Bycatch Mortality Research Articles

Review of: "Review of: Estimates of Atlantic Goliath Grouper (Epinephelus Itajara) Bycatch Mortality in Commercial Fisheries of the Southeastern Us From 2002 to 2022"

Review of: "Review of: Estimates of Atlantic Goliath Grouper (Epinephelus Itajara) Bycatch Mortality in Commercial Fisheries of the Southeastern Us From 2002 to 2022"

Evidence from interpretable machine learning to inform spatial management of Palau's tuna fisheries

AbstractStatic and dynamic area‐based management tools hold substantial potential to balance socioeconomic benefits derived from fisheries and costs from bycatch mortality of at‐risk species. Palau longline fisheries have high bycatch of at‐risk species including the olive ridley marine turtle and silky and blue sharks. This study analyzed a two decades‐long time series of observer and electronic monitoring datasets from the Palau distant‐water and locally‐based pelagic longline fisheries. An interpretable or explainable machine learning‐based modeling approach was used to derive spatially resolved species‐specific catch rate predictions. These models were conditioned on a suite of potentially informative environmental, bathymetric, ocean‐climate metric, vessel, monitoring system, and set‐specific operational predictors. Overall, there would be limited ecological tradeoffs from focusing fishing effort within primary catch rate hotspots for target bigeye and yellowfin tunas. Mean field prediction surfaces also defined catch rate hotspots for at‐risk species of silky and blue sharks, olive ridley turtle, and pelagic stingray, which did not overlap the hotspots for target species. The predicted target species hotspots, however, overlap olive ridley and pelagic stingray warmspots. Results also identify opportunities for temporally dynamic spatial management to control catch rates of target and bycatch species. Management of fishery operational predictors of fishing depth and soak duration present additional opportunities to balance catch rates of at‐risk bycatch and target species. A transition to employing fleetwide or vessel‐based output controls that effectively constrain the fishery would alter the spatial management strategy to focus on zones with the lowest ratio of at‐risk bycatch to commercial catch. Our findings support evidence‐informed evaluation of spatial management strategies and complementary measures to meet objectives for balancing socioeconomic benefits derived from target species catch with costs to threatened species.

An evaluation of the cetacean bycatch monitoring programme on board the Spanish set gillnet and bottom pair trawl fleets in the Bay of Biscay

Abstract Bycatch of cetaceans by European Union (EU) fleets is mostly monitored by onboard observers under the EU Data Collection Framework (DCF) programme. There is concern that the resulting estimates of cetacean bycatch mortality are biased and imprecise, because bycatch events are rare, sampling coverage is limited, and DCF observers prioritize the sampling of the discard of commercial species. Here, we explore the utility of the recently established Spanish cetacean bycatch monitoring programme to estimate cetacean bycatch mortality and compare it to the EU DCF programme. Data from October 2020 to September 2021 from this new Spanish programme were analysed to explore the following issues: (i) representativeness of the sampling frame; (ii) estimation of bycatch per unit of effort (BPUE); and (iii) sampling coverage. The probabilistic sampling design of the programme, with a record of refusals, has allowed identification and correction of biases in BPUE estimation. BPUE estimates for the bottom pair trawl fleet were recalculated to account for the fact that although boats work in pairs, each pair is monitored by a single observer located in a single boat. In relation to the sampling coverage, our results point to a need for levels up to 31% to achieve statistically robust bycatch rates. The current Spanish EU DCF at-sea sampling programme (which includes the sampling of bycatch of non-target protected species) and the Spanish (dedicated) cetacean bycatch monitoring programme provided similar results on cetacean bycatch for some of the ICES Divisions and Metiers monitored but sampling coverage of both is insufficient to obtain robust estimates.

Wire-snare bushmeat poaching and the large African carnivore guild: Impacts, knowledge gaps, and field-based mitigation

Wire-snare poaching for bushmeat is increasingly recognized as a serious threat to the species comprising the large African carnivore guild (African lion, African wild dog, spotted hyena, cheetah and leopard), with impacts described primarily through prey depletion and snaring by-catch mortality or injury. However, the species-specific impacts of snaring on this guild and on intraguild dynamics are not well-understood. These guilds evolved through competition and predation and have a diverse array of behaviors, space use, movements, diet, morphology, and densities; thus, it is logical to expect snaring impacts through prey depletion and by-catch will also be variable, subtle, and complex, yet significant. Utilizing the scientific literature and ongoing, long-term research we: 1) Summarize and describe the known and potential impacts of snaring by-catch and prey depletion on specific species in the guild and on intraguild dynamics, 2) Identify knowledge gaps and propose areas of future research to better understand and address snaring threats, and 3) Describe a successful strategy collaboratively merging conservation science work with resource protection, that has been implemented across carnivore strongholds in Zambia to combat the immediate threats from snaring on large carnivore populations. Collectively, this work can help mitigate snaring impacts while longer-term community-based solutions to the bushmeat crisis are developed and implemented.

Evaluating uncertainty to improve a common monitoring method and guide management decisions for diamond‐backed terrapins

AbstractDesigning monitoring with adequate certainty to evaluate management actions can be challenging, especially for elusive species in relatively inaccessible habitats. The diamond‐backed terrapin (Malaclemys terrapin) is considered a high priority species for management in all states within its range. Among key threats to terrapin populations is bycatch mortality in crab pots, prompting states throughout the species' range to consider crabbing regulations and creating a need for a monitoring approach able to evaluate the success of any new crabbing regulations to benefit terrapins. Because terrapins occupy extensive and often difficult to access estuarine habitat, it is hard to collect sufficient data to estimate population responses accurately and precisely for management. To assist state managers with decisions regarding monitoring bycatch reduction regulations, we modeled 12 years of terrapin capture‐recapture data. We used estimates from those models to simulate capture‐recapture data pre‐ and post‐implementation of bycatch reduction that resulted in a 0.20 absolute increase in mean apparent survival probability. Results indicated weak reliability of a monitoring approach using only manual seining of tidal creeks to detect a real management effect, with a positive management effect detected at the 95% certainty level only 34% of the time. When we considered 85% and 75% certainty thresholds, we detected a positive effect on survival among 61% and 75% of simulations, respectively. Low within‐year recapture probability and the effect of tide amplitude on terrapin availability indicate there is low feasibility of improving monitoring precision at a single site, requiring monitoring of more sites to improve confidence in the detection of the management effect. The number of sites monitored depends on the acceptable level of certainty. We recommend that researchers and management entities that use seining assess the level of certainty they require to evaluate management actions and increase the number of sites sampled to meet that level of certainty. Additionally, the use of multiple monitoring methods and integrated models should be explored to reduce uncertainty and to allow for easier monitoring of more populations over broader spatial scales.

An interview-based investigation of marine megafauna bycatch in the northern South China Sea

There is still limited information available regarding the patterns and extent of bycatch mortality in heavily-fished marine systems. To address this gap, we conducted an interview-based study to investigate the bycatch of five marine megafaunal species/categories in the northern South China Sea. Approximately two-thirds of the interviewed fishers reported encountering bycatch events, with sea turtles being the most commonly reported megafauna (33.6 % of respondents), followed by Indo-Pacific finless porpoises and whale sharks (both 12.4 %), and Indo-Pacific humpback dolphins (8.5 %). Dugongs were only reported by one respondent. Bycatch of these taxa was mainly associated with gillnets, trawl nets, and seine nets. Our estimates indicated a total of 7464 bycatch events annually, resulting in 1391 deaths for these five taxa. These numbers underscore the alarming impact of fisheries on marine megafauna, particularly on small cetaceans, which accounted for 66.1 % of the reported annual deaths, including 690 finless porpoises and 230 humpback dolphins. Lower bycatch levels were reported for all taxa in winter, potentially due to temporary reductions in fishery activity and/or animal migrations. Random forest model revealed that bycatch of humpback dolphins, finless porpoises, and sea turtles was primarily influenced by water depth and distance from the coast, while whale shark bycatch was influenced by administrative region. This study demonstrates that local ecological knowledge can provide a rapid means of obtaining basic information on the bycatch of marine megafauna, and serves as a sobering reminder that fisheries-related mortality contributes to the population declines of coastal small cetaceans.

Decadal increase in vessel interactions by a scavenging pelagic seabird across the North Atlantic

Fisheries waste is used by many seabirds as a supplementary source of food,1 but interacting with fishing vessels to obtain this resource puts birds at risk of entanglement in fishing gear and mortality.2 As a result, bycatch is one of the leading contributors to seabird decline worldwide,3 and this risk may increase over time as birds increasingly associate fishing vessels with food. Light-level geolocators mounted on seabirds can detect light emitted from vessels at night year-round.4 We used a 16-year time series of geolocator data from 296 northern fulmars (Fulmarus glacialis) breeding at temperate and arctic colonies to investigate trends of nocturnal vessel interactions in this scavenging pelagic seabird. Vessel attendance has progressively increased over the study period despite no corresponding increase in the number of vessels or availability of discards over the same time frame. Fulmars are highly mobile generalist surface feeders,5 so this may signal a reduction in available prey biomass in the upper water column, leading to increased reliance on anthropogenic food subsidies6 and increased risk of bycatch mortality in already threatened seabird populations. Individuals were consistent in the extent to which they interacted with vessels, as shown in other species,7 suggesting that population-level increases may be due to a higher proportion of fulmars following vessels rather than changes at an individual level. Higher encounter rates were correlated with lower time spent foraging and a geographically restricted overwintering distribution, suggesting an energetic advantage for these scavenging strategists compared with foraging for natural prey.

Quantifying longline bycatch mortality for pelagic sharks in western Pacific shark sanctuaries.

Marine protected areas are increasingly touted for their role in conserving large marine predators such as sharks, but their efficacy is debated. Seventeen "shark sanctuaries" have been established globally, but longline fishing continues within many such jurisdictions, leading to unknown levels of bycatch mortality levels. Using public data from Global Fishing Watch and Regional Fisheries Management Organizations, we quantified longline fishing within eight shark sanctuaries and estimated pelagic shark catch and mortality for seven pelagic shark species. Sanctuary mortality ranged from 600 individuals (Samoa) to 36,256 individuals (Federated States of Micronesia), equivalent to ~5% of hypothesized sustainable levels for blue sharks to ~40% for silky sharks, with high mortality levels in the Federated States of Micronesia, Palau, and the Marshall Islands. Unsustainable mortality rates were exceeded for silky sharks in two sanctuaries, highlighting a need for additional stock assessments and implementation of bycatch reduction measures. Big data integration workflows represent a transformative tool in fisheries management, particularly for data-poor species.

Hidden demographic impacts of fishing and environmental drivers of fecundity in a sea turtle population.

Fisheries bycatch is a critical threat to sea turtle populations worldwide, particularly as turtles are vulnerable to multiple gear types. The Canary Current is an intensely fished region, yet there has been no demographic assessment integrating bycatch and population management information of the globally significant Cabo Verde loggerhead turtle (Caretta caretta) population. Using Boa Vista island (Eastern Cabo Verde) subpopulation data from capture-recapture and nest monitoring (2013-2019), we evaluated population viability in combination with estimated regional bycatch rates (2016-2020) in longline, trawl, purse-seine and artisanal fisheries. We further evaluated current nesting trends in the context of bycatch estimates, existing hatchery conservation measures, and environmental (net primary productivity) variability in turtle foraging grounds. Current bycatch mortality rates present a risk of near extinction this century, when projecting the Boa Vista subpopulation with matrix population models. Bycatch reduction could increase finite population growth rate by 1.95%, and 1.76% for longline fisheries alone. Hatchery conservation increases hatchling production and reduces extinction risk, but cannot achieve population growth alone. Short-term increases in nest counts (2013-2021), putatively driven by temporary increases in net primary productivity, may be crucially masking ongoing long-term population declines. When fecundity was linked to net primary productivity, our hindcast models simultaneously predicted these opposing long-term and short-term trends. Consequently, our results show conservation management must diversify from land-based management. The masking effect we found has broad-reaching implications for monitoring sea turtle populations worldwide, demonstrating the importance of directly estimating adult survival and that nest counts might inadequately reflect underlying population trends. This article is protected by copyright. All rights reserved.

Acoustic telemetry provides insights for improving conservation and management at a spawning aggregation site of the endangered Nassau grouper (Epinephelus striatus)

Nassau grouper (Epinephelus striatus, Serranidae) were one of the most important fishery species in the Caribbean, but are now listed as critically endangered. Population collapse in most places occurred primarily by fishers targeting their spawning aggregation sites, where thousands of fish gathered during full moon periods from December to April. Identifying management options for protecting this vulnerable period is crucial for their survival. In the US Virgin Islands, we tagged 24 Nassau grouper with acoustic transmitters at a fish spawning aggregation (FSA) site called the Grammanik Bank, a 1.5 km2, marine protected area (MPA) closed each year from February to April. We tracked their movements over a seven-year period and found that the small MPA protected Nassau grouper only 65% of time during the spawning season. Nassau grouper arrived at the FSA from January to May, with a peak in March, and made daily roundtrip migrations (1-25 km) from the FSA site. Females swam twice as far as males, while males were more sedentary and remained longer (22 d) at the FSA than females (9 d); behavioural characteristics that may cause differential bycatch mortality among sexes. We also found nearly half of all fish were temporary residents within 5 km of the FSA site for 2 to 6 months after the spawning season. This mismatch of MPA size and timing of closed season provided the impetus for developing new guidelines for protection. The large acoustic array allowed us to calculate spatially relevant areas useful for conservation including courtship arena (1.2 km2), staging area (4.7 km2) and primary migration pathways (9.6 km2) used by Nassau grouper during spawning season. To improve conservation goals of this endangered species, increasing the Grammanik Bank seasonal closure to five months and its size to 6.4 km2 would increase protection to >96%. These spatial and temporal changes to existing management regulations incorporate the best available data to improve conservation goals by reducing potential bycatch mortality, eliminating disturbance of spawning fish, and protecting critical spawning habitat for Nassau grouper. These guidelines may also be applied more broadly to other Nassau grouper FSA sites located on large insular or continental shelfs areas throughout the Caribbean.

Reconsidering the longline ban in the Galapagos Marine Reserve

In 2000, longline fishing was banned inside the Galapagos Marine Reserve to prevent illegal fishing of sharks and bycatch of endangered, threatened, and protected (ETP) species. Despite local management institutions possess one of the most sophisticated control and surveillance systems in the Eastern Tropical Central Pacific, statistical and anecdotic evidence suggest the longline ban has been ineffective in eradicating illegal, unreported and unregulated (IUU) fishing for the last two decades. This short communication examines the legal, institutional, and socio-economic factors that have prevented the effective implementation of the longline ban and proposes an ecosystem approach to fisheries to maintain bycatch mortality rates below biologically based limits, facilitating the recovery of ETP species while safeguarding a sustainable development of the Galapagos small-scale tuna fishery. Significant investments in science, technology, and innovation are necessary to encourage gradual and adaptive improvements in fishing practices to reduce IUU fishing and bycatch.

Bycatch of the Indo-Pacific bottlenose dolphin (Tursiops aduncus) in gillnet fisheries off Amakusa-Shimoshima Island, Japan

A year-round resident population of the Indo-Pacific bottlenose dolphin (Tursiops aduncus) inhabits the waters off Amakusa-Shimoshima Island (32°25′N, 130°05´E), in Japan. The effect of bycatch in gillnet fisheries on the Amakusa population was examined. Population size in 2007 and 2008 was estimated at 230 individuals (CV = 2.5%) and 216 individuals (CV = 2.1%), respectively, based on a mark-recapture technique. The magnitude of bycatch was evaluated by analysing interview surveys (263 gillnetters) during these two years. Minimum numbers of dolphin bycatch were 12 individuals in 2007 and 14 individuals in 2008. Most of the dolphins, which were captured by bottom-set gillnets, were considered to be Indo-Pacific bottlenose dolphins for the following reasons: (1) two individuals were identified based on DNA analysis; (2) only Tursiops sp. and finless porpoises were found in the sighting survey by ferry boats, and fishermen can distinguish between the two; and (3) the seasonal and spatial distribution of bycatch corresponded well to habitat use patterns of the Amakusa population. If the US potential biological removal (PBR) approach is used it estimates two individuals per year, which is much lower than the minimum bycatch numbers of 12–14 individuals per year (5.2–6.5% of abundance estimates). Reducing bycatch mortality caused by bottom-set gillnets is essential for the effective conservation of Indo-Pacific bottlenose dolphins in Amakusa, Japan.

Environmental Characteristics Associated with the Presence of the Pelagic Stingray (Pteroplatytrygon violacea) in the Pacific High Sea

The pelagic stingray (Pteroplatytrygon violacea), perhaps the only stingray to inhabit open ocean waters, is highly interactive with longline and purse seine fisheries. The threat to P. violacea posed by high bycatch mortality has received widespread attention. To date, the environmental preference of P. violacea, which is important in designing conservation and management measures, has not been well studied. Based on data collected during a 2016–2019 survey in the Pacific Ocean by national observers of tuna longline fisheries, the relationship between the presence of P. violacea and spatiotemporal and environmental variables was first analyzed using the Generalized Additive Model. The results showed that geographic location (latitude and longitude) was the most influential variable. Monthly, P. violacea is frequently present in the Pacific high sea from December to May. The El Niño–Southern Oscillation had a significant impact on the presence of P. violacea in the Pacific high sea, with both the cold (Ocean Nino Index <−0.5) and warm (Ocean Nino Index >1) phases leading to a decrease in its presence. Regarding the environmental factors, we found that high presence was associated with low salinity (33.0~34.5 psu), a relatively high concentration of chlorophyll (0.2–0.35 mg/m3), and warm water (>20 °C). P. violacea was most likely observed in the waters offshore, closer to seamounts, and with water depths between 4000 and 5000 m. Four areas, including those east of the Solomon Islands and east of Kiribati, areas west of the Galapagos Islands, and areas near the coastal upwelling of northern Peru, related to upwelling systems or seamounts, were identified as the potential key habitats of P. violacea. Predicted distribution maps showed a significant seasonal variation in the presence of P. violacea. Moreover, the yearly change in the presence of P. violacea in the Pacific high sea indicated a possible decreasing trend in recent years. The information first provided here is essential for developing conservation and management measures for P. violacea to prevent the unavoidable ecological consequences of bycatch or other anthropogenic factors.

Fishermen’s perception of interactions between fisheries and cetaceans in the Bulgarian Black Sea area

The eastern part of Bulgaria entirely borders the Black Sea, which defines it as a maritime country. The Bulgarian coast is an important fishing region for local people and has a high potential for interactions between fisheries and cetaceans. Depredation caused by cetaceans and damage to fishing gears can potentially lead to substantial economic loss for fishermen, while cetacean bycatch raises conservation concerns. Over the period 2016 - 2019, we conducted face-to-face interviews with fishermen in local fishing areas to better understand the fisheries – cetacean interactions in the Bulgarian part of the Black Sea. The research objectives were to identify the current fishermen’s attitudes toward cetaceans, understand the damage caused by local marine mammals to the different types of fishing gear, and gather fishermen’s proposals to resolve the problems. To record all of this, a specially designed structured survey was conducted. The results showed that fishermen’s attitude towards cetaceans was mostly positive and was not influenced by the type of fishing gear used. We found that fisheries - cetacean interactions are frequent, especially with dalyans gear and set gillnets. Cetacean bycatch mortality was reported to be highest for set gillnets, and the most vulnerable species was the Phocoena phocoena. Although interview data may be biased due to differences in perceptions and experience of the interviewees, and therefore should be interpreted with caution, this method allowed us to cover multiple types of interactions between cetaceans and Bulgarian fisheries.

Spatiotemporal relationships of threatened cetaceans and anthropogenic threats in the lower Yangtze system

The impacts of fisheries interactions on cetaceans can be challenging to determine, often requiring multiple complementary investigative approaches. The Yangtze finless porpoise (Neophocaena asiaeorientalis asiaeorientalis), one of the few Critically Endangered cetaceans, is endemic to the middle-lower Yangtze drainage, a system impacted by multiple anthropogenic pressures. Bycatch mortality is implicated in regional porpoise decline, but the significance and dynamics of porpoise interactions with fishing activities and other threats remain poorly understood. We conducted boat-based surveys to map seasonal distributions and spatial congruence of porpoises and two potential threats (fishing and sand-mining), and an interview survey of fishing communities to understand temporal patterns and drivers of regional fishing activity, across Poyang Lake and the adjoining Yangtze mainstem. Variation in harmful and non-harmful gear use (non-fixed nets versus static pots and traps) between these landscapes might be an important factor affecting local porpoise status. Within Poyang Lake, spatial correlations between porpoises and threats were relatively weak, seasonal porpoise and threat hotspots were located in different regions, and two protected areas had higher porpoise encounter rates and densities than some unprotected sections. However, porpoise hotspots were mostly in unprotected areas, threats were widely observed across reserves, and more fishing and sand-mining was seasonally observed within reserves than within unprotected areas. Compared to null distributions, porpoises were detected significantly closer to fishing activities in summer and further from sand-mining in winter, indicating possible spatial risks of gear entanglement and disturbance. Reported porpoise bycatch deaths are associated with fixed and non-fixed nets, hook-based gears, and electrofishing. Longitudinal patterns in reported gear use indicate that hook-based fishing has decreased substantially and is generally conducted by older fishers, and significantly fewer respondents now practice fishing as their sole source of income, but electrofishing has increased. This combined research approach indicates a continued potential risk to porpoises from changing fisheries interactions and other threats, highlighting the importance of fishing restrictions and appropriate support for fishing communities impacted by this legislation. A potential “win–win” for both biodiversity and local livelihoods could be achieved through wider use of static pots and traps, which are not associated with bycatch deaths.

Spatial and temporal genetic stock composition of river herring bycatch in southern New England Atlantic herring and mackerel fisheries

Anadromous river herring (alewife and blueback herring) persist at historically low abundances and are caught as bycatch in commercial fisheries, potentially preventing recovery despite conservation efforts. We used newly established single-nucleotide polymorphism genetic baselines for alewife and blueback herring to define fine-scale reporting groups for each species. We then determined the occurrence of fish from these reporting groups in bycatch samples from a Northwest Atlantic fishery over four years. Within sampled bycatch events, the highest proportions of alewife were from the Block Island (34%) and Long Island Sound (22%) reporting groups, while for blueback herring the highest proportions were from the Mid-Atlantic (47%) and Northern New England (24%) reporting groups. We then quantified stock-specific mortality in a focal geographic area (∼3500 km2, including Block Island Sound) of high bycatch incidence and sampling effort, where the most accurate estimates of mortality could be made. During this period, we estimate that bycatch took about 4.6 million alewife and 1.2 million blueback herring, highlighting the need to reduce bycatch mortality for the most depleted river herring stocks.

A Comparison of Circle and J Hook Performance Within the Grenadian Pelagic Longline Fishery

The use of circle hooks in pelagic longline fisheries has often been proposed as a minimally disruptive means to minimize bycatch mortality. However, most studies have been in large scale fisheries, not the small-vessel fleets seen in many Caribbean countries. Size 16/0 circle hooks and 9/0 J hooks were alternated over 26 sets to assess the performance of circle hooks in the Grenadian pelagic longline fishery. Catch, mortality, hook location, length, and grade of fish were compared between hook types. No species-specific differences in at-vessel mortality rate or yellowfin tuna grade were found between hook types. However, significantly fewer billfish collectively (t = 2.36, p = 0.028), and sailfish specifically (t = 3.04, p = 0.005), were caught on circle hooks. Additionally, tunas caught with circle hooks had a 69% greater chance of external hooking compared to J hooks (χ2 = 4.38, p = 0.036). All other species analyzed had similar catch rates regardless of hook type (p < 0.05), including yellowfin tuna. The results of this study suggest that the Grenadian pelagic longline fishery – and other Caribbean fleets, by extension – could reduce impacts on billfish stocks by using 16/0 circle hooks without incurring negative effects on yellowfin tuna catch rate or grade. With many bycatch species populations at depressed levels, these results also provide further evidence that circle hooks should be the recommended terminal gear type when using a bycatch mitigation approach to manage pelagic longline fisheries while maintaining target catches for local and export markets.

Bycatch in a Coastal Black Drum Trotline Fishery

AbstractBycatch remains a challenging issue for many fisheries across the globe. Due to the cost and effort of (bycatch) observer programs, many fisheries—particularly those not in U.S. federal waters—operate with little information about bycatch. The Black Drum Pogonias cromis and Sheepshead Archosargus probatocephalus fishery in Louisiana comprises one of the largest fisheries by volume and in recent years has exceeded US$5 million in annual value. The vast majority of Black Drum and Sheepshead are harvested by the baited trotline fishery that operates in state waters. Very little is known about the bycatch from this fishery; however, because of the magnitude of the fishery quantifying bycatch is needed for the management of this resource. We observed 59 baited trotline sets on 13 different dates spanning 2 years of sampling (2020–2021). The total bycatch (n = 1,392) was similar in number to the harvest of the target species (n = 1,265). However, most bycatch species were nongame fishes and were released alive. Of all the bycatch caught, only 4% was dead and the majority of that 4% (47 out of 57 fish) consisted of Gafftopsail Catfish Bagre marinus and Hardhead Catfish Ariopsis felis, both species for which there is little fishery and no known issues with the populations. We also found that catch rates for bycatch did not vary by season or between the two areas fished. The catch rate of bycatch did not increase as the catch rate of target species increased; although, as expected, bycatch mortality did significantly increase as water temperature increased. Overall, the baited trotline fishery for Black Drum and Sheepshead catches about one individual of bycatch for one target individual; however, the overall extremely low bycatch mortality (of common species) suggests that this fishery operates with few adverse effects on nontarget species.

Analysis of At-Haulback Mortality and Influencing Factors of Indian Ocean Swordfish (Xiphias gladius)

The at-haulback mortality of swordfish (Xiphias gladius), a highly migratory fish, in the Indian Ocean during tuna longline fishery is a concern of the Indian Ocean Tuna Commission Fisheries Management. We obtained the data of 1144 swordfish recorded in 1925 operations in the Indian Ocean by Chinese tuna longline observers from 2012–2018. A generalized linear model was used to analyze the at-haulback mortality of swordfish and the potential influencing factors. The overall mortality rate of swordfish was 64.0%, and the average female size was 166.3 cm (SD = 32.5 cm), with an observed at-haulback mortality rate of 64.3%. The average male size was 155.1 cm (SD = 25.6 cm), which was smaller than females. The observed at-haulback mortality rate was 63.6%. No significant difference was observed between the sexes. Quarter, longitude, hook type, and Lower Jaw-Fork Length (LJFL) had a significant effect on the at-haulback condition when fish were retrieved onboard. Longitude and LJFL had a significant effect on the at-haulback mortality of swordfish. When the capture position was closer to the east, the at-haulback mortality decreased and LJFL increased. The interaction term of hook type and LJFL had a significant effect on at-haulback mortality. When using the circle hook and other hook types, the greater the LFJL, the greater the mortality rate; the opposite was true for Japanese tuna hooks. We provide information for understanding the at-haulback mortality of swordfish bycatch by Chinese tuna longline fishing fleets in the Indian Ocean and its influencing factors, which will help the future conservation and management of swordfish.

Maximizing salmonid bycatch survival with passively operated commercial fish traps

Commercial fish traps were recently implemented at a limited scale in the lower Columbia River in efforts to selectively harvest hatchery-origin fishes with minimal mortality of wild-origin salmonids listed under the U.S. Endangered Species Act. Prior studies of a modified passive trapping technique demonstrated nearly 100 % survival among adult salmonids released as bycatch. Building upon this research, additional survival studies for adult coho salmon, spring-run Chinook salmon, and summer-run Chinook salmon were conducted between 2019 and 2021 at two separate fish trap sites in the lower Columbia River. Over three years of study, we used mark-recapture and net pen holding methodologies to estimate post-release survival of Chinook salmon and coho salmon, respectively. Evaluating detections of Chinook salmon tagged and released from a passively operated fish trap in 2019, detection at Bonneville Dam over a mean 6.5 d upriver migration (167 km) was 1.000 (95 % CI: S ≥ 0.970) for the sample genetically assigned to populations originating upriver of Bonneville Dam. Through two separate net pen holding studies, post-release survival of coho salmon was estimated at 1.000 (95 % CI: S ≥ 0.975) over 4 d in 2020 and 0.965 (95 % CI: 0.948 ≤ S ≤ 0.969) over 6 d in 2021. Given that these analyses lacked control groups to adjust survival estimates for confounding mortality effects, study results are inherently conservative. Ultimately, the findings of our research support the conclusions of prior studies and further suggest that passively operated fish traps may allow for selective harvesting of targeted fish stocks with little to no mortality of adult salmonid bycatch.