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Fisheries bycatch is one of the biggest threats to marine mammal populations. A literature review was undertaken to provide a comprehensive assessment and synopsis of gear modifications and technical devices to reduce marine mammal bycatch in commercial trawl, purse seine, longline, gillnet and pot/trap fisheries. Successfully implemented mitigation measures include acoustic deterrent devices (pingers) which reduced the bycatch of some small cetacean species in gillnets, appropriately designed exclusion devices which reduced pinniped bycatch in some trawl fisheries, and various pot/trap guard designs that reduced marine mammal entrapment. However, substantial development and research of mitigation options is required to address the bycatch of a range of species in many fisheries. No reliably effective technical solutions to reduce small cetacean bycatch in trawl nets are available, although loud pingers have shown potential. There are currently no technical options that effectively reduce marine mammal interactions in longline fisheries, although development of catch and hook protection devices is promising. Solutions are also needed for species, particularly pinnipeds and small cetaceans, that are not deterred by pingers and continue to be caught in static gillnets. Large whale entanglements in static gear, particularly buoy lines for pots/traps, needs urgent attention although there is encouraging research on rope-less pot/trap systems and identification of rope colours that are more detectable to whale species. Future mitigation development and deployment requires rigorous scientific testing to determine if significant bycatch reduction has been achieved, as well as consideration of potentially conflicting mitigation outcomes if multiple species are impacted by a fishery.

1. Assessment and monitoring of exploited fish populations are challenged by costs, logistics and negative impacts on target populations. These factors therefore limit large-scale effective management strategies. 2. Evidence is growing that the quantity of eDNA may be related not only to species presence/absence, but also to species abundance. In this study, the concentrations of environmental DNA (eDNA) from a highly prized sport fish species, Lake Trout Salvelinus namaycush (Walbaum 1792), were estimated in water samples from 12 natural lakes and compared to abundance and biomass data obtained from standardized gillnet catches as performed routinely for fisheries management purposes. To reduce environmental variability among lakes, all lakes were sampled in spring, between ice melt and water stratification. 3. The eDNA concentration did not vary significantly with water temperature, dissolved oxygen, pH and turbidity, but was significantly positively correlated with relative fish abundance estimated as catch per unit effort (CPUE), whereas the relationship with biomass per unit effort (BPUE) was less pronounced. 4. The value of eDNA to inform about local aquatic species distribution was further supported by the similarity between the spatial heterogeneity of eDNA distribution and spatial variation in CPUE measured by the gillnet method. 5. Synthesis and applications. Large-scale empirical evidence of the relationship between the eDNA concentration and species abundance allows for the assessment of the potential to integrate eDNA within fisheries management plans. As such, the eDNA quantitative method represents a promising population abundance assessment tool that could significantly reduce the costs associated with sampling and increase the power of detection, the spatial coverage and the frequency of sampling, without any negative impacts on fish populations.

Recent research on ocean health has found large predator abundance to be a key element of ocean condition. Fisheries can impact large predator abundance directly through targeted capture and indirectly through incidental capture of nontarget species or bycatch. However, measures of the global nature of bycatch are lacking for air-breathing megafauna. We fill this knowledge gap and present a synoptic global assessment of the distribution and intensity of bycatch of seabirds, marine mammals, and sea turtles based on empirical data from the three most commonly used types of fishing gears worldwide. We identify taxa-specific hotspots of bycatch intensity and find evidence of cumulative impacts across fishing fleets and gears. This global map of bycatch illustrates where data are particularly scarce—in coastal and small-scale fisheries and ocean regions that support developed industrial fisheries and millions of small-scale fishers—and identifies fishing areas where, given the evidence of cumulative hotspots across gear and taxa, traditional species or gear-specific bycatch management and mitigation efforts may be necessary but not sufficient. Given the global distribution of bycatch and the mitigation success achieved by some fleets, the reduction of air-breathing megafauna bycatch is both an urgent and achievable conservation priority.

Balasubramanian, A.; Meenakumari, B.; Pravin, P.; Boopendranath, M.R.; Rajani, M., and Erzini, K., 2024. Mesh and hook selectivity of larger carangid Caranx ignobilis (Forsskal, 1775), captured along the Kanyakumari coast of India. Journal of Coastal Research, 40(3), 554–562. Charlotte (North Carolina), ISSN 0749-0208. The larger carangid Caranx ignobilis is one of the key commercial fisheries operated by traditional fishermen on the Kanyakumari coast of Tamil Nadu, India, using larger meshed gillnet and drift hand lines. No selectivity study was conducted for this species in chosen study area; selectivity studies were conducted for C. ignobilis using different larger meshed gillnets having mesh sizes of 13.5, 14.0, 14.5 and 15.0 cm and hooks No. 5,6,7, and 8. Catch data obtained from gillnets and hooks were appropriated into various uninormal models (i.e. normal scale, normal location, log-normal, and gamma model) and a binormal model for finding the best fit that employs the share each length catch total (SELECT) methodology incorporated in Generalised Including Log-Linear N Estimation Technique (GILLNET) software. The estimated parameters of these models were evaluated using the statistical tools (i.e. model deviance, dispersion parameter, residual plots) to determine the best fit of the selectivity data. The binormal model under equal fishing power and fishing power proportional to hook size were found as best fit for the gillnet and hook selection data respectively. For all meshes and hooks tested, mesh size of 14.5 cm and hook No. 5 showed good catching efficiency capturing the respective modal length of fish 63.3 cm and 62.4 cm, which revealed that modal length of fish caught in the experimental mesh size is greater than the gears used by the traditional fishermen. Gillnet catch data appropriately converged into unimode in the selection curve, whereas the hook catch data appeared with bimode under the binormal model. Over dispersion was common phenomena in catch data obtained from gillnet and hooks because of the dispersed nature of population and the abundance of a larger size of fish in the study area.

The current level of annual incidental bycatch of gray seals Halichoerus grypus in the New England sink gillnet fishery is the highest for all marine mammal species in the USA. One way to evaluate the risk of bycatch is to examine the risk of encounter between an animal and fishing gear based on the animal’s habitat use in relation to fishing activity. Here we used satellite telemetry deployed on 30 gray seal pups in 2019 and 2020 to measure the risk of encounter with large-mesh sink gillnet fishing effort throughout the Gulf of Maine and southern New England. We estimated relative encounter risk within 30 min grid cells in each calendar quarter based on the overlap of seal presence and fishing effort, and then validated the expected risk based on bycatch events reported by independent observers on board fishing vessels. The relative risk of seals encountering gillnet fishing gear was highest off southeastern Massachusetts in spring. Patterns in the estimated encounter risk fit our expectation that relatively high levels of habitat use and fishing effort correspond to increased encounter risk. The approach taken here can be used to identify times and areas of high encounter risk to justify altered fishing practices for purposes of avoidance, or to target observer monitoring intended to characterize and quantify bycatch. Mitigation strategies will need to be continuously monitored and updated to incorporate new information as conflicts with fisheries and gray seals are likely to continue.

The ocean is a dynamic environment inhabited by a diverse array of highly migratory species, many of which are under direct exploitation in targeted fisheries. The time-scales of variability in the marine realm coupled with the extreme mobility of ocean-wandering species such as tuna and billfish complicates fisheries management. Developing eco-informatics solutions that allow for near real-time prediction of the distributions of highly mobile marine species is an important step towards the maturation of dynamic ocean management and eco-logical forecasting. Using 25 yr (1990–2014) of NOAA fisheries' observer data from the California drift gillnet fishery, we model relative probability of occurrence (presence–absence) and catchability (total catch per gillnet set) of broadbill swordfish Xiphias gladius in the California Current System. Using freely available environmental data sets and open source software, we explore the physical drivers of regional swordfish distribution. Comparing models built upon remotely sensed data sets with those built upon a data-assimilative configuration of the Regional Ocean Modelling System (ROMS), we explore trade-offs in model construction, and address how physical data can affect predictive performance and operational capacity. Swordfish catchability was found to be highest in deeper waters (>1,500 m) with surface temperatures in the 14–20°C range, isothermal layer depth (ILD) of 20–40 m, positive sea surface height (SSH) anomalies, and during the new moon (<20% lunar illumination). We observed a greater influence of mesoscale variability (SSH, wind speed, isothermal layer depth, eddy kinetic energy) in driving swordfish catchability (total catch) than was evident in predicting the relative probability of presence (presence–absence), confirming the utility of generating spatiotemporally dynamic predictions. Data-assimilative ROMS circumvent the limitations of satellite remote sensing in providing physical data fields for species distribution models (e.g., cloud cover, variable resolution, subsurface data), and facilitate broad-scale prediction of dynamic species distributions in near real time.

Fish community responses to long-term mass removal of planktivorous fish from eutrophicated Lake Tuusulanjärvi were examined during 1996–2018 by gill net test fishing, hydroacoustics with simultaneous trawling, virtual population analysis (VPA) and fish growth measurements. The gillnet catches of white bream and roach increased first while bream and bleak decreased but recovered in the early 2000s. Perch and pikeperch increased but ruffe decreased and remained low. According to hydroacoustics, pelagic fish biomass decreased. Smelt was the most abundant species at the beginning and the end of the monitoring period but bream dominated the fish biomass in most years. VPA calculations indicated a five- and threefold increase in the biomass of bream and white bream, respectively, during 2005–2011. Significant increases appeared in the growth of perch, bream and roach. Overall, the responses in the fish community structure and fish abundance to the food web management were slight and mostly masked by changes in reproduction and growth of fish and annual variability in environmental conditions such as temperature and water turbidity. Gillnet test fishing and echo sounding complemented each other well in monitoring the effects of management fishing. Where bream is one of the dominant species VPA is also recommended.

The selectivity of greater lizardfish (Saurida tumbil). which is one of the most abundant economic species caught by gillnets in the northeast of the Oman Sea. Iranian waters, was studied. Sampling was conducted from February to March 2021. Four types of gillnets with mesh sizes of 4.8. 6.3. 10.0. and 15.3 cm were used and 857 fish specimens were collected. The catch patterns, including (snagged, gilled. wedged, and entangled) for 5. tumbil were observed in gillnets. For 4.8 and 6.3 cm mesh sizes, more than 70% of the catch was mainly obtained by gilled. followed by wedged, and no found of entanglement. For 10.0 and 15.3 cm mesh sizes, the catch of 10.0 mesh size included 16% of fish caught from wedging, though most of the fish was caught by entanglement. In particular, all catches at 15.3 cm were due to entanglement. Estimation of gillnets selectivity for 5. tumbil was performed using the SELECT method. The SELECT method was used to fit three various gillnet selectivity models (log-normal, skew-normal, and bi-normal). Gillnets selectivity was best estimated by a bi-modal Selection curve. The mean lengths ±SE were estimated as 31.48±0.71. 40.3±0.97.40.1±0.75 and43.9±1.05 cm for 4.8. 6.3.10.0 and 15.3 cm mesh sizes, respectively. Mean lengths increased with increasing the mesh size. Most of the fish caught in the 4.8 and 6.3 cm mesh sizes were below the first maturity length (Lmso). Considering the relative efficiency set as 0.5. that was L50 (50% retention length), the optimal mesh size was determined to be 10.0 cm. Therefore, to protect 5. tumbil stock and the sustainability of the fishing resource, the gillnet mesh size should be at 10.0 cm to manage 5. tumbil in this area.

Gillnet fisheries are widely thought to pose a conservation threat to many populations of marine mammals, seabirds, and turtles. Gillnet fisheries also support a significant proportion of small-scale fishing communities worldwide. Despite a large number of studies on protected-species bycatch in recent decades, relatively few have examined the underlying causes of bycatch and fewer still have considered the issue from a multitaxon perspective. We used 3 bibliographic databases and one search engine to identify studies by year of publication and taxon. The majority of studies on the mechanisms of gillnet bycatch are not accessible through the mainstream published literature. Many are reported in technical papers, government reports, and university theses. We reviewed over 600 published and unpublished studies of bycatch in which causal or correlative factors were considered and identified therein 28 environmental, operational, technical, and behavioral factors that may be associated with high or low bycatch rates of the taxa. Of the factors considered, 11 were associated with potential bycatch reduction in 2 out of the 3 taxa, and 3 factors (water depth, mesh size, and net height) were associated with trends in bycatch rate for all 3 taxa. These findings provide a basis to guide further experimental work to test hypotheses about which factors most influence bycatch rates and to explore ways of managing fishing activities and improving gear design to minimize the incidental capture of species of conservation concern while ensuring the viability of the fisheries concerned. Se cree extensamente que las pesquerías que utilizan redes significan una amenaza para la conservación de muchas poblaciones de mamíferos marinos, aves marinas y tortugas. Las pesquerías con redes también sustentan a una proporción significativa de comunidades pesqueras a nivel mundial. A pesar del gran número de estudios sobre la captura incidental de especies protegidas realizados en la década reciente, relativamente pocos han examinado las causas subyacentes de la captura incidental y muchos menos han considerado el tema desde una perspectiva multi-taxón. Utilizamos tres bases de datos bibliográficas y un buscador para identificar a los estudios por año de publicación y por taxón. La mayoría de los estudios sobre los mecanismos de la captura incidental con redes de pesca no son accesibles por medio de la literatura publicada convencionalmente. Muchos estudios están reportados en artículos técnicos, reportes del gobierno y tesis universitarias. Revisamos más de 600 estudios publicados y no-publicados sobre la captura incidental, en los que los factores correlativos o causales fueron considerados e identificados dentro de 28 factores ambientales, operativos, técnicos y de comportamiento que pueden asociarse con las tasas altas o bajas de captura colateral de los taxa. De los factores considerados, once estuvieron asociados con la reducción potencial de la captura incidental en dos de los tres taxa, y tres factores (profundidad del agua, tamaño de la malla y altura de la red) estuvieron asociados con las tendencias de la tasa de captura incidental para los tres taxa. Estos hallazgos proporcionan una base para guiar más allá al trabajo experimental para probar hipótesis sobre cuáles factores influyen sobre las tasas de captura incidental y para explorar formas de manejo de las actividades de pesca y el mejoramiento del diseño del equipo para minimizar la captura incidental de las especies de interés de conservación, a la vez que se asegura la viabilidad de las pesquerías involucradas.

Marine megafauna are critical for marine ecosystem health and their removal can cause food webs to collapse. Methods to reduce marine megafauna mortality can result in conflict between scientists, conservationists, fishers and fisheries management due to real or perceived effects on target catch, income and food security. Sensory deterrents have been used in attempts to mitigate bycatch and retain target catch quantity and quality. Here, we completed a systematic review of 116 papers, plus 25 literature reviews published between 1991 and 2022, to investigate potential for sensory deterrents to mitigate bycatch across four marine megafauna taxonomic groups (marine mammals, sea turtles, seabirds and elasmobranchs). Lights on gillnets are the only technology so far to result in significant bycatch reductions across all four taxonomic groups. It is difficult to make generalisations about the efficacy of sensory deterrents and their ability to deliver consistent bycatch reductions. The efficacy of each method is context dependent, varying with species, fishery and environmental characteristics. Further research is recommended for field studies assessing bycatch mitigation in all sensory deterrents, including combinations of deterrents, to assess effects on target and non-target species. The associated issues of habituation, habitat exclusion and foraging around fishing gear are important, although reducing mortality of vulnerable species should remain the highest priority for conservation and preserving ecosystems that fishers depend on. Multiple complementary measures will be required to achieve consistent bycatch reduction targets in many fisheries, of which sensory deterrents could play some part if implemented appropriately.