Drift Gillnet Fishery Research Articles

Bycatch in drift gillnet fisheries: A sink for Indian Ocean cetaceans

AbstractIn 1992, the UN banned the use of large‐scale pelagic driftnets on the high seas (UNGA Resolution 46/215). Three decades later, however, drift gillnets remain one of the primary fishing gears in the Indian Ocean, accounting for approximately 30% of tuna catches in this ocean. Recent estimates indicate that several million small cetaceans have been killed in Indian Ocean gillnets over the past few decades. National agencies and the regional fisheries management organization charged with managing tuna fisheries, the Indian Ocean Tuna Commission, have yet to effectively document the bycatch of small cetaceans in these fisheries. Here, we review current information on cetacean bycatch in Indian Ocean drift gillnets and propose potential solutions to this important conservation issue.

Estimating abundance of an elusive cetacean in a complex environment: Harbor porpoises (Phocoena phocoena) in inland waters of Southeast Alaska

The harbor porpoise (Phocoena phocoena) is common in temperate waters of the eastern North Pacific Ocean, including Southeast Alaska inland waters, a complex environment comprised of open waterways, narrow channels, and inlets. Two demographically independent populations are currently recognized in this region. Bycatch of porpoises in the salmon drift gillnet fisheries is suspected to occur regularly. In this study, we apply distance sampling to estimate abundance of harbor porpoise during ship surveys carried out in the summer of 2019. A stratified survey design was implemented to sample different harbor porpoise habitats. Survey tracklines were allocated following a randomized survey design with uniform coverage probability. Density and abundance for the northern and southern Southeast Alaska inland water populations were computed using a combination of design-based line- and strip-transect methods. A total of 2,893 km was surveyed in sea state conditions ranging from Beaufort 0 to 3 and 194 harbor porpoise groups (301 individuals) were detected. An independent sighting dataset from surveys conducted between 1991 and 2012 were used to calculate the probability of missing porpoise groups on the survey trackline (g[0]=0.53, CV=0.11). Abundance of the northern and southern populations were estimated at 1,619 (CV=0.26) and 890 (CV=0.37) porpoises, respectively. Bycatch estimates, which were only obtained for a portion of the drift gillnet fishery, suggest that mortality within the range of the southern population may be unsustainable. Harbor porpoises are highly vulnerable to mortality in gillnets, therefore monitoring abundance and bycatch is important for evaluating the potential impact of fisheries on this species in Southeast Alaska.

Comparing observed and unobserved fishing characteristics in the drift gillnet fishery for swordfish

Central to the success of partial coverage observer programs is the assumption that observed fishing behaviors are representative of unobserved fishing behaviors. Traditionally, this was a difficult assumption to test beyond comparing catch compositions for observed and unobserved trips. With the proliferation of vessel monitoring systems (VMS) however, a suite of trip- and set-level fishing characteristics can be engineered from vessel locations that facilitate quantitative comparisons between observed and unobserved behaviors. The U.S. West Coast drift gillnet (DGN) fishery for swordfish (Xiphias gladius) is a small fishery with a long history of bycatch concerns and an observer coverage rate around 20–30 %. We developed an integrated data set composed of VMS, logbook, landings, and observer data for the DGN fishery from 2013 to 2019. We used a suite of machine learning models to classify unobserved fishing behaviors and we characterized these fishing behaviors at the trip- and set-level. Analyses of the trip- and set-level metrics (e.g., trip duration, fishing depth, sea surface temperatures, distance from port, and catch per unit effort) revealed that operational characteristics of the fishery were largely similar for observer and unobserved trips and vessels, but also highly variable. In the fall season, observed trip distances were an average of 53 % (std. error 20 %) longer than unobserved trips for the same vessels. These longer trips occurred in water that was, on average, about 39 % (std. error 16 %) deeper. However, characteristics like amounts of fishing effort, revenue, and water temperatures were generally similar between observed and unobserved trips. We present a methodical workflow for data integration and feature engineering and a simple approach for assessing differences across fleet behaviors in space and time that could be applied to many fisheries with limited observer coverage. This approach could also be used to target specific strata within fisheries that may benefit from increases or shifts in observer coverage.

Setting the net lower: A potential low‐cost mitigation method to reduce cetacean bycatch in drift gillnet fisheries

Abstract Bycatch is the most significant threat to marine megafauna (sea turtles, marine mammals, elasmobranchs, seabirds) worldwide, and the leading cause of the decline of several cetacean species. The bycatch issue in the Indian Ocean is poorly understood, but high bycatch levels in gillnet fisheries have been documented for the past two decades, in both small‐scale and semi‐industrial fisheries. Unfortunately, methods to reduce bycatch are often unavailable, financially non‐viable or socially unacceptable to fishermen. Using a network of trained boat captains in the tuna drift gillnet fishery in the Arabian Sea, targeted catch and bycatch data were collected from 2013 to 2017 off the coast of Pakistan (northern Indian Ocean). Two fishing methods using multifilament gillnets were used: surface deployment and subsurface deployment (i.e. headline of net set below 2 m depth). Predicted catch rates for targeted species did not differ significantly between the two fishing practices, although a drop in tuna (6.2%) and tuna‐like (10.9%) species captures was recorded in subsurface sets. The probability of cetacean bycatch, however, was 78.5% lower in subsurface than in surface sets. Cetacean bycatch in tuna drift gillnet fisheries has the potential to be significantly reduced at a relatively low cost for fishers. However, further research with an appropriate sampling design and a large sample size is required to confirm the efficacy of the proposed mitigation method. The acceptability and adoption of subsurface setting by fishers also needs to be further investigated. Despite some limitations, this preliminary study also highlights the importance of crew‐based observer data as an alternative source of data when observers cannot be deployed on fishing vessels.

Profile and consumption risk assessment of trace elements in megamouth sharks (Megachasma pelagios) captured from the Pacific Ocean to the east of Taiwan

Focusing on 27 rare filter-feeding megamouth sharks (Megachasma pelagios) captured as a by-catch of drift gillnet fishery in the Pacific Ocean to the east of Taiwan, this study analyzes the concentrations of 24 elements in their muscle, discusses the bioaccumulation of each element and the correlation between different elements, and assesses the potential health risks of consuming megamouth shark muscle. Among the 24 elements, mean concentrations of Ga, Ag, Li, Bi, Hg, Co, and Cd were relatively low ranging from 10-3 to 10-1mg/kg, those of Pb, Ba, Mn, Ni, As, Cr, B, Sr, Cu, and Zn ranged from 10-1-101mg/kg, and those of Fe, Ca, Al, K, Mg, Ti, and Na were relatively high ranging from 101 to 103mg/kg. The toxic element content index was most significantly correlated with the concentration of Cu. Hence, this study recommends that the concentration of Cu could be used as an indicator of metal accumulation in megamouth shark muscle. The log bioconcentration factor (BCF) ranged from less than 0 to 7.85 in shark muscle. For elements with a concentration of less than 100μg/L in seawater, the log BCF was inversely proportional to their concentration in seawater. According to the correlation analysis, the accumulation of elements in muscle of megamouth sharks is primarily affected by the concentrations of dissolved elements in seawater, except that the accumulation of Hg, As, Cu, Ti, Al, and Fe appears to be mainly affected by feeding behaviors. The assessment of the health risk of consuming megamouth shark muscle showed that its total hazard index was greater than 1. This suggests that the long-term or high-frequency consumption of megamouth shark muscle may cause health hazards due to the accumulation of trace elements, particularly those with a large contribution of health risk, including As, Hg, and Cu.

ObsCovgTools: Assessing observer coverage needed to document and estimate rare event bycatch

Observer program design and evaluation often overlook the challenges of documenting rare-event bycatch. To support and facilitate consideration of threatened, endangered, and protected species bycatch in evaluating observer programs and assessing fisheries impacts, we developed a software tool to assess observer coverage with respect to several objectives for documenting or estimating rare-event bycatch. The ObsCovgTools package for the R programming language, also available as an online application, predicts observer coverage performance for a given total fishery effort in relation to three metrics: (1) the conditional probability of observing any bycatch given that bycatch occurred in the fishery and the probability of any bycatch in the total fishery effort, (2) the upper confidence limit for total bycatch when none is observed, and (3) precision (coefficient of variation) of the bycatch estimate. We describe the tool; explore how specific observer coverage targets for these metrics vary with total effort, BPUE, and dispersion index; and apply it to evaluate observer coverage in the California drift gillnet fishery. Our results underscore the importance of considering effort as well as percentage in assessing how well an observer program documents bycatch. We caution that rare species interactions may not be documented in many observer programs, and should be anticipated through a complementary risk assessment approach. The tool’s modular design and open source programming approach encourage adaptation and augmentation to address additional objectives or complexities in sampling design or estimation.

Change of relative fishing power index from technological development in the small yellow croaker drift gillnet fishery

Change of relative fishing power index from technological development in the small yellow croaker drift gillnet fishery

Convergence of fishers' knowledge with a species distribution model in a Peruvian shark fishery

Limited data on the spatial, environmental, and human dimensions of small‐scale fisheries hinder conservation planning, so the incorporation of fishers' local ecological knowledge may be a valuable way to fill data gaps while legitimizing management decisions. In Peru, vulnerable and poorly assessed juvenile smooth hammerhead sharks (Sphyrna zygaena) are the most commonly caught shark species in a small‐scale drift gillnet fishery. We conducted semistructured interviews with 87 hammerhead fishers in three major Peruvian ports to elucidate the spatiotemporal niche of the hammerhead fishery and environmental drivers of juvenile hammerhead catch. We also built a biophysical model of hammerhead distribution that correlated remotely sensed environmental variables with a spatially explicit fishery observer dataset. Overall, we found a consensus between fishers' knowledge and species distribution modeling. Sea surface temperature and chlorophyll‐a emerged as important environmental drivers of juvenile hammerhead catch, with both fishers' knowledge and the biophysical model identifying similar habitat preferences (~20–23°C and log chl‐a >−1.6 mg/m3). Participatory mapping of fishing grounds also corresponded to the spatiotemporal patterns of predicted hammerhead distribution. This study points to the utility of combining fishers' knowledge and biophysical modeling for spatial, temporal, and/or dynamic management of these sharks in Peru and in other data‐poor fisheries globally.

Community-level effects of spatial management in the California drift gillnet Fishery

Fisheries management interventions that protect certain species by redistributing fishing effort may generate unintended consequences for other species. In the California drift gillnet fishery for swordfish and sharks, a large spatial closure was implemented in 2001 to protect endangered leatherback turtles, which limited fishing effort to the Southern California Bight. Leatherback bycatch has since decreased, but the effects on other species have not been comprehensively examined. Here, we explore the effects of this closure on the community catch composition in the fishery and find that other protected species may have benefited, while catch per unit effort of major target species increased or was not significantly affected over the long term. However, a time-series analysis reveals that changes in catch trends across twenty species began at least five years before the closure was implemented, suggesting that previous regulatory measures or other drivers may also contribute to these trends. These results highlight the importance of comprehensive approaches that include the historical context when evaluating management outcomes.

Marine megafauna catch in southwestern Indian Ocean small-scale fisheries from landings data

The measurable impacts of small-scale fisheries on coastal marine ecosystems and vulnerable megafauna species (elasmobranchs, marine mammals and sea turtles) within them are largely unknown, particularly in developing countries. This study assesses megafauna catch and composition in handline, longline, bottom-set and drift gillnet fisheries of the southwestern Indian Ocean. Observers monitored 21 landing sites across Kenya, Zanzibar and northern Madagascar for 12 months in 2016–17. Landings (n = 4666) identified 59 species, including three sea turtles, two small cetaceans and one sirenian (Dugong dugon). Primary gear threats to investigated taxa were identified as bottom-set gillnets (marine mammals, sea turtles and batoids), drift gillnets (marine mammals, batoids and sharks) and longlines (sharks). Overall, catch was dominated by small and moderately sized coastal requiem sharks (Carcharhiniformes) and whiprays (Dasyatidae). Larger coastal and oceanic elasmobranchs were also recorded in substantial numbers as were a number of deeper-water species. The diversity of catch demonstrates the potential for small-scale fisheries to have impacts across a number of ecosystems. From the observed catch rates we calculated annual regional elasmobranch landings to be 35,445 (95%CI 30,478–40,412) tonnes, 72.6% more than officially reported in 2016 and 129.2% more than the 10-year average (2006–16), constituting 2.48 (95%CI 2.20–2.66) million individuals. Productivity-Susceptibility Analyses indicate that small and moderately sized elasmobranchs are most vulnerable in the small-scale fisheries. The study demonstrates substantial underreporting of catches in small-scale fisheries and highlights the need to expand efforts globally to assess the extent and impact of small-scale fisheries on vulnerable marine species and their respective ecosystems.

Environmental indicators to reduce loggerhead turtle bycatch offshore of Southern California

Extreme climatic events are expected to become more frequent under current conditions of increasing global temperatures and climate variability. A key challenge of fisheries management is understanding and planning for the effect of anomalous oceanic conditions on the distributions of protected species and their interactions with fishing gear. Atypical marine states can cause non-target species to shift outside of their normal distribution patterns, leading to unwanted bycatch events that threaten fisheries sustainability. Environmental indicators can serve as early warning signals that allow for proactive management responses before significant bycatch occurs. Marine heatwaves in the Pacific have caused shifts in the distributions of endangered loggerhead turtles (Caretta caretta), increasing overlap with California’s Drift Gillnet fishery and thereby the risk of turtle bycatch events. To reduce bycatch, a fishery closure offshore of Southern California – The Loggerhead Conservation Area – Is enacted when an El Niño event has been declared and local sea surface temperatures (SSTs) are warmer than normal. However, this regulation was based on qualitative assessment of bycatch that occurred during past El Niño events, and no explicit threshold for SST anomalies was defined. Additionally, closures enacted under the current regulation rely on structured expert decision-making. Providing a quantitative indicator could help to refine future decisions. We developed and evaluated potential new indicators to guide the Loggerhead Conservation Area closure timing based on thermal indices in three different regions: the equatorial Pacific, regional areas offshore of Southern California, and temperate pelagic areas off the US west coast. Our objectives were to: 1) quantify thermal indicators and their respective thresholds to guide closure timing, and 2) hindcast closure scenarios based on these indicator thresholds to evaluate efficacy in terms of opportunity costs to fishers and ability to avoid turtle interactions. The best indicator in terms of avoiding historical turtle interactions while minimizing opportunity cost to fishers was a six-month average local SST anomaly indicator with closures enacted above a threshold of 0.77 °C. This result can improve upon the current closure guidelines by providing a quantified and spatially-explicit indicator and threshold to supplement the structured decision-making process. Our analysis demonstrates a novel approach to developing fisheries management strategies for species with a paucity of data. Issues with data comprehensiveness are frequently present in fisheries management exercises, and precautionary approaches are needed to allow adherence with legislation while considering the best available science.

Reducing bycatch while sustaining harvests

Ecology Reducing bycatch—the capture of nontarget marine species—remains challenging for global fisheries, which provide essential sources of protein for billions of people. Even in well-managed fisheries, the ensnaring of species such as turtles, seabirds, and sharks is a problem. For many species, the solutions used to mitigate bycatch have not been successful. Focusing on the California drift gillnet fishery, Hazen et al. developed a multispecies ocean management approach that combines several types of historical data on multiple species' movement to predict daily catch. With this tool, they discovered that operating the California drift gillnet fishery on the basis of daily oceanographic conditions, instead of more static approaches based on rigid boundaries, could greatly reduce bycatch while maintaining fish harvests. Sci. Adv. 10.1126/sciadv.aar3001 (2018).

An adaptable agent-based model for guiding multi-species Pacific salmon fisheries management within a SES framework

Informing fishery management decisions using coupled socio-ecological systems (CSES) models requires model construction that captures the systems interactions with high precision. Ecological uncertainty in fishery models is easily reduced using existing scientific literature, but social drivers are often poorly defined or understood. The lack of knowledge about fishermen behavior results in inaccurate models of questionable utility for fishery managers. We designed and constructed a high fidelity agent based model (ABM) using the socio-ecological framework that reduces social system uncertainty by capturing complex behaviors using data-driven bounded rationality and feedback. The resulting generalized ABM of CSES dynamics was instantiated to Pacific salmon fisheries at Kenai river in Upper Cook Inlet, Alaska. The data-driven model construction fuses multiple data-sets for classification of social and ecological fishery regimes into stochastic distributions; the agent behaviors were generalized by evolving parametrized equations using data-driven machine learning; multiple non-trivial metrics on multiple scales verified model's accuracy and predictive capacity. The verified model of CSES dynamics at the Kenai river revealed recent instability in the dipnet fishery coupled dynamics, historic instability in the drift gillnet fishery coupled dynamics due to a compensatory and aggressive fishing strategy, and in the future the model will be used for scenario-based studies to understand the outcomes of alternative management strategies.

Initial validation of blubber cortisol and progesterone as indicators of stress response and maturity in an otariid; the California sea lion (Zalophus californianus)

Chronic stress can have detrimental effects on an individual’s health and reproductive success. The use of cortisol quantification as an indicator of stress in free-ranging cetaceans and phocids is increasing but no studies have applied this technique on blubber in otariids. We measured cortisol concentrations in blubber samples obtained from California sea lions, Zalophus californianus, stranded in San Diego County and those incidentally killed in the California drift gillnet fishery. We also measured progesterone concentrations to assess female reproductive status and, in males, as a potential secondary measure of adrenal steroid production. Blubber cortisol and progesterone values were compared across demographic groups (sex and maturity), season, and proportion blubber lipid extracted. Stranded animals (247.3±70.767SEng/gblubber) had significantly higher cortisol concentrations compared to fishery bycaught (8.1±2.108SEng/gblubber) animals. These findings are likely driven by inherent differences in the cause of death and associated nutritional state coupled with the mean duration of expiration for these two groups of animals (i.e., the duration from an animal’s initial perception of the threat-to-self until death). The duration of transition from healthy state to death in stranded animals is on the order of many hours to weeks while in fishery bycaught animals, this transition occurs much more rapidly (i.e., seconds to tens of minutes). The presumed longer duration of the mortality event in stranded animals gives sufficient time for elevated cortisol to diffuse into the blubber. No significant differences between demographic groups, or season were found. However, blubber cortisol declined inversely with proportion blubber lipid extracted, suggesting utility in assessing long-term nutritional status. Blubber progesterone was significantly higher in mature females than immature females (153.8±54.546SEng/gblubber and 9.7±3.60SEng/gblubber respectively), containing on average 15 times more progesterone, irrespective of pregnancy state. Additionally, a significant relationship between mean cortisol and progesterone was found in males with >35% blubber lipid (p<0.0001). This study is an initial step in validating blubber cortisol and progesterone concentrations as a potential marker of stress response and reproductive state, respectively, in otariids. Especially when paired with dart biopsying, this approach could represent a relatively rapid way to assess baseline stress, nutritional status and reproductive states in otariids while minimizing the effects of sampling.

From Sunrise to Sunset in the California Drift Gillnet Fishery: An Examination of the Effects of Time and Area Closures on the Catch and Catch Rates of Pelagic Species

From Sunrise to Sunset in the California Drift Gillnet Fishery: An Examination of the Effects of Time and Area Closures on the Catch and Catch Rates of Pelagic Species

Oceanographic determinants of ocean sunfish (Mola mola) and bluefin tuna (Thunnus orientalis) bycatch patterns in the California large mesh drift gillnet fishery

The reduction of incidental capture of non-target species in a fishery (bycatch) is a key objective of ecosystem-based fisheries management (EBFM) and critical to the conservation of many marine species. Predicting bycatch events can inform targeted ecosystem-based management approaches to reduce risk. Here, the probability of ocean sunfish (Mola mola) and bluefin tuna (Thunnus orientalis) bycatch in the California large-mesh drift gillnet fishery is predicted using a suite of remotely-sensed environmental variables. Generalized additive models (GAMs) were used to model bycatch probability for these species in 8045 observed sets from 1990 to 2011, and predictive capabilities were assessed using k-fold cross validation. Bycatch probabilities for both species were elevated in regions of cool sea surface temperatures (<17°C), likely related to seasonal upwelling dynamics in the California Current, a major eastern boundary upwelling ecosystem. Mola bycatch occurred primarily in late fall, at moderate eddy kinetic energy values (0.006–0.008m2/s2) and in areas of high seafloor rugosity. Bluefin tuna bycatch rates were higher west of the Southern California Bight, also in late fall, and appear to be associated with the seasonal upwelling frontal zone. These models can be used with near-real time satellite data by both fishers and managers for bycatch avoidance, providing a tool for more dynamic ocean management strategies.

Predicting overlap between drift gillnet fishing and leatherback turtle habitat in the California Current Ecosystem

AbstractConcern over bycatch of protected species has become a key factor in shaping fisheries management decisions. In 2001, the National Marine Fisheries Service established an annual closure of a large mesh drift gillnet fishery targeting swordfish from central Oregon to central California between August 15 and November 15 because of concerns of bycatch of endangered leatherback turtles (the Pacific Leatherback Conservation Area, PLCA). The spatio‐temporal constraints of the PLCA were developed to encompass nearly all previously observed leatherback turtle bycatch events in the fishery. The PLCA has been effective at reducing bycatch of leatherback turtles but has reduced fishing opportunities. In this study, we examined whether the timing of the current PLCA closure is optimal for leatherback turtle conservation, by developing statistical models of leatherback turtle presence inside the PLCA based on environmental variables. We also examined finer‐scale spatiotemporal patterns of potential overlap between the fishery and leatherback turtle foraging habitat using Maxent and Random Forests applied to logbook data and leatherback turtle telemetry data. Our results suggest that the temporal extent of the current static closure period is the shortest and most effective for protecting the turtles while allowing fishing during low bycatch‐risk periods. We also found that it is possible to predict foraging habitat of leatherback turtles and fishing effort using environmental variables. Identification of spatial and temporal hotspots of potential overlap between fishing effort and leatherback turtle distribution can form a basis for dynamic management approaches.

Ecology of the Ocean Sunfish, Mola mola, in the southern California Current System

The common ocean sunfish, Mola mola, occupies a unique position in the eastern Pacific Ocean and the California Current Large Marine Ecosystem (CCLME) as the world's heaviest, most fecund bony fish, and one of the most abundant gelativores. M. mola frequently occur as bycatch in fisheries worldwide and comprise the greatest portion of the bycatch in California's large-mesh drift gillnet fishery. In this first long-term tagging study of any ocean sunfish species in the eastern Pacific, 15 M. mola (99cm to 200cm total length) were tagged in the southern California Bight (SCB) between 2003 and 2010 using 14 satellite pop-off archival tags (PATs) and one Fastloc Mk10 GPS tag. Ten tags provided positional data for a cumulative dataset of 349 tracking days during the months of July through March. Thirteen tags provided temperature and depth data. All M. mola remained within ~300km of the coast, and nearly all exhibited seasonal movement between the SCB and adjacent waters off northern and central Baja California, Mexico. No tagged individuals were tracked north of the SCB. Tag depth data showed diel vertical migration and occasional deep (>500m) dives. Data from the Fastloc GPS tag allowed close examination of the relationship between the movements of the largest tagged ocean sunfish (2m TL) and fine-scale oceanographic features. Near-instantaneous satellite sea surface temperature images showed this individual associated with upwelling fronts along its migration path, which exceeded 800km and ranged from 6 to 128km from the coast. Tag depth data showed active use of the water column within the frontal zones. Synthetic aperture radar (SAR) images demonstrated that surface slicks, which often indicate convergent circulation, coincided with this type of front. Zooplankton tows in the southern region of tracking off central Baja California, Mexico revealed dense populations of salps toward the warm side of these fronts. Satellite tag and ecosystem data suggest that bio-physical interactions in coastal upwelling fronts create favorable foraging habitat.

Factors affecting local permit ownership in Bristol Bay

Since the first limited entry Bristol Bay drift and set gillnet permits were issued in 1975, many local residents have sold their permits to non-resident fishermen. Declining local permit ownership destabilizes the main economic base of the region. Previous studies have documented the decline of locally owned permits and have proposed social and economic hypotheses that could explain why local permit ownership is declining in limited entry fisheries in Alaska. To stem the outward flow of permits, the Bristol Bay Economic Development Corporation operates a Permit Loan Program to assist Bristol Bay watershed residents in buying permits. Despite its generous benefits, it has not reversed the decline in local permit ownership. This paper examines why residents enter or exit the fishery. Hypotheses on permit retention were tested through interviews with Bristol Bay fishery participants. Interviewees indicate stronger cultural and familial ties than economic factors to fishing. As local permit ownership has declined, these ties are being lost, leading to declining interest in the fishery. Commercial fishing is a competitive and costly enterprise. Successful participants in the fisheries, especially the drift gillnet fishery, are financially savvy with supplemental non-fishing income that outcompete residents by catching most of the fish. The Bristol Bay Economic Development Corporation Permit Loan Program appears unable to stop the local loss of permits. There are no obvious ways to expand local permit ownership and retaining local permits remains a major challenge for the region. However, it is important for local residents to participate in the fishery because it keeps communities economically and socially healthy, gives residents access to their local resources and strengthens their voice in managing their local resources.

Bayesian inference and assessment for rare-event bycatch in marine fisheries: a drift gillnet fishery case study.

Fisheries bycatch is a global threat to marine megafauna. Environmental laws require bycatch assessment for protected species, but this is difficult when bycatch is rare. Low bycatch rates, combined with low observer coverage, may lead to biased, imprecise estimates when using standard ratio estimators. Bayesian model-based approaches incorporate uncertainty, produce less volatile estimates, and enable probabilistic evaluation of estimates relative to management thresholds. Here, we demonstrate a pragmatic decision-making process that uses Bayesian model-based inferences to estimate the probability of exceeding management thresholds for bycatch in fisheries with < 100% observer coverage. Using the California drift gillnet fishery as a case study, we (1) model rates of rare-event bycatch and mortality using Bayesian Markov chain Monte Carlo estimation methods and 20 years of observer data; (2) predict unobserved counts of bycatch and mortality; (3) infer expected annual mortality; (4) determine probabilities of mortality exceeding regulatory thresholds; and (5) classify the fishery as having low, medium, or high bycatch impact using those probabilities. We focused on leatherback sea turtles (Dermochelys coriacea) and humpback whales (Megaptera novaeangliae). Candidate models included Poisson or zero-inflated Poisson likelihood, fishing effort, and a bycatch rate that varied with area, time, or regulatory regime. Regulatory regime had the strongest effect on leatherback bycatch, with the highest levels occurring prior to a regulatory change. Area had the strongest effect on humpback bycatch. Cumulative bycatch estimates for the 20-year period were 104-242 leatherbacks (52-153 deaths) and 6-50 humpbacks (0-21 deaths). The probability of exceeding a regulatory threshold under the U.S. Marine Mammal Protection Act (Potential Biological Removal, PBR) of 0.113 humpback deaths was 0.58, warranting a "medium bycatch impact" classification of the fishery. No PBR thresholds exist for leatherbacks, but the probability of exceeding an anticipated level of two deaths per year, stated as part of a U.S. Endangered Species Act assessment process, was 0.0007. The approach demonstrated here would allow managers to objectively and probabilistically classify fisheries with respect to bycatch impacts on species that have population-relevant mortality reference points, and declare with a stipulated level of certainty that bycatch did or did not exceed estimated upper bounds.