Fishery Selectivity Research Articles

Performance of length limits to sustain a data- and capacity-limited fishery

Length limit regulations are widely recommended to sustain fish biomass and fishery yields, particularly for data- and capacity-limited fisheries. However, previous findings were generally derived from equilibrium conditions, used hypothetical fisheries, and/or did not explicitly quantify the performance of length limits under a combination of implementation levels, increasing fishing effort, and discard mortality. Here, we apply the management strategy evaluation (MSE) framework to test the performance of three length-limit management procedures (MPs)—maturity and high maturity length limits (i.e., minimum length limits set at and above the length at maturity, respectively) and slot length limit (i.e., excludes both immature and very large fish from retention)—in Iran's largely unregulated, data- and capacity-limited kingfish fishery. We subject these MPs to different conditions, including strong and weak implementations, discard mortality, and an expansion in future fishing efforts, and quantify their overall performance in terms of fishing mortality, biomass, and yield. Our results show that strong implementation of length limits results in the largest gains: the lowest future long-term fishing mortality and the highest biomass and yields relative to the maximum sustainable yield (MSY) level. When illegal-sized fish are selected but discarded with high discard mortality under strong implementation, or retained under weak implementation, the benefits of length limits are severely reduced. Still, length limits are projected to result in much lower fishing mortality and maintain biomass and fishery yields near MSY levels compared to the current fishery's selectivity and retention. Over the entire projection period, the high maturity length limit provided the highest probability of sustainable biomass and the highest mean relative yield under weak implementation and a growing future fishing effort. Our results underscore the benefits of implementing length limit regulation in sustaining data- and capacity-limited fisheries, particularly if current fishing conditions involve selecting and retaining small and immature fish under a high and growing fishing mortality rate.

Value of data in stock assessment models with misspecified initial abundance and fishery selectivity

AbstractThe age‐structured assessment model available in the MSEtool R package assesses stock status and exploitation for varying data availability, from limited to rich datasets. We investigated model accuracy in relation to data availability, population exploitation levels, initial population assumption and fishery selectivity misspecification. Estimates were accurate in all conditions when data were available for a stock in an unfished state. However, for estimates to be accurate without complete exploitation data, total catch and abundance index data needed to span more than two stock generations. When the data time series was shorter than two generations, fishery mean lengths spanning one generation improved relative estimates (e.g. depletion), but precise estimates of unfished recruitment required fishery age‐ or length‐structured data.

Recreational fisheries selectively capture and harvest large predators

AbstractSize‐ and species‐selective harvest inevitably alters the composition of targeted populations and communities. This can potentially harm fish stocks, ecosystem functionality, and related services, as evidenced in numerous commercial fisheries. The high popularity of rod‐and‐reel recreational fishing, practiced by hundreds of millions globally, raises concerns about similar deteriorating effects. Despite its prevalence, the species and size selectivity of recreational fisheries remain largely unquantified due to a lack of combined catch data and fisheries‐independent surveys. This study addresses this gap by using standardised monitoring data and over 60,000 digital angling catch reports from 62 distinct fisheries. The findings demonstrate a pronounced selectivity in recreational fisheries, targeting top predators and large individuals. Catch‐and‐release practices reduced the overall harvest by 60% but did not substantially alter this selectivity. The strong species‐ and size‐specific selectivity mirror patterns observed in other fisheries, emphasising the importance of managing the potential adverse effects of recreational fisheries selective mortality and overfishing.

Selectivity-based management for reversing overexploitation of demersal fisheries in North-western Mediterranean Sea

Bottom trawling in the Mediterranean Sea has compromised the availability of demersal fishery resources, primarily due to decades of overexploitation beyond sustainability thresholds. Low selective bottom trawl nets have exacerbated this situation by catching large numbers of under-sized or immature individuals of targeted and not-targeted commercial species that are subsequently discarded. EU legislation requires EU Mediterranean countries to recover demersal fish stocks to sustainable levels by January 2025. This is a challenging requirement under current management scenarios that mainly focus on reducing fishing hours, compromising the activity of the fleets. While social networks among fishing fleets and fisherman associations are essential for facilitating the transition to sustainable exploitation, improving the selectivity of bottom trawl gears is proposed as a viable option beyond reducing fishing hours. This study presents the benefits of improving bottom trawlers net selectivity in multi-species NW Mediterranean coastal and deep-sea fisheries to reduce the catch of non-commercial sizes and discards. Data from selectivity experiments and literature review, fish market landings and a regional monitoring program in the FAO geographical sub-area 6 were used. Results indicate that in the short term, enhanced selectivity leads to an increase in the proportion of fish that are closer to their minimum conservation reference sizes or sizes at maturity, while minimising the impact on commercial catches. Sustainability may be achieved, though beyond 2025, with measures complementing selectivity. These include the adoption of semi-pelagic otter doors to mitigate the impact of trawlers on the seabed and the expansion of no-take marine reserves to ensure habitat recovery and spillover effects.

The effect of process variability and data quality on performance of a state-space stock assessment model

State-space modeling is an emerging approach to age structured fisheries stock assessment that can accommodate multiple sources of variability in processes like recruitment, abundance, and selectivity. By maximizing the marginal likelihood by treating yearly deviations as random effects and then integrating them from the likelihood, these models can estimate multiple process variances. Several fisheries software packages have been developed that use a state-space framework with marginal likelihood, which has increased their popularity and usage across the U.S. Atlantic coast, Canada, and Europe. However, robust testing is still needed to gauge the applicability of these models and understand how they perform under a range of realistic variability in the process or observation error. Using an assessment model fit to Gulf of Maine Haddock as a baseline, we used a simulation-estimation procedure to test if state-space stock assessment models could produce approximately unbiased and precise estimates over a range of process variances that extended from zero to well above the levels estimated in the Gulf of Maine Haddock assessment, or when observations were noisier (i.e., more variable around their true value) than had been assumed in the assessment. We fit alternative estimation models that differed in which processes errors were included (of recruitment, expected survival, and fishery selectivity). State-space models which specify random effects in all three processes produced approximately unbiased and precise estimates of biomass and exploitation for most operating models and therefore are recommended except when variability in expected survival is absent (or very low), in which case the model is unlikely to converge. A conventional statistical-catch-at-age model with recruitment estimated as a fixed effect for each year, deterministic expected survival, and constant selectivity produced estimates that were comparable to the best state-space model, but do not provide internal estimates of process variances and did not perform well when recruitment was highly variable. This work will facilitate the use of state-space stock assessment models and choosing the parameterization that will produce the most accurate output to inform future predictions and management.

Modeling potential consequences of climate‐driven sex reversal on Southern and Summer Flounder population dynamics

AbstractObjectiveWe investigate the interacting effects of fishery selectivity and sexual dimorphism by using the fisheries for Southern Flounder Paralichthys lethostigma and Summer Flounder P. dentatus as case studies. In both species, females are larger than males, and temperature‐sensitive sex determination can lead to an increased proportion of males at sexual differentiation in warmer temperatures.MethodsWe developed a size‐, age‐, and sex‐structured population dynamics model for each species to understand how temperature‐induced sex reversal will interact with harvesting to impact population productivity. We considered scenarios with realistic sex ratios and varying levels of fishing pressure.ResultOur models of these species show that population egg production, abundance, biomass, and catch will all be constrained if sex ratios at differentiation become more male‐biased. For both species, climate‐induced changes to the sex ratio in early‐life stages could have a greater impact on relative catch than fishing mortality. We demonstrate that the spawning potential ratio may not be sensitive to climate‐induced changes in the sex ratio unless we account for changes in the sex ratio at differentiation in the historic baseline reproductive potential.ConclusionThese findings underscore the importance of utilizing sex‐specific stock assessments for species with temperature‐sensitive sex determination mechanisms.

Are rigid sorting devices necessary to control size selectivity in demersal trawl fisheries?

Diamond mesh codends are the most common size selectivity devices in trawls for releasing bycatch and undersized individuals. However, codend selectivity can vary due to the flexibility of the netting meshes, making discrimination by size between retained and released fish difficult to control. In some trawl fisheries, rigid sorting devices have been introduced to make size selection more controlled with sharper discrimination by size. However, the resulting size selectivity often does not show a sharp size selection. In the present study, we tested and compared the size selectivity performance of two “diamond-mesh codend designs” in the Barents Sea gadoid-trawl fishery: a four-panel codend, which was more rigid than a traditional two-panel codend design due to the additional selvedges, and a fully rigid codend design, that included a metal frame. The aim was to investigate the effect of added codend rigidness on the size selectivity of cod (Gadus morhua), haddock (Melanogrammus aeglefinus) and redfish (Sebastes spp.). In addition, the obtained results were compared to earlier research on size selectivity in this fishery including codends with different levels of rigidness and sorting grids. The results demonstrated that using a fully rigid codend did not result in a sharper size selectivity compared to a four-panel codend. Further, there was no indication that a rigid sorting grid makes size selectivity sharper than what can be obtained with a four-panel diamond mesh codend alone. There was also no proof that other codend stabilizing mechanisms such as shortened lastridge ropes could make the size selection sharper compared to the four-panel codend.

Addressing complex fleet structure in fishery stock assessment models: Accounting for a rapidly developing pot fishery for Alaska sablefish (Anoplopoma fimbria)

Fisheries management operates under uncertainty, often driven by the dynamic nature of marine ecosystems and associated fisheries. Stock assessment models, which form the scientific basis of decision-making in fisheries management, strive for realistic representations of biological and fishery processes. However, data limitations and knowledge gaps necessitate simplifying assumptions for representing these complex bio-socioeconomic systems, which can increase uncertainty in the assessment process. Addressing time-varying fishery dynamics (i.e., due to regulatory changes or alterations in harvester behavior) is a common and particularly challenging problem for stock assessment models. Time-varying fishery selectivity is widely utilized to address changes in fishery dynamics but may not be adequate when regulatory changes substantially alter gear usage and associated assessment fleet structures. We explore the implications of accounting for, or ignoring, complex temporal changes in fleet structure and selectivity within stock assessment models by utilizing a recent and high-value case study, Alaska sablefish (Anoplopoma fimbria). Our findings demonstrate that the treatment of fleet structure (i.e., adding fleet complexity to account for gear transitions) did not greatly influence estimates of spawning biomass trajectories. However, associated selectivity assumptions had substantial impacts on sustainable harvest recommendations. We recommend that the treatment of fleet structure and associated selectivity assumptions should incorporate a priori considerations and subject-matter expertise of fishery and biological dynamics to ensure pragmatic and appropriate model parameterizations. Moreover, we advocate for multi-fleet models as a useful diagnostic tool for validating model estimates from single-fleet assessments when uncertainty in fleet dynamics exist.

Data weighting: An iterative process linking surveys, data synthesis, and population models to evaluate mis-specification

Integrated stock assessments specify a distribution for multiple data types, and these distributions control the relative leverage assigned to each datum. A decade of research has demonstrated that (1) proper data weighting is necessary to avoid bias resulting from overweighting noisy age- and length-composition data; (2) sampling data can be pre-processed to estimate the likely sampling variance for composition data; and (3) using random effects to estimate time-varying parameters can improve the fit to data while also changing statistical leverage, and thereby serve a similar role to reweighting data. However, there are also unresolved questions including: (A) Is it more appropriate to model age and length data as proportions-at-age and as an index for the total, or as a series of indices-at-age? (B) Are correlated residuals appropriately addressed via data weighting or do they require additional model changes (i.e., time-varying parameters)? (C) How to efficiently communicate information about sampling imprecision and model errors between sampling and stock-assessment teams? (D) How does model-based expansion of sampling data affect data weighting? and (E) How to address alternative hypotheses about factors driving poor fit to data? Here, we argue that stock assessment errors can be classified using four categories: sampling bias (e.g., changes in survey coverage), sampling imprecision (e.g., finite sample sizes), assessment model bias (e.g., incorrect demographic assumptions) and assessment model imprecision (e.g., random effects). This categorization has several implications with resulting practical recommendations. For example, we define Percent Excess Variance (PEV) from the ratio of input sample size (the measured variance of sampling imprecision) and effective sample sizes (the variance of assessment-model residuals). We propose calculating PEV as standardized diagnostic measuring the net effect of survey bias and assessment model bias and imprecision. We demonstrate PEV in a simulation experiment fitted using the Woods Hole Assessment Model (WHAM) conditioned upon Gulf of Alaska walleye pollock, where unacknowledged variation in fishery selectivity results in a PEV of 77 % and this is eliminated when correctly specifying a time-varying estimation model. We also argue that model-based expansion of data inputs using auxiliary information can mitigate sampling bias, while also measuring sampling imprecision for spatially unrepresentative surveys. Similarly, including random effects can mitigate model bias while increasing model imprecision when the demographic model has little explanatory power. Finally, we observe that down-weighting compositional data for a given fleet fails to propagate information about model residuals when interpreting abundance indices or reference points for that same fleet. When PEV is large for important fleets, we therefore encourage focused research to explain the sources of these errors rather than simply downweighting without propagating information about residuals. However, we acknowledge a continuing role for automated data weighting for less important fleets, although we recommend explicit hypotheses about potential sources of errors in those cases.

A model of size-spectrum dynamics to estimate the effects of improving fisheries selectivity and reducing discards in Mediterranean mixed demersal fisheries

Size-spectrum models are good candidates to examine the effects of fishery management because predation and fishing are largely body-size dependent. We examine the effects of increasing trawl fisheries’ selectivity through the application of a size-spectrum model to a “continental shelf system” in the NW Mediterranean. This system is sustained by detritus, as background resource, and by benthic invertebrates that channel the energy to the fish community. The “continental shelf system” in our model consists of three components: 1) demersal and benthic fish and invertebrate species modelled by size spectrum dynamics, 2) carrion and 3) detritus. The model was able to exemplify the effects of changes in fishing patterns on the different biological components of the shelf system. According to the model outputs, in the short term the main target species, hake and red mullet, would be the main beneficiaries of the increased net selectivity and reduction of fishing effort. Discards reduction would have negligible effects. Despite the positive outcomes, this exercise was not exempt of challenges, mainly due to the data-demanding nature of the approach applied to a system with high diversity of life-histories and feeding strategies of invertebrate and fish species. And yet, our work is a first and crucial step to understand the size-spectrum dynamics of a continental shelf ecosystem subjected to fishery activities.

Unveiling unselective fishing in China: A nationwide meta‐analysis of multispecies fisheries

AbstractUnderstanding and managing fishery selectivity to target species and desirable size are instrumental to fisheries management. China, as the world's largest producer of marine capture fisheries, has been widely perceived to possess unselective domestic fisheries. To date, this perception remains largely anecdotal and conjectural, hindering the development of evidence‐based and effective management solutions. Here, we conducted a literature review to examine the magnitude and scale of unselective fisheries in China. By collating and analysing 140 fishery‐level and 807 species‐level records from 66 peer‐reviewed publications from 2010 to 2021, we found that primary target species were absent in 59% of fisheries, while unidentifiable low‐value and juvenile mixed catch were universal. Key commercial taxa were subject to nationwide multi‐gear and multispecies fisheries, each involving an average of 3.33 types of gear and accounting for less than 25% of catch individually. The ‘permissible gears’ defined by the national gear regulatory catalogue were selective over target species and caught negligible by‐products, though they were used less frequently, representing only 24% of catch records. While unselective fishing can provide seafood supplies for China's large population and potentially facilitate balanced harvest, management actions are needed to control the fishing pressure on primary target species and by‐product species. Amid the ongoing fisheries management reform in China, we proposed management recommendations tailored to China's needs and social contexts, including accounting for the trade‐off between socio‐economic and ecological goals, contemplating impacts of unselective fishing when implementing TAC programmes, and strengthening fisheries monitoring to inform management at multiple scales.

An assessment of sampling approaches for estimating growth from fishery-dependent biological samples

Abstract Accuracy of fish growth estimates depends greatly on how samples are collected. A total of three common sampling strategies are random, fixed otolith sampling (FOS), and proportional otolith sampling (POS). Random sampling is inefficient and rarely random. While POS has been shown to produce more accurate estimates of growth compared to FOS under ideal conditions, it is unclear how variables such as sample size, fishery selectivity, and fishing mortality influence sampling bias. Simulated age-at-length observations from two tropical deepwater species were used to evaluate the accuracy of von Bertalanffy growth parameter estimates comparing FOS and POS for a fast-growing, short-lived fish (Prisitipomoides auricilla) and a slow-growing, long-lived fish (Etelis coruscans). A total of 24 scenarios were tested to examine the influence of sample size, minimum size selected in the fishery, fishery exploitation rates, and supplemental sampling on growth parameter estimates. Furthermore, tests were applied to real fishery age–length observations for P. auricilla. POS consistently performed better than FOS, especially when the catch was representative of the population. However, FOS performed better when catch was not representative. This research demonstrates the importance of considering effects of fishery selectivity, fishing mortality, and sample size on sampling strategy and provides a tool to select an appropriate approach.

Demographic Consequences of Small-Scale Fisheries for Two Sex-Changing Groupers of the Tropical Eastern Pacific

The effects of fishing on the demography and population ecology of sex-changing fishes are largely unknown, despite the fact that their fisheries provide important economic and nutritional resources in coastal communities throughout the tropics, especially in Latin America. Species with female-first sex change often have naturally skewed sex ratios in the adult population, and fishing pressure can alter this natural bias, limiting egg production and fertilization success. How fishing alters demography and population vital rates depends on which sizes and sexes are selected. We consider two types of fishery selectivity “asymptotic (selecting the largest fish) and plate-sized (selecting fish between in a narrow, but small, size range)” that to represent fisheries for two important fish species of the Tropical Eastern Pacific, the Pacific goliath grouper (Epinephelus quinquefasciatus) and the endemic sailfin grouper (Mycteroperca olfax) of the Galapagos Islands, known locally as bacalao. Each of these large, long-lived species support small scale fisheries of significant value, but there is limited information on fishing effort, selectivity, or population trends. Using a population model, we estimate how the biology of these species contributes to their risk of overexploitation under different possible scenarios of fishing and reproductive biology. Specifically, we consider how variation in growth rates and fertilization rates interact with selectivity to affect age structure and sex ratios. We compare two metrics of population status: the spawning potential ratio (SPR), and the relative standing biomass after fishing (BF/B0). In our modeled populations, when fertilization rates were reduced, fishing rapidly decreased the spawning potential of both species, but did not affect biomass at moderate levels of fishing mortality. However, we predict low fertilization success, fast somatic growth, and asymptotic selectivity of fisheries for sex-changing species decreases both spawning potential and biomass, even at low levels of exploitation, suggesting these factors can cause rapid depletion of sex-changing species. Our findings highlight key gaps in our knowledge of spawning behavior and fertilization success of sex-changing fishes that must be filled if we are to sustainably manage these culturally and economically significant fisheries.

Combining telemetry and fisheries data to quantify species overlap and evaluate bycatch mitigation strategies in an emergent Canadian Arctic fishery

Greenland halibut Reinhardtius hippoglossoides are bottom-dwelling flatfish that support commercial and community fisheries in Baffin Bay, Canada. Recently, exploratory inshore summer fisheries have raised concerns surrounding the bycatch of Greenland sharks Somniosus microcephalus and Arctic skate Amblyraja hyperborea, which are susceptible to overfishing due to their conservative life history traits. To explore fisheries selectivity and opportunities for bycatch mitigation, we combined pop-up satellite archival tags (PSATs) and fisheries data to assess habitat overlap and catch trends across these 3 species. PSAT data showed variable inter-specific overlap, with Greenland sharks primarily inhabiting depths <1000 m (725 ± 193 m), Greenland halibut inhabiting a narrower depth range (1030 ± 113 m), and Arctic skates overlapping depths (950 ± 225 m) of both species. However, fisheries data suggested high inter-specific overlap at deepest depths, with peak catch-per-unit-effort (CPUE) of all species at depths 800-1000 m. A marked decline in Greenland shark CPUE was observed throughout the fishing season which was best explained by cumulative fishing pressure. Combined tagging and fisheries data suggest that targeting specific seasonal habitat will not decrease bycatch, and inshore summer longline fisheries should be evaluated in the context of potentially high elasmobranch mortality, with enforced bycatch handling practices and alternative mitigation measures (e.g. gear modification or reduced soak times) required.

Protected areas and frugivorous fish in tropical rivers: Small‐scale fisheries, conservation and ecosystem services

Abstract Frugivorous fish provide often conflicting ecosystem services of seed dispersal and food provisioning in tropical rivers. Fishing may reduce the size and abundance of frugivorous fish, thus affecting their potential as seed dispersers, which could affect the conservation of these fish and of floodplain forests. The goal of this study was to assess the influence of co‐managed protected areas in the form of extractive reserves (RESEX) and small‐scale fisheries on frugivorous fish in the Tapajós and Negro rivers, in the Brazilian Amazon. The study examined whether: (i) frugivorous fish are important for fisheries and selectively caught; and (ii) frugivorous fish abundance, size and fisheries catch per unit of effort are higher inside the RESEX than outside. The analyses included fisheries‐dependent data (3,753 fish landings) and independent data (12,730 sampled fish) collected in 16 fishing communities (eight for each river). In both rivers, frugivorous fish are among the 10 species caught the most and frugivorous biomass was proportionately higher in landings than in samplings, indicating fisheries selectivity towards these fishes. In both rivers, catches of frugivorous fish were higher outside the RESEX than inside. Catch per unit effort and the proportion of frugivores in the catch were higher outside the RESEX in the Tapajós River but did not vary between sites inside and outside the RESEX in the Negro River. Frugivorous fish were larger inside the RESEX in the Negro, but smaller inside the RESEX in the Tapajós. The results indicated that the ecosystem services of seed dispersal and food source provided by frugivorous fish are not in conflict in the tropical rivers studied. Therefore, these clearwater and blackwater rivers in the Brazilian Amazon show a balance between fisheries and conservation of frugivorous fish, which play an essential role in the functioning of tropical floodplain forest ecosystems.

Size-selective competition between cod and pelagic fisheries for prey

Abstract Predators often predate on a limited size range of prey, which may or may not overlap with size ranges of same prey targeted by fisheries. When they do overlap, the effect of competition over that prey is immediate, as the predator removes prey, which are at the same time suitable for the fishery. However, if the predator consumes the same prey species as the fishery, but targets smaller prey sizes, this predation on smaller sizes may result in a potential loss of future, rather than current, fishing opportunities. Comparative analyses of predator size preference and fisheries selectivity are scarce, despite their relevance in the context of integrated management of fish populations. We evaluated how size-selective cod predation influences the dynamics of sprat and herring in the Baltic Sea, as well as the competition with pelagic fisheries through immediate and delayed effects. We found a large overlap (30–60%) between prey lengths targeted by cod and fisheries, which was largest in the 1970s–1980s, when cod had higher abundance and was larger in size. Cod generally consumes herring and sprat, which are smaller than those caught by the fisheries, causing both immediate and delayed effects on prey biomass available for the fisheries.

Incorporating demographic information into spawner–recruit analyses alters biological reference point estimates for a western Alaska salmon population

Changes over time in age, sex, and length-at-age of returning Pacific salmon have been widely observed, suggesting concurrent declines in per capita reproductive output. Thus, assessment models assuming stationary reproductive output may inaccurately estimate biological reference points that inform harvest policies. We extended age-structured state-space spawner–recruit models to accommodate demographic time trends and fishery selectivity to investigate temporal changes in reference points using Kuskokwim River Chinook salmon (Oncorhynchus tshawytscha). We illustrate that observed demographic changes have likely reduced per capita reproductive output in an additive manner, for example, models including changes in both length-at-age and age composition showed larger declines than models incorporating only one time trend. Translated into biological reference points using a yield-per-recruit algorithm, we found escapement needed for maximum sustained catch has likely increased over time, but the magnitude further depended on size-selective harvest (i.e., larger increases for reference points based on larger mesh gillnets). Compared to traditional salmon assessments, our approach that acknowledges demographic time trends allows more complete use of available data and facilitates evaluating trade-offs among gear-specific harvest policies.

A bioeconomic approach towards improved fishery management of Monomia haanii in the southern Taiwan Strait, China

Red swimming crab, Monomia haanii, is one of China’s most valuable crab export products, but the fishery is managed with very limited scientific information. There is currently no analytical stock assessment for this little studied, but highly sought-after species. Here, a new equilibrium-based bioeconomic model was developed and applied to the M. haanii fishery in the southern Taiwan Strait, China to assess the performance of the current management approach in terms of revenue, yield, and egg production. Because the species is short-lived and grows rapidly, simulations were conducted to evaluate whether temporal allocation of fishing effort can improve performance along these dimensions by optimizing the tradeoffs between growth and mortality. Overall, we found that M. haanii is both highly productive and heavily exploited. Rapid harvest of sublegal individuals and females in the first half of the fishing season (August-December) after the national summer fishing moratorium, results in reduced yield, revenue, and egg production relative to management approaches that either spread fishing mortality throughout the year enabling more somatic growth, or target larger crabs. These outcomes were robust over a variety of model sensitivity scenarios, including changes in the timing of settlement, growth rate, and fishery selectivity. Evaluating such tradeoffs will be essential for sustainable management of Chinese swimming crabs in the face of heavy fishing pressure and environmental change.

Urban market amplifies strong species selectivity in Amazonian artisanal fisheries

Abstract Despite Amazonia possessing the highest freshwater biodiversity on Earth, urban landing data show how huge fishing pressure is placed on only a dozen species. However, truly characterising the fishery and understanding the drivers of species selectivity is challenging, given the neglect of artisanal fishing activity, who may catch most of the Amazon’s fish. We register the catch of 824 fishing trips by interviewing artisanal fishers in their rural riverside communities. We use these data to characterise the artisanal fishery of the Rio Purus, the main fish source sub-system for the Amazon’s largest city (Manaus), and investigate the factors determining catch composition. Fishers caught 80 fish species, yet just four species made up over half of the harvested biomass. Urban markets appear to drive greater selectivity, with a significantly lower species diversity in commercial compared to subsistence catches. Fish catch composition varied significantly both seasonally and with geographical remoteness from Manaus. The spatial turnover in catch composition appears to be driven by urban access, with more commercially important species dominating where Manaus-based fish-buyers frequent. Our data may partially explain observed overfishing in some commercially important species, particularly as most Amazonians now live in urban areas.

Deconstructing size selectivity to evaluate the influence of fishery management

Fisheries are managed through various means to regulate harvest and maintain the sustainability of the stock. These regulations also influence which size fishes are removed from the population: A minimum mesh or hook size directly affects the size of fish retained by the fishing gear. A minimum fish size leads to discards, and any survival of released fish decreases the selectivity for sub-legal size classes. Closed areas and seasons indirectly influence size selectivity by altering the relative availability of each size class. Collectively, these regulations control the population-level size selectivity of the fishery, which is the product of gear selection (modulated by discard mortality) and spatiotemporal availability. Stock assessment models are particularly vulnerable to incorrectly specified selectivity, yet it is difficult to independently validate the structure of this variable because it is influenced by several processes. We analyzed the Gulf of Maine fishery for Atlantic cod (Gadus morhua), which has been intensively regulated through a complex management system for several decades, and deconstruct the mechanisms through which regulations have influenced fishery selectivity. Gear selection-at-size was estimated for four fleets under two regulatory regimes by comparing the size distribution of survey and fishery catches, sampled from the same time and area. Availability-at-size was estimated by weighting predictions from a random forest model of fish density-at-size by the spatiotemporal pattern of fishing effort. Several regulations caused fishery selectivity to be distinctly dome-shaped, and regulatory changes altered the shape of this curve over time. The approach of deconstructing selectivity into its gear and availability components can be adapted to other stocks to elucidate the underlying mechanisms, inform assessment models, and evaluate alternate regulation scenarios.