Sturgeon Acipenser oxyrinchus oxyrinchus Mitchill, 1815

BackgroundApplication of Social Network Analysis (SNA) to acoustic telemetry is a useful approach to examine social behavior in fish. Atlantic Sturgeon (Acipenser oxyrinchus oxyrinchus) are ancient, long-lived anadromous finfish. Although Atlantic Sturgeon have been the subject of numerous telemetry studies, none have used SNA to analyze their co-occurrence behavior. During 2010–2014 Atlantic Sturgeon (n = 103) that were later genetically identified as being from the Saint John River, Canada and the Kennebec River, US were captured by otter trawl and brush weir in Minas Basin, Bay of Fundy, Canada, and acoustically tagged. Using data from moored acoustic receivers within foraging habitat in Minas Basin, we tested if Atlantic Sturgeon formed social associations that were random or structured during 2012 to 2014; and whether these co-occurrences consisted of individuals from the same river of origin or capture date.ResultsIrrespective of genetic origin and initial capture date, Atlantic Sturgeon formed co-occurrences in Minas Basin that were significantly different than would be observed by chance during 2012 and very close to significant during 2013. Analysis demonstrated that some Atlantic Sturgeon preferentially co-occur within their primary feeding habitat.ConclusionsThe current threats to Atlantic Sturgeon aggregations within the Bay of Fundy, Nova Scotia, include bycatch in fisheries throughout their coastal migration routes and more recently the development of tidal turbines along their migratory corridor to their summer aggregation site. It is important to determine if Atlantic Sturgeon form aggregations with conspecifics from the same population to inform management decisions regarding threats to groups of individuals. This study indicated that Atlantic Sturgeon may form preferential co-occurrences within their feeding aggregation and co-occurrences that were identified were not dependent upon population of origin or initial capture date.

Juvenile fish assemblage recruitment dynamics in a mid-Atlantic estuary: before and after Hurricane Sandy

Hurricanes can have long-term effects on estuarine fauna. Understanding these effects is important as climate change may influence the severity and frequency of these storms. On 29 October 2012, Hurricane Sandy, a large storm spanning roughly 1850 km in diameter, made landfall in Brigantine, New Jersey (USA), approximately 20 km south of Barnegat Bay, during an ongoing study of the bay’s ichthyofauna, providing an opportunity to observe fish recruitment dynamics coincident with hurricane passage. The objective of this study was to measure variance in the Barnegat Bay pre-Sandy fish assemblage relative to that of 1 and 2 yr after the storm. Barnegat Bay fishes were surveyed with an extensive otter trawl study in April, June, August, and October of 2012 (pre-Sandy), 2013 (1 yr post-Sandy), and 2014 (2 yr post-Sandy). Species composition of the fish assemblage was similar across years. Analyzed structural characteristics (abundance, diversity, richness) of the fish assemblage were occasionally more likely to occur or were larger pre-Sandy and 2 yr post-Sandy relative to 1 yr post-Sandy, but this trend was inconsistent across seasons and between structural characteristics. Furthermore, odds of occurrence and length frequency distributions for many resident species and sentinel fall/winter spawners did not indicate that variance could be definitively explained as a hurricane effect. The capability of fish to relocate from areas of temporarily unsuitable habitat and annual new recruitment of larvae and juveniles to the bay likely contributed to the observed stability in the fish assemblage.

Different bottom trawl fisheries have a differential impact on the status of the North Sea seafloor habitats

AbstractFisheries using bottom trawls are the most widespread source of anthropogenic physical disturbance to seafloor habitats. To mitigate such disturbances, the development of fisheries-, conservation-, and ecosystem-based management strategies requires the assessment of the impact of bottom trawling on the state of benthic biota. We explore a quantitative and mechanistic framework to assess trawling impact. Pressure and impact indicators that provide a continuous pressure–response curve are estimated at a spatial resolution of 1 × 1 min latitude and longitude (∼2 km2) using three methods: L1 estimates the proportion of the community with a life span exceeding the time interval between trawling events; L2 estimates the decrease in median longevity in response to trawling; and population dynamic (PD) estimates the decrease in biomass in response to trawling and the recovery time. Although impact scores are correlated, PD has the best performance over a broad range of trawling intensities. Using the framework in a trawling impact assessment of ten métiers in the North Sea shows that muddy habitats are impacted the most and coarse habitats are impacted the least. Otter trawling for crustaceans has the highest impact, followed by otter trawling for demersal fish and beam trawling for flatfish and flyshooting. Beam trawling for brown shrimps, otter trawling for industrial fish, and dredging for molluscs have the lowest impact. Trawling is highly aggregated in core fishing grounds where the status of the seafloor is low but the catch per unit of effort (CPUE) per unit of impact is high, in contrast to peripheral grounds, where CPUE per unit of impact is low.

Comprehensive bycatch assessment in US fisheries for prioritizing management

Wild-capture fisheries help provide food security to billions of people, yet bycatch of non-target species threatens ecosystem health and fishery sustainability. Appropriate monitoring and fisheries management can mitigate bycatch but require standardized bycatch data to be robustly recorded and effectively disseminated. Here we integrated and analysed 30,473 species-specific bycatch records from 95 US fisheries in 2010–2015. We examined patterns in fish and invertebrate, marine mammal, seabird and sea turtle bycatch and developed a standardized scoring system, the relative bycatch index, to assess bycatch performance of each fishery. The estimated amount of fish and invertebrate discards totalled 1.93 million tonnes (4.26 billion pounds) over the 6-year period. We found that the national discard rate is 10.5%, considerably lower than past estimates. Results from our relative bycatch index analysis can be used to facilitate management intervention strategies for particular fisheries or gear types, such as shrimp and otter trawls and several pelagic longline and gillnet fisheries, which had the poorest bycatch performance. These findings underscore the need for continued, high-quality, easily accessible bycatch information to better support fisheries management in the United States and globally. The sustainability of wild fisheries is limited by the capture of non-target species. This study estimates that such ‘bycatch’ from US fisheries totalled 1.93 million tonnes in 2010–2015, with a 10.5% discard rate. Despite considerable improvements, certain longline, trawl and gillnet fisheries have persistent bycatch issues.

A meta-analysis of plaice size-selection data in otter trawl codends

The influence that mesh size, the number of open meshes around the circumference, twine thickness and catch size have on the size-selection of plaice (Pleuronectes platessa) in otter trawl codends is investigated by synthesising the data from 164 hauls using 25 gear configurations during 9 trips.All these variables have an effect on selection. The 50 % retention length (L50) increases by 1.9 cm for every 10 mm increase of mesh size and decreases by 0.24 cm for every doubling of catch. The selection range decreases by 3.8 % for each increase of 10 open meshes and increases by 12.3 % for each 1 mm increase of twine thickness. The mesh size and twine thickness effects have strong support from the data. The open meshes and catch size effects have less support and should be treated with some caution.There is good correspondence between the predictions of the analysis presented here and the L50 estimates of beam trawl codend selectivity trials, which is noteworthy given the many operational differences between the two fishing methods.

울진 연안에서 기선저인망에 어획된 어류의 종조성 및 계절변동

Seasonal fluctuation in the abundance and species composition of fish were investigated using small otter trawl in the coastal waters off Uljin, Korea, in 2016. A total of 6,017 fishes were sampled and classified into 48 species, 33 families and 11 orders. The dominant orders were Perciformes including 20 species and 16 families followed by Scorpaeniformes including 8 species and 5 families, Tetraodontiformes including 5 species and 3 families, Clupeiformes including 4 species, and 2 families, Pleuronectiformes including 4 species and 2 families, and Zeiformes including 2 species and 2 family. Among the appeared 48 species, Engraulis japonicus was the dominant species by 17.6%, followed by Hexagrammos otakii (5.3%), Pterogobius zonoleucus (5.3%), Ditrema temmincki (4.5%), and Hexagrammos agrammus (4.2%). The diversity was the highest in summer (3.26), while the lowest in winter (2.89). The evenness was the highest in winter (0.94), while the lowest in summer (0.90). The dominance was the highest in fall (27.0%), while the lowest in winter (20.9%). The richness wast the highest summer (4.67), while the lowest in winter (3.13). The similarity of community structure was 72.77% between summer and fall.

Change of relative fishing power index from technological development in the otter trawl fishery

Change of relative fishing power index from technological development in the otter trawl fishery

Artificial light improves escapement of fish from a trawl net

AbstractThe elimination of unwanted catch in mixed species fisheries is technically challenging given the complexity of fish behaviour within nets. Most approaches to date have employed technologies that modify the nets themselves or use physical sorting grids within the gear. There is currently increasing interest in the use of artificial light to either deter fish from entering the net, or to enhance their escapement from within the net. Here, we evaluated the differences in catch retained in a standard otter trawl, relative to the same gear fitted with a square mesh panel, or a square mesh panel fitted with LEDs. We found that the selectivity of the gear differed depending on water depth. When using a square mesh panel in shallow depths of 29–40 m the unwanted bycatch of whiting and haddock was reduced by 86% and 58% respectively. In deep, darker water (45–95 m), no change in catch was observed in the square-mesh panel treatment, however when LEDs were added to the square-mesh panel, haddock and flatfish catches were reduced by 47% and 25% respectively. These findings demonstrate the potential to improve the performance of bycatch reduction devices through the addition of light devices to enhance selectivity. The results also highlight species-specific and site-specific differences in the performance of bycatch reduction devices, and hence a more adaptive approach to reduce bycatch is probably required to maximize performance.

Survival of undersized plaice (Pleuronectes platessa) discarded in the bottom otter trawl and Danish seine mixed fisheries in Skagerrak

The landing obligation of the new Common Fisheries Policy includes a “high survival”-exemption, which allows to continue discarding of fish with scientifically proven high chances to survive the process of capture, handling and release. This study presents survival rates of undersized plaice (Pleuronectes platessa) caught by two commercial fishing vessels targeting plaice in Skagerrak during the summer, a bottom otter trawler and a Danish anchor seiner. Instead of being discarded, the fish were assessed and monitored in on-land observation tanks for 14 days. For each gear, a Weibull mixture model was fitted to the data to estimate survival probabilities, including the uncertainties from the haul selection and conditions of the captivity experiment, and to determine which biological, environmental and operational factors influence survival. Generally, fish caught by the trawler showed lower survival (44%, 95%-confidence interval: 37–52%) than those caught by the seiner (78%, 95%-confidence interval: 67–87%). Air exposure was found to be the most important factor in determining survivability of fish discarded from the trawler. For the seiner, bottom temperature was identified as an additional important determinant.

Fish communities and environmental variables during dry season in Pampanga estuary (Philippines)

Baseline assessment of fish communities and selected environmental variables (salinity, water temperature, water transparency, current speed, total suspended solid, chlorophyll a) was conducted in Pampanga estuary (Philippines) during a specific dry season (April 2010). The aims were: (a) to examine the variations of fish diversity and community structure, in relation to neap and spring tides and estuary zones and; (b) to examine the influence of environmental variables on fish communities. Reconnaissance showed that salinity was measurable at 6.8 kilometers from the rivermouth, for the surface and mid-water layers, and at 15.9 kilometers for the bottom layers, indicating the occurrence of a salt wedge. Bottom salinity was used to establish the sampling stations, grouped into 3 zones (lower 1–3, middle 4–6, upper 7–9). Sampling of fish, using otter trawl, and measurement of environmental variables were done under four tidal conditions at each station (neap low and high tide; spring low and high tide). Neap tide showed low and homogenous horizontal salinity, higher temperature, slower current speed, high TSS and chlorophyll a, whereas, spring tide showed higher salinity, horizontal saline stratification, lower temperature, faster current speed, low TSS and chlorophyll a. A total of 330 fish were captured from 32 trawl samples, encompassing 20 families and 27 species. Number of species generally did not vary between neap (20) and spring tide (18), but showed a decreasing pattern from lower to the upper estuary. Only 36% of total species were shared between neap and spring tides. Fish abundance was significantly affected by neap and spring tide, and zonation, whereas fish biomass was solely affected by zonation. Arius manillensis was the most typifying species, suggesting high probability of encounter. Chlorophyll a, TSS, salinity and current speed have a significant influence on the variations of abundance, biomass and frequency of occurrence of particular fish species.

Historical dynamics of the demersal fish community in the East and South China Seas

Taiwan has a long history of fishery operations and contributes significantly to the global fishery harvest. The East and South China seas are important fishing grounds for which publicly available data are very limited. More efforts are needed to digitise and analyse historical catch rate data to illuminate species and community changes in this region. In this study we digitised historical records of catch and effort from government fishery reports for nine commercial species caught by otter trawl, and reported quarterly from 1970 to 2001, from the East and South China seas. We analysed the four seasons and present abundance indices, distributions and among-species correlations for nine commercially important species from 1970 to 1988 (a period with high fishing effort) using a multispecies spatiotemporal model that estimates both covariation in multispecies catch rates, attributed to spatial habitat preferences and environmental responses, and indices representing trends in abundance and distribution. We found substantial spatial, temporal and spatiotemporal variation in the distribution of fishes and season-specific patterns. We recommend collaborative work from various adjacent countries to digitise historical records of fishing catch rates, because more records would potentially address scientific disagreements regarding trends in the abundance and distribution of commercial fishes in this region through comparative studies.

Bottom trawling activity, main fishery resources and associated benthic and demersal fauna in a mud volcano field of the Gulf of Cádiz (southwestern Iberian Peninsula)

Bottom trawling activity, commercial species and benthic and demersal assemblages were studied integrating Vessel Monitoring System data (VMS), fishing logbook data and otter trawl samples in a mud volcano (MV) field of the Spanish margin of the Gulf of Cádiz (GoC) in 2011. High trawling activity was detected around Anastasya MV and between the Cádiz and Guadalquivir diapiric ridges, and low trawling activity around Gazul MV and in the deepest areas (Huelva Channel and southwards). A total of 11938 individuals from 126 benthic and demersal species were collected in the samples. Multivariate analyses mainly grouped samples according to depth and sediment type and 3 different assemblages were detected. The Stratum 1 assemblage (Gazul MV, 441–470 m depth, muddy sand, low trawling activity and high near-bottom current speed) was dominated by Parapenaeus longirostris, Leptometra phalangium and Plesionika antigai, among others, and displayed high species richness and abundance. The Stratum 2 assemblage (Anastasya and Tarsis MVs, Triangle, and the External zone, 528–622 m, mud/sandy mud, medium to high trawling activity and low near-bottom current speed) was dominated by P. longirostris, Plesionika martia, Nezumia aequalis and small chondrichthyans (mainly sharks), among others, and displayed low species richness, abundance and biomass. The Stratum 3 assemblage (Huelva Channel > 700 m, sand, low trawling activity and high near-bottom current speed) was dominated by similar species to Stratum 2, but also included large demersal sharks (Centrophorus granulosus, Galeorhinus galeus, Dalatias licha) that resulted in a high biomass. Depth and near-bottom current speed showed strong relationships with the benthic and demersal assemblages in Principal Component Analyses. The occurrence of medium to intense trawling activity in and around some MVs (Habitat 1180, Council Directive 92/43/ECC-Habitats Directive) should imply an effective regulation that allows a balance between sustainable fisheries and habitat conservation.

Quantifying long-term discards from Queensland’s (Australia) east Coast otter trawl fishery

AbstractThis article estimated the weight of annual discards in the Queensland east coast otter trawl fishery. Discards data were analysed using three generalized linear mixed models to derive adjusted discard rates, expressed as kg per retained catch, kg per boat day, and kg per area swept by trawls, respectively. Model explanatory terms included trawl fishing sector, presence/absence of bycatch reduction devices (BRDs), lunar phase, and whether the data were obtained from commercial vessels during their normal fishing activities or during research charters. Adjusted discard rates were then used to derive three estimates of annual discards and the average used to examine a long-term discard trends from 1988 to 2014. Total discards declined significantly from a peak of approximately 67 000 t in 1997 to approximately 21 000 t from 2011 to 2014, largely due to a decline in fishing effort, and to a lesser degree, the effects of BRDs that were mandated in the fishery in the early 2000s.

Stimulating release of undersized fish through a square mesh panel in the Basque otter trawl fishery

Discards of regulated species in the Basque mixed trawl fishery are a challenge. In 2006, a square mesh panel (SMP) was introduced in the fishery to increase the release efficiency of undersized fish. However, studies have shown that the selectivity in this fishery is based on codend selectivity and the release through the SMP is inefficient due to low contact between fish and the SMP. In order to improve contact, we tested four different gear configurations that use different stimulators to lead fish to the panel: without stimulation, with stimulation based on ropes, with stimulation based on ropes and floats, and with stimulation based on LED lights. The experiment was carried out on three of the potential choke species for the fishery: hake (Merluccius merluccius), horse mackerel (Trachurus trachurus), and blue whiting (Micromesistius poutassou). The results showed that stimulators did not significantly improve the release efficiency of hake and horse mackerel through the panel. For blue whiting, stimulation with floats had a significant positive effect on release efficiency, whereas LED light-based stimulation had the opposite effect. In general, the contribution of the SMP to the overall release efficiency of the selective system (SMP + codend) was low. Underwater recordings confirmed that the stimulators generally were not able to lead fish towards the SMP.

Roles of fishing and climate change in long-term fish species succession and population dynamics in the outer Beibu Gulf, South China Sea

A prevailing, controversial hypothesis is that fishing pressure has played a greater role than climatic and environmental drivers, in changing fish species succession and biomass fluctuation in the South China Sea (SCS). Based on otter trawl survey data from 1959 to 2010 in the outer Beibu Gulf (OBG), northern SCS, large seasonal and interannual variation is reported for fish species composition, the proportional abundances of dominant taxa, and fish biomass. Generalized additive models are developed to quantify relationships between fish biomass and the external factors of fishing pressure and climate change. Fishing pressure proved to be the main driver of sharp declines in demersal fish stocks, with high-value species being replaced by low-value ones over time. Abrupt decreases in fish biomass during the years of 1993 and 1998 correspond to El Nino events, with climate change possibly the main driver of proportional representation of pelagic species in fisheries trawl catch. The need to differentiate impacts of fishing and environmental drivers on fish species with different life history strategies is stressed to better understand fish community dynamics.

여수연안에서 소형기선저인망에 어획된 어류의 종조성 및 계절변동

여수연안에서 소형기선저인망에 어획된 어류의 종조성 및 계절변동