Otter Trawl Research Articles

Amending survey design to improve statistical inferences: Monitoring recruitment of juvenile reef fish in the eastern Gulf of Mexico

Many species of interest to management and conservation remain data-limited, and the data that are available are often unable to produce statistically reliable population trends. We examined 10 years of juvenile reef fish catch data from two gear-specific, fishery-independent surveys in the eastern Gulf of Mexico to assess our ability 1) to characterize population trends for various reef species sampled using haul seines or otter trawls and 2) to amend our survey, as logistics would allow, to improve those characterizations. Species richness, catch-per-unit-effort, and length-frequency distributions were generally similar between gear types, suggesting a single survey (i.e., one gear type) may be sufficient. Simulation-based power analyses for the reef fish species indicated that overall, otter trawl data provided greater power for detecting trends over a 10-year period (41–75 % probability to detect a 50 % change in abundance) than haul seine data (27–52 %). Likewise, otter trawl data provided greater power for detecting trends from one year to the next (22–53 % probability to detect a 50 % change in abundance in one year) than haul seine data (15–33 %). Simplifying data collection from two surveys to a single, trawl-only survey (and approximately doubling the number of trawl tows) was more efficient, more effective, and more powerful (64–91 % for the amended design with more tows) in detecting abundance trends. Furthermore, by increasing sample size but retaining all other design elements, the data collected during the trawl survey before and after the change remain comparable; the time series was not interrupted. These changes increase our confidence in estimating population trends, predicting productivity, and informing management and conservation decisions.

Identifying partners at sea from joint movement metrics of pelagic pair trawlers

Abstract Here, we present an approach to identify partners at sea based on fishing track analysis, and describe this behaviour in several fleets: pelagic pair trawlers, large and small bottom otter trawlers, mid-water otter trawlers, all in the North-East Atlantic Ocean, anchovy purse-seiners in the South-East Pacific Ocean, and tuna purse-seiners in the western Indian Ocean. This type of behaviour is known to exist within pair trawlers, since these vessels are in pairs at least during their fishing operations. To identify partners at sea, we used a heuristic approach based on joint-movement metrics computed from vessel monitoring system data and Gaussian mixture models. The models were fitted to joint-movement metrics of the pelagic pair trawlers, and subsequently used to identify partners at sea in other fleets. We found partners at sea in all of the fleets except for the tuna purse-seiners. We then analysed the connections between vessels and identified exclusive partners. Exclusiveness was more common in pelagic pair trawlers and small bottom otter trawlers, with 82% and 74% of the vessels involved in partnerships having exclusive partners. This work shows that there are collective tactics at least at a pairwise level in diverse fisheries in the world.

Long-term and seasonal changes in the life-history biology of the abyssal holothurian Pseudostichopus aemulatus from the Porcupine Abyssal Plain (North-East Atlantic)

Long-term interannual variations in the benthic community structure are well-known from abyssal plains in the North Atlantic and North Pacific, where rapid responses to changes in the environment by first-order opportunists modify overall species composition. To increase our knowledge of the long-term variations of deep-sea benthic communities in the North-East Atlantic, we studied the response of a second-order opportunist echinoderm, the holothurian Pseudostichopus aemulatus, before, during and after the “Amperima Event” – a four-year episode (1996–1999) where the first-order opportunistic holothurian, Amperima rosea, increased dramatically in density. We used a collection of 14 sampling times of deep-sea community assemblage over a sixteen-year period to understand the temporal variation (i.e., interannual and seasonal) in population structure (i.e., density and individual sizes) and reproduction (i.e., mean oocyte sizes, sex ratios, gonadal indices, and fecundity) in P. aemulatus. Samples were collected from the Porcupine Abyssal Plain (PAP) in the NE Atlantic Ocean at a depth of ca. 4850 m between 1989 and 2005 with a semi-balloon otter trawl (OTSB14) and an epibenthic sledge (BN1.5/C) at varying seasons. We measured individual body size, dissected organisms to prepare histological slides of their gonads, and measured the Feret oocyte diameter in females. We found changes in the life-history biology of P. aemulatus during the “Amperima Event”. Population density increased gradually from September 1997 until April 1999, when P. aemulatus reached a peak in density coinciding with the smallest mean body size observed (1998 and 1999) suggesting recruitment of juveniles. In 1997, before these changes in population structure, mean oocyte sizes were at their largest, suggesting a strong reproductive effort with individuals either preparing to spawn or spawning. This reproductive effort was minimal in the years after 1999, when we found the largest proportions of previtellogenic oocytes in individuals that were probably allocating their energy into growth rather than reproduction given their smaller size. The presence of gametes of all sizes across seasons and a broad range of individual sizes in various sampling times suggests no seasonal reproductive patterns. We suggest that feeding guild differences between various holothurian species explain the diverse life-history responses observed among holothurians competing for resources in a food-poor environment.

ICHTHYOFAUNA IN THE INNER CONTINENTAL SHELF NEXT TO THE PARANAGUÁ ESTUARINE COMPLEX, SOUTHERN BRAZIL

The composition and structure of fish assemblages in the inner continental shelf of the Paraná State are affected by cold fronts and the rainfall regime. Data from fishing activities in this region, as well as the analysis of environmental characteristics, are the main tools available for understanding fish dynamics, under influence of increasing human activities. In order to better understand patterns and temporal variations in fish assemblages in the inner continental shelf of the Paraná State, a total of 24 double trawls were performed with an otter trawl between August 2000 and July 2001, in two sampling areas, the North area in front of the northern mouth of the Paranaguá Estuary Complex, and the South area, in front of the Leste beach. A total of 45,277 fish specimens belonging to 35 families and 97 species were caught. Sciaenidae was the most abundant family, with 37.1% of the total number of individuals caught, and with the highest richness (18 species). Statistical analyses evidenced significant differences in environmental characteristics and in fish fauna, and that both areas disturbances were observed in the fish assemblages during the dry and wet season, being more intense in the Southern area, disturbances that would be related to the shrimp fishing present in the two areas sampled in this study.

Notes on reproduction in the deep-sea cup coral Balanophyllia malouinensis (Squires 1961) from the Southern Ocean

Deep-sea scleractinian cup corals are prominent members of Southern Ocean megabenthic communities, though little is known about their life history. This study used paraffin histology and scanning electron microscopy to characterize the reproduction of the scleractinian coral Balanophyllia malouinensis (Squires, 1961) from Burdwood Bank in the Drake Passage. Samples were taken in April and May via otter trawls, Blake trawls, and dredges on three separate cruises: one on the RV Lawrence M. Gould in 2006, and two on the RV Nathaniel B. Palmer in 2008 and 2011. B. malouinensis is gonochoric with rare hermaphroditism. All four spermatocyst stages were seen simultaneously in males; similarly, most females contained oocytes at varying stages of development, though seasonality or periodicity could not be determined without a wider range of sample dates. Average fecundity was 241 ± 184 oocytes/individual (n = 38) and not correlated to cup size. Maturing larvae were found in the mesenteries and coelenteron of females, indicating B. malouinensis broods its larvae. This study was the first to characterize the reproduction of a deep-sea Balanophyllia species and adds to a small but growing body of work seeking to understand the unique benthic communities of Burdwood Bank.

Utility of gillnets for selectively targeting penaeids off Iran

In response to concerns over the mortality of discards and other negative environmental impacts of otter trawling for penaeids, the utility of bottom-set gillnets deployed by artisanal vessels to target Penaeus monodon, Metapenaeus affinis and other retained species by-product was assessed in the Sea of Oman. Two gillnets (each comprising several 80-m panels for total lengths of up to 1.3 km) were diurnally fished at two locations (off Beris and Govater, eastern Iran) within established penaeid fishing grounds. During 30 days, the total catch was 2721 kg, comprising penaeids (981 kg), eight retained by-product species (598 kg and mostly Otolithes ruber, Cynoglossus bilinearus and Carangoides talamparoides) and 19 discarded species (1142 kg). The catch-per-unit-of-effort (CPUE, kg/gillnet-panel · hour-fished) of targeted penaeids and by-product varied significantly between areas (0.5–1.0 kg), but nevertheless reached economically viable levels. The CPUEs of total bycatch were similar at both sites (<0.5 kg) and no marine turtles were caught. The concomitant ratios of retained-catches-to-bycatch were considerably less than those for regional penaeid-trawl fisheries. Nevertheless, while gillnet selectivity was satisfactory for M. affinis, most of the P. monodon were smaller than their carapace length at maturity at both fished sites. The results support using gillnets to target penaeids and by-product species as an alternative to otter trawling in the Sea of Oman; albeit with ongoing assessments of modifications to maximise selectivity.

A review of bycatch reduction in demersal fish trawls

Otter trawling for fish is one of the world’s most productive yet problematic fishing methods due to its bycatch and discards; issues that have been mitigated in some fisheries by developing more selective trawls. This paper systematically reviews efforts published in international peer-reviewed papers over the past 30 years to identify beneficial (and limiting) factors and propose a way forward in this field. In total, 203 papers were assessed, encompassing many of the world’s fishing regions, and involving > 147 species, although 74% of efforts occurred in Europe mainly focussing on haddock (Melanogrammus aeglefinus) (64 papers) and cod (Gadus morhua) (59 papers). Common, simple modifications have involved increasing lateral-mesh openings to match the morphology of unwanted catches via larger diamond-shaped mesh, or simply turning meshes 45° or 90°, either throughout codends or as strategic windows in the posterior trawl. In some fisheries, more complex grids have improved size or species selection. Fewer attempts have been made to modify the anterior trawl, but varying sweep/bridle lengths, horizontal separator panels and longer headropes have realized benefits depending on species-specific behavioural responses. While the utility of many modifications is indisputable, experimental designs (mostly involving covers, but also alternate hauls and paired comparisons) have, in many cases, suffered low replication and/or confounding variables. These deficits may have compromised some results and contributed to repeated efforts in particular fisheries. We conclude that rigorous empirical assessments, initially focusing on the posterior trawl, but eventually encompassing anterior changes, combined with straightforward interpretation of results for stakeholders, are as important as the simplicity and reliability of modifications. Finally, by assessing the utility, applicability, advantages and disadvantages of the modifications developed, we provide a framework which could be followed in future work to reduce bycatch in these fisheries.

Evaluation of impacts of bottom fishing on demersal habitats: A case study off the Pacific coast of north-eastern Japan

Physical disturbance of the seafloor induced by bottom trawling is of increasing concern from the viewpoints of ecosystem conservation and sustainable fishery. We developed a method for evaluating the impacts of several kinds of trawl gears on the different seafloor habitats using bathymetry and sediment data and scientific survey data off the Pacific coast of north-eastern Japan. Ten types of habitats were classified by the cluster analysis applied to seafloor bathymetry, topography, and sediment hardness over the study area. Our mapping of these habitats demonstrated that the intensity of bottom fishing activities on the seafloor was different among habitat types and related to the abundance of commercial species living in each habitat. The ecological influences of fishing on these habitats were examined by focusing on the changes in demersal species diversity and biomass detected through scientific surveys. A decline in species diversity was more evident in the habitat types with complex topography and hard sediment in the intensely trawled areas. Comparison of fishing gear suggested that the otter trawl may be the most influential owing to the significant change in the demersal ecosystem.

Time of day affects squid catch in the U.S. Illex illecebrosus squid fishery

A mid-water otter trawl fishery targeting Illex illecebrosus operates in the northwestern Atlantic Ocean. The majority of I. illecebrosus are captured during mid-June to early September. Diel migratory behavior limits the fishery to daylight hours, but does the time-of-day affect catch? Illex illecebrosus were collected from each tow from a subset of the trawler fleet fishing on the outer continental shelf of the Mid-Atlantic Bight over the fishing season to determine the influence of time-of-day on catch. The male-to-female ratio was not influenced by time-of-day of capture. The size–frequency​ distribution of I. illecebrosus varied between tows within the same day of capture. Time-of-day of capture influenced average weight and, to a lesser degree, mantle length. Shorter and lighter squid were caught in the middle of the day. Larger and heavier squid were caught during the morning and afternoon. Data collection on commercial fishing vessels required that squid be frozen onboard, thawed onshore, and then weighed and measured. Differences between fresh and frozen squid were significant, but small, which permitted combining fresh and frozen measurements from different boats to track temporal and spatial trends in squid size. As time-of-day differences in catch were significant, obtaining unbiased size data from fishing vessel-based reports requires that samples be collected from the morning, afternoon, and evening tows during the trip.

Bottom temperature warming and its impact on demersal fish off the Pacific coast of northeastern Japan

Global climate change occurs not only at the ocean surface but also at the ocean bottom, which is the main habitat of demersal fish. To clarify the current status of bottom temperature warming off the Pacific coast of northeastern Japan, we examined gridded bottom temperature fields from 2003 to 2019. These fields were created by a newly developed gridding method using flexible Gaussian filter weighting with time, distance, and depth. Spatially averaged bottom temperature had a strong, significant warming trend of 0.083 to 0.115°C yr-1 in depth zones of 150-300 m, indicating bottom temperature warming. Corresponding to the warming, increases in landing amounts were found for warm-water species such as searobin in the middle region of our study area (37°50’-39°N). Seasonal catch amounts suggest that ribbon fish and swimming crab recently began to overwinter and reproduce in the area. The distribution shifts of non-target species in fisheries were also analyzed using bottom otter trawl survey data from the area from 2003 to 2019. Northward distribution shifts and increases in density were observed in blackbelly lantern shark and bighand grenadier, indicating that bottom temperature warming led to habitat expansion. Conversely, darkfin sculpin and jelly eelpout shifted northward with decreasing density, suggesting that bottom temperature warming had a negative effect on them. Deepsea bonefish shifted deeper into colder waters with increasing density and mean body weight. Thus, changes and responses of demersal fish to bottom temperature warming in the area were revealed.

Life-history characteristics of the eastern shovelnose ray,

The eastern shovelnose ray (Aptychotrema rostrata) is a medium-sized coastal batoid endemic to the eastern coast of Australia. It is the most common elasmobranch incidentally caught in the Queensland east coast otter trawl fishery, Australia’s largest penaeid-trawl fishery. Despite this, age and growth studies on this species are lacking. The present study estimated the growth parameters and age-at-maturity for A. rostrata on the basis of sampling conducted in southern Queensland, Australia. This study showed that A. rostrata exhibits slow growth and late maturity, which are common life-history strategies among elasmobranchs. Length-at-age data were analysed within a Bayesian framework and the von Bertalanffy growth function (VBGF) best described these data. The growth parameters were estimated as L0 = 193 mm TL, k = 0.08 year–1 and L∞ = 924 mm TL. Age-at-maturity was found to be 13.3 years and 10.0 years for females and males respectively. The under-sampling of larger, older individuals was overcome by using informative priors, reducing bias in the growth and maturity estimates. As such, the results can be used to derive estimates of natural mortality for this species.

Feeding habits of Urotrygon microphthalmum (Myliobatiformes: Urotrygonidae) caught off northeastern Brazil

Abstract The feeding habits of Urotrygon microphthalmum, a Critically Endangered (CR) species, were investigated through stomach contents analysis from specimens caught on bottom double rigged otter trawls in Pernambuco state, Brazil, between March of 2010 and March of 2012. A total of 338 stomachs were analyzed, and 31 food items were identified in the diet of U. microphthalmum. The species ingests mainly shrimps. The diets between males and females were not different, and an ontogenetic diet shift was not observed. The estimated species’ trophic level is 3.5, classifying it as a secondary order consumer.

ENVIRONMENTAL DRIVERS OF FISH ASSEMBLAGES FROM THE SHALLOW INFRALITTORAL AREAS OF THE PARANAGUÁ BAY, SOUTHERN BRAZIL

The composition of estuarine fish fauna is dynamic and complex. Despite the variation in fish fauna in estuaries, it is possible to observe some species associated with microhabitats. The assemblage composition of these communities is apparently derived from habitat selection. The aim of this study was to assess the structure of the demersal fish fauna of the Paranaguá Bay and its relationship with biotic and abiotic factors. To this end, monthly collections were conducted using an otter trawl, from October 1999 to September 2000, at five sites in the shallow infralittoral. Biotic (organic matter, and invertebrates) and abiotic factors (salinity, temperature, pH, dissolved oxygen, seston, and nutrients) were recorded during sampling. A total of 6,623 individuals were captured, belonging to 2 species of Chondrichthyes and 53 species of Actinopterygii (26 families). Sciaenidae was the most representative in terms of richness, with 11 species, and Ariidae was the most representative in terms of abundance. The catfish Cathorops spixii was the most abundant species, representing 21 % of the entire catch. There were significant differences in the composition and abundance of the fish fauna among the studied areas, as evidenced by the Shannon-Wiener diversity index. Such differences seem to be related to the complexity of habitats, which occur due to different types of bottom cover found at each sampling site. The abiotic variables indicated little influence on the structure of the fish assembly. These results contribute to a better understanding of the fish fauna from the shallow infralittoral areas of the Paranaguá Bay, and provide data to decision makers for better implementation of management and conservation measures.

Acoustic backscatter analysis of ground-fishing activity in the German North Sea sector

The physical impact of demersal fishing was studied in three different areas of the German North Sea sector by use of a multibeam echosounder. The areas represent typical shallow seas siliciclastic habitats in variable distance to the coastline. Their seabed substrate is dominated by coarse silt close to the shore and by fine sand in the most distal area on the Dogger Bank. The study documents the utility of the multibeam as a suitable tool to map large seafloor areas, as well as to quantify and qualify the footprints of the fishing groundgear. Besides the depth and amplitude information, the multibeam data allows the analysis of the incidence angle-dependent variation of the backscatter signal that provides additional information on the seabed impedance, roughness, volume scatter and substrate type. Two main types of destructive seafloor pattern were observed in the three areas. The dominant type are pairs of parallel furrows exhibiting widths of 6–9 m and depths of 2–6 cm and is related to beam trawling. Separation distance of the pair furrows is 16–23 m. The second type are pairs of narrow furrows (3–4 m) with much larger separation distance of 110–120 m. They are attributed to otter board trawling.A comparison of their mechanical ramification shows that the fishing gear incises the seafloor and modifies seafloor structures and properties. Our investigation documents that beam trawling has the greatest physical impact and exhibits the highest furrow density in the three study areas. The created furrows can persist for at least 4 months in a substrate of very fine sand. Depending on the hydrodynamic regime, the furrows can be later refilled by finer sediments, or levelled out by storm wave-induced turbulence reaching the seafloor. The hydrofoils used in otter trawl to spread the net likewise incise the seafloor, but the furrows are much narrower. However, the ground rope of the net towed between the boards scratches the seafloor and levels the microscale topography along track widths of ca. 100 m.

High survivability of discarded invertebrates from bottom trawling fisheries

The impacts of bottom trawling in coastal ecosystems are significant, not only for targeted species but also for non-commercial ones. However, the critical gaps in scientific knowledge associated with these impacts on untargeted but functionally important species remain to be filled. This is notably the case for a great proportion of discarded invertebrates, for which only a handful of studies have investigated their survival. In this study, 600 individuals from six different benthic invertebrate species were collected in commercial conditions in the Bay of Biscay for short-term survival experiments. Overall, the observed survival after 100–130 h and the predicted survival via mixture models were very high (>93%) for Asterias rubens, Aphrodita aculeata, Buccinum undatum and Pagurus sp. Survival of Maja brachydactyla was lower though still high (>80% overall) and Atelecyclus undecimdentatus was more vulnerable to trawling and handling, with ∼50% of survival. Both showed 100% mortality when presenting carapace cracks and survival of M. brachydactyla was lower when missing appendages. No biotic nor abiotic parameters were correlated to survival, except injury class for M. brachydactyla and A. undecimdentatus. This study shows an overall high survival but highlights the fact that otter trawl fisheries may differentially affect discarded benthic invertebrates, therefore bringing light for future studies on longer-term impacts on benthic communities and coastal ecosystems. Furthermore, we recommend incorporating survival rates of discarded invertebrates in ecosystem-level modelling studies and encourage the use of detailed information and biomass reports of benthic invertebrates for a better management of natural populations and fisheries resources.

Bottlenose dolphins (Tursiops truncatus) in the north‐western Adriatic Sea: Spatial distribution and effects of trawling

Abstract Many species and populations of odontocetes have modified their behaviour to take advantage of feeding opportunities provided by fishing activities, with depredation of fishing gear being the most common type of adaptation. The northern Adriatic Sea has been identified as an important marine mammal area because of a regular occurrence of common bottlenose dolphins. Boat surveys were conducted within a 3,000 km2 sector of the Adriatic Sea off the coast of Veneto, Italy, between April and October 2018–2019. Based on 76 days at sea, 10,711 km of navigation, and 81 h 26 min of dolphin tracking, this study contributes novel quantitative information on dolphin spatial distribution, and on their occurrence in the wake of beam trawlers, otter trawlers, and midwater pair trawlers. A combined generalized additive model and generalized estimation equation framework indicated that trawling—along with other physiographic, biological and anthropogenic variables—influenced dolphin distribution. In days of trawling, the chance of encountering dolphins increased by ~4.5 times (95% confidence interval 1.8–11.0) near active beam trawlers, by ~16.0 times (7.1–36.0) near otter trawlers, and by ~28.9 times (12.0–69.6) near midwater pair trawlers. Spatial modelling was used to create maps of predicted distribution, suggesting differences in habitat use between trawling and no‐trawling days. Spatial modelling for all days identified a dolphin distribution hotspot of 832 km2, situated off the Po river delta. Evidence contributed by this study can be used to inform management action within one of the world's areas most heavily impacted by fishing and other human encroachment. Such management action would help enforce the European Union's Habitats Directive and Marine Strategy Framework Directive, while also informing EU's Maritime Spatial Planning.

Investigating the representativeness of onboard sampling trips and estimation of discards based on clustering

Onboard observer programs are key to collect data for fishery dynamics analysis and species bycatch and discard estimation. Major sources of bias of onboard sampling for catch data collection concern the vessel selection method, representativeness of catch sampling data and changes in fishing behavior in the presence of observers. This study was motivated by the worldwide issue of low number of vessels included in the sampling frame due to several types of refusals, and the representativeness of the samples for obtaining accurate catch estimates. Analyses were performed using trips from a bottom otter trawl (target fleet) and from the sampling frame, with and without observers onboard. Multivariate analysis of trips was conducted using logbook data, including landings, trip duration, fishing hours and spatial information, but not discards. Two groups of fishing trips were identified within the target fleet during the study period (2012−2015) with distinct fishing regimes, total landings and main landed species. Vessels from the sampling frame were representative of the target fleet and no observer effects occurred in the sampled trips. These findings support the assertion that the sampling frame used is an indicator fleet and analogous to a reference fleet. Cluster-based discard approaches improved the discard estimates precision, when compared to the fleet-based, in particular the cluster stratified approach. Our study suggests that the implementation of a threshold on the species prevalence in discards may potentially introduce bias in the estimates. Discard estimation without applying a threshold, with fleet- or cluster-based approach, should be ultimately dictated by end-users need, based on the trade-off between bias and variance.

Exploring epidermal mucus protease activity as an indicator of stress in Atlantic sturgeon (Acipenser oxyrinchus oxyrhinchus).

Atlantic sturgeon are anadromous fish that spend much of their life in near-shore environments. They are designated as "threatened" by the Committee on the Status of Endangered Wildlife in Canada and listed by the IUCN as "near threatened." In Canada, Atlantic sturgeon support small commercial fisheries in the Saint John River, New Brunswick, and the St. Lawrence River, Quebec. While occupying the marine environment, the species is susceptible to various anthropogenic stressors, including by-catch in trawl fisheries and through interactions with coastal engineering projects such as tidal power development. Atlantic sturgeon are also susceptible to implantation of acoustic tags used by researchers to study their movement ecology. These stressors can cause physiological and behavioural changes in the fish that can negatively impact their viability. Because the species are commercially important, and are also of conservation concern, it is important to understand stress responses of Atlantic sturgeon to better mitigate the effects of increased industrial activity in the coastal zone. This study used proteomics to identify and characterize protease activity and identify putative novel protein biomarkers in the epidermal mucus of Atlantic sturgeon. Changes in protein profiles in Atlantic sturgeon epidermal mucus as a result of by-catch and surgery stress were investigated using one-dimensional sodium dodecyl sulphate-polyacrylamide gel electrophoresis and mass spectrometry. Proteolytic activity was identified and characterized using inhibition zymography, which provided information on the classes of proteases that are associated with stress. Samples were collected from Atlantic sturgeon on the Minas Basin, Nova Scotia, Canada, after capture by brush weir and otter trawl, and after surgical implantation of a V16-69 kHz VEMCO acoustic tag. Significant proteins found in the epidermal mucus include various inflammatory proteins, with calmodulin and complement 9 found ubiquitously, and more rarely lysosome C, identified in a brush weir capture sample. Serum albumin, a blood plasma protein, was another ubiquitous protein and verifies how the sample collection method provides a picture of the internal systems. Protease activity was dominantly exhibited by matrix metalloproteases and serine proteases in all sample collections, with serine proteases more active in otter trawl captures than in brush weir captures. By identifying potential protein biomarkers of stress, this study is an example of a non-invasive method for measuring stress in Atlantic sturgeon. Understanding the defence mechanism and release of non-specific biomarkers can be used to improve conservation regulations, as well as to contribute to the limited scientific knowledge on the stress response of Atlantic sturgeon.

Hydrodynamic performance of a newly designed biplane-type hyper-lift trawl door for otter trawling

A biplane-type hyper-lift trawl door is newly designed to achieve stable handling while maneuvering, deploying, and stowing with excellent hydrodynamic performances for both bottom and midwater trawling. Flume-tank experiments were carried out to investigate the effect of the aspect ratio (λ, defined as the span b divided by chord c) on the hydrodynamic characteristics of the biplane-type hyper-lift trawl door with a camber ratio of 20% within a wide range of angles of attack (α). The maximum lift coefficients (CLmax) were 2.10 (λ = 1.0, α = 42°), 2.11 (λ = 1.6, α = 37°), and 2.06 (λ = 2.0, α = 28°). With a decreasing stall angle, the maximum lift coefficient remained constant (greater than 2). Considering the compactness and maneuverability of the biplane-type hyper-lift trawl door, an aspect ratio of 2 was adopted for the following experiments. The gap-chord ratio (G/c, G the gap between the fore and rear wings perpendicular to the chord direction) and stagger angle (θ, the included angle between the gap and the line connecting the leading edges of the fore and rear wings) expressing the positional relationship between the fore and rear wings of the biplane-type hyper-lift trawl door were set as 0.75, 0.9, and 1.0 and 20°, 30°, and 40°, respectively, to investigate the hydrodynamic characteristics further. The maximum lift coefficients and stall angles for the five biplane-type models were approximately 2.05 and 30°, respectively. Interestingly, these values were larger than the maximum lift coefficient and stall angle for a monoplane hyper-lift trawl door with the same aspect ratio (= 2) (CLmax = 1.78, α = 22°). This implies that the biplane-type structure contributes to enhancing the lift force and increasing the stall angle. The hydrodynamic forces of the fore and rear wings of the biplane-type hyper-lift trawl door were then calculated using a computational fluid dynamics approach. The lift coefficient for the rear wing was significantly higher than that for the fore wing before an angle of attack of 30° was reached. Nevertheless, the lift force distribution ratio for both the fore wing and the rear wing relative to the entire otter board approached 50%. On the other hand, the stall angle for the rear wing was approximately 30°, and that of the fore wing was around 38°. The fluid between the fore and rear wings tended to flow downstream and to successfully inhibit flow separation on the suction side of the fore wing.

Benchmarking global fisheries discards

Discarding by fisheries is one of the most wasteful human marine activities, yet we have few estimates of its scale. Reliable estimates of global discards are essential for sustainable fisheries management. Using United Nations Food and Agriculture Organization databases on country-specific landings, we estimated the discard rate and magnitude for global marine and estuarine capture fisheries using fishery-specific discard rates derived from direct observations and global gear-specific discard rates estimated within a Bayesian modelling framework. An estimated 9.1 million tonnes are discarded annually (95% uncertainty interval: 7–16 M t)—or 10.8% of the global catch (95% UI: 10–12%). Encouragingly, this is about half of the annual global discard rate estimated in the late 1980s. Trawl fisheries, especially demersal otter trawls, warrant intensified efforts to reduce discards. Periodic benchmarks of global discards are needed to assess the performance of reduction efforts.