Fishery Yields Research Articles

Dietary differences among commercially important fishes in Lake Tanganyika assessed using stable isotope analysis

The pelagic fishery of Lake Tanganyika, which is largely made up of the three species Lates stappersii, Limnothrissa miodon, and Stolothrissa tanganicae, has been in decline, and there is no clear understanding of the primary underlying causes. It has been suggested that climate change has altered the primary productivity of the system, but detailed knowledge of the system's food web is required to elucidate the effect on higher trophic levels. The aim of this study is to determine the diet of the three commercially important fish species. Muscle tissue samples for stable isotope analysis were taken from February through April 2017, supplemented with stomach samples from L. stappersii for use in stomach content analysis. The stomach analysis showed an ontogenetic change in the diet composition of L. stappersii, shifting from copepods to fish larvae, supplemented with shrimp, to whole fish prey as the fish grew larger. Stable carbon and nitrogen isotope values from fish muscle were consistent with this observation, and also seemed to indicate that fish of similar sizes had similar diets, irrespective of species. This suggests that the diet of the pelagic fish species might be better explained by size, rather than species. The isotope data revealed a short range of δ15N values from primary consumers to fish, which may help explain the high fisheries productivity of Lake Tanganyika, and the strong impact of primary productivity changes on fisheries yield.

THE FISHERY EXPLOITATION OF ACOUPA WEAKFISH OFF SÃO PAULO COAST, SOUTHEASTERN BRAZIL

The Acoupa weakfish Cynoscion acoupa (Lacepí­¨de, 1801) is a large inshore sciaenid of commercial and social importance found along the entire Brazilian coast. In spite of this, there is few information on the species, both in terms of biological aspects and fishery yields, particularly off southeastern Brazil. Within this context, this study aimed to an evaluation of Acoupa weakfish fishery production off São Paulo coast (23°22’í 25°18’S) based on publicly available statistical data collected between 1998 and 2016. Acoupa weakfish fishery in this State takes place on both industrial and artisanal scales, and employs at least 22 different fishing gears. The pair-trawling fishery was the most important Acoupa weakfish producer during the period surveyed, with a sharp decline in the catches from 2007 onward due to the establishment of Marine Protected Areas off São Paulo coast, which banned trawling in depths shallower than 26 meters. Acoupa weakfish yields were relatively low in São Paulo when compared with other demersal fishes exploited, despite the high retail prices commanded by the species in regional markets and restaurants.

Fish spawning strategies in the northwestern Mediterranean Sea

The Mediterranean is globally considered an oligotrophic sea. However, there are some places or certain seasons in which mechanisms that enhance fertility may occur. These mechanisms, and related processes, are especially relevant in maintaining fish populations when they take place during the period of larval development. This contribution analyzes how environmental conditions occurring in the NW Mediterranean, at local and seasonal scales, determine the temporal and spatial patterns of fish reproductive activity in the region. The structure of the bathymetry, diversity of adult fish habitats and hydrodynamic mechanisms conditioning the primary production of the region (e.g., shelf-slope density front and associated current, continental water inflows, winter mixing, stratification of the water column) determine the location of spawning and the distribution patterns of fish eggs and larvae. A pronounced seasonal variability regarding both the number of species and the number of fish larvae in the plankton can be observed throughout the year. Most NW Mediterranean fish reproduce during the spring–summer stratification period, when the phytoplankton biomass values at the upper layers of the water column are lower than in winter. The development of the Deep Chlorophyll Maximum in this period and the high zooplankton biomass associated to it offers an important food source for the larvae. Additionally, during this period the inputs of continental waters are one of the fertilization mechanisms of surface waters and some species, as anchovy, takes advantage of this situation. Autumn–winter is the period with lower ichthyoplankton diversity, being dominated by sardine. Vertical mixing during winter is one of the mechanisms that enhance productivity. Overall, fish species show reproductive strategies and larval fish behavior that allow them to take advantage of the available resources throughout the seasonal cycle. These strategies, together with the high ecological efficiency of oligotrophic systems, contribute to the relatively high yield of Mediterranean fisheries. In a context of global change, understanding of the mechanisms relating environmental changes to the extent of spatial and temporal location of suitable spawning habitats of fish is a key first step to predicting and projecting such future changes, and thereby adapting to these changes.

Diet and movements of pikeperch (Sander lucioperca) in a large oligotrophic lake with an exceptionally high pikeperch yield

AbstractThrough the recent increases in abundance, pikeperch (Sander lucioperca) has not only become a valuable target for recreational and commercial fishing, but also the most important top predator in many lakes. Generally oligotrophic Lake Höytiäinen (283 km2) sustains one of the most economically valuable pikeperch stocks in Finland, and its annual fisheries yield (up to 5.5 kg/ha) is exceptionally high among Finnish lakes. Here, we followed seasonal movements of fish using conventional tagging, and studied short‐term and long‐term diet of pikeperch by stomach content and stable isotope analyses to examine if the movements were motivated by prey availability. A clear seasonal migration pattern was observed: in summer, 80% of the tag recoveries were obtained from the shallow and mesotrophic northern and western areas, whereas in autumn and winter, the deep and oligotrophic southern area dominated in the recaptures. The diet of pikeperch consisted of the most abundant fish species in the lake, perch (Perca fluviatilis) and roach (Rutilus rutilus), as well as insects. Stable isotope analysis indicated a shift from a dominance of pelagic food sources to littoral food sources during the growing season of pikeperch. Our results suggest that the pikeperch migrations facilitate efficient material transfer from the mesotrophic part while the deeper and oligotrophic part offers a favourable overwintering environment.

Toward a Coordinated Global Observing System for Seagrasses and Marine Macroalgae

In coastal waters around the world, the dominant primary producers are benthic macrophytes, including seagrasses and macroalgae, that provide habitat structure and food for diverse and abundant biological communities and drive ecosystem processes. Seagrass meadows and macroalgal forests play key roles for coastal societies, contributing to fishery yields, storm protection, biogeochemical cycling and storage, and important cultural values. These socio-economically valuable services are threatened worldwide by human activities, with substantial areas of seagrass and macroalgal forests lost over the last half-century. Tracking the status and trends in marine macrophyte cover and quality is an emerging priority for ocean and coastal management, but doing so has been challenged by limited coordination across the numerous efforts to monitor macrophytes, which vary widely in goals, methodologies, scales, capacity, governance approaches, and data availability. Here, we present a consensus assessment and recommendations on the current state of and opportunities for advancing global marine macrophyte observations, integrating contributions from a community of researchers with broad geographic and disciplinary expertise. With the increasing scale of human impacts, the time is ripe to harmonize marine macrophyte observations by building on existing networks and identifying a core set of common metrics and approaches in sampling design, field measurements, governance, capacity building, and data management. We recommend a tiered observation system, with improvement of remote sensing and remote underwater imaging to expand capacity to capture broad-scale extent at intervals of several years, coordinated with stratified in situ sampling annually to characterize the key variables of cover and taxonomic or functional group composition, and to provide ground-truth. A robust networked system of macrophyte observations will be facilitated by establishing best practices, including standard protocols, documentation, and sharing of resources at all stages of workflow, and secure archiving of open-access data. Because such a network is necessarily distributed, sustaining it depends on close engagement of local stakeholders and focusing on building and long-term maintenance of local capacity, particularly in the developing world. Realizing these recommendations will produce more effective, efficient, and responsive observing, a more accurate global picture of change in vegetated coastal systems, and stronger international capacity for sustaining observations.

Relative Impacts of Simultaneous Stressors on a Pelagic Marine Ecosystem

Climate change and fishing are two of the greatest anthropogenic stressors on marine ecosystems. We investigate the effects of these stressors on Hawaii's deep-set longline fishery for bigeye tuna (Thunnus obesus) and the ecosystem which supports it using a size-based food web model that incorporates individual species and captures the metabolic effects of rising ocean temperatures. We find that when fishing and climate change are examined individually, fishing is the greater stressor. This suggests that proactive fisheries management could be a particularly effective tool for mitigating anthropogenic stressors either by balancing or outweighing climate effects. However, modeling these stressors jointly shows that even large management changes cannot completely offset climate effects. Our results suggest that a decline in Hawaii's longline fishery yield may be inevitable. The effect of climate change on the ecosystem depends primarily upon the intensity of fishing mortality. Management measures which take this into account can both minimize fishery decline and support at least some level of ecosystem resilience.

Balanced harvest: concept, policies, evidence, and management implications

Balanced harvest has been proposed to reduce fishing impact on ecosystems while simultaneously maintaining or even increasing fishery yield. The concept has attracted broad interest, but also received criticisms. In this paper, we examine the theory, modelling studies, empirical evidence, the legal and policy frameworks, and management implications of balanced harvest. The examination reveals unresolved issues and challenges from both scientific and management perspectives. We summarize current knowledge and address common questions relevant to the idea. Major conclusions include: balanced harvest can be expressed in several ways and implemented on multiple levels, and with different approaches e.g. métier based management; it explicitly bridges fisheries and conservation goals in accordance with international legal and policy frameworks; modelling studies and limited empirical evidence reveal that balanced harvest can reduce fishing impact on ecosystem structure and increase the aggregate yield; the extent of balanced harvest is not purely a scientific question, but also a legal and social choice; a transition to balanced harvest may incur short-term economic costs, while in the long-term, economic results will vary across individual fisheries and for society overall; for its application, balanced harvest can be adopted at both strategic and tactical levels and need not be a full implementation, but could aim for a “partially-balanced” harvest. Further objective discussions and research on this subject are needed to move balanced harvest toward supporting a practical ecosystem approach to fisheries.

Optimized fishing through periodically harvested closures

Abstract Periodically harvested closures are a widespread, centuries‐old form of fisheries management that protects fish between pulse harvests and can generate high harvest efficiency by reducing fish wariness of fishing gear. However, the ability for periodic closures to also support high fisheries yields and healthy marine ecosystems is uncertain, despite increased promotion of periodic closures for managing fisheries and conserving ecosystems in the Indo‐Pacific. We developed a bioeconomic fisheries model that considers changes in fish wariness, based on empirical field research, and quantified the extent to which periodic closures can simultaneously maximize harvest efficiency, fisheries yield and conservation of fish stocks. We found that periodic closures with a harvest schedule represented by closure for one to a few years between a single pulse harvest event can generate equivalent fisheries yield and stock abundance levels and greater harvest efficiency than achievable under conventional fisheries management with or without a permanent closure. Optimality of periodic closures at maximizing the triple objective of high harvest efficiency, high fisheries yield, and high stock abundance was robust to fish life history traits and to all but extreme levels of overfishing. With moderate overfishing, there emerged a trade‐off between periodic closures that maximized harvest efficiency and no‐take permanent closures that maximized yield; however, the gain in harvest efficiency outweighed the loss in yield for periodic closures when compared with permanent closures. Only with extreme overfishing, where fishing under nonspatial management would reduce the stock to ≤18% of its unfished level, was the harvest efficiency benefit too small for periodic closures to best meet the triple objective compared with permanent closures. Synthesis and applications. We show that periodically harvested closures can, in most cases, simultaneously maximize harvest efficiency, fisheries yield, and fish stock conservation beyond that achievable by no‐take permanent closures or nonspatial management. Our results also provide design guidance, indicating that short closure periods between pulse harvest events are most appropriate for well‐managed fisheries or areas with large periodic closures, whereas longer closure periods are more appropriate for small periodic closure areas and overfished systems.

Source water variability as a driver of rockfish recruitment in the California Current Ecosystem: implications for climate change and fisheries management

Elucidating connections between ocean climate variability and change and recruitment of juvenile fishes to adult populations is critical for understanding variability in stock–recruit dynamics. Recruitment to adult rockfish populations in the California Current Ecosystem (CCE) is highly variable, leading to short- and long-term changes in abundance, productivity, forage availability, and potential fisheries yield. We used regional ocean model output, oceanographic data, and a 34-year time series of pelagic juvenile rockfish to investigate the interaction between changes in CCE source waters, as reflected by physical water mass properties, and recruitment variability. Specifically, variability of “spiciness” on upper water isopycnals explains a substantial fraction of the variation in pelagic juvenile rockfish abundance. High rockfish abundances correspond to cooler, fresher waters with higher dissolved oxygen (i.e., “minty”) conditions, indicative of Pacific subarctic water. By contrast, years of low rockfish abundance are associated with warmer, more saline, and more oxygen-deficient (i.e., “spicy”) conditions, reflecting waters of subtropical or equatorial origin. Transport and source waters in the CCE are key factors determining density-independent processes and subsequent recruitment to adult populations.

Effectiveness of small locally-managed marine protected areas for coral reef fisheries management in the Philippines

The Philippines has more than 1600 locally managed marine protected areas (MPAs), the most in the world. However, their effectiveness for coral reef fisheries management is often questionable because most of these MPAs are small and ineffectively managed. In this study, we assessed the fish biomass of commercially important coral reef fishes (e.g. surgeonfish (family Acanthuridae), parrotfish (subfamily Scarinae), snapper (family Lutjanidae), grouper (subfamily Epinephelinae), sweetlips (family Haemulidae), goatfish (Mullidae) and emperor (family Lethrinidae)) in 57 locally managed MPAs in the Philippines. We used the fish biomass level at the nationally managed, large (332.0 km2), remote, old and well enforced (i.e. strictly protected for >20 years) Tubbataha Reefs National Marine Park (TRNMP) as a proxy for “unfished” ecosystems (Bo). We considered fish biomass levels between 25 and 50% of Bo as biomass within the maximum sustainable yield for multi-species coral reef fisheries (BMMSY) (McClanahan et al., 2014). Results showed that fish biomass levels in 7%, 25% and 68% of the surveyed MPAs were “above BMMSY”, “within BMMSY” and “below BMMSY”, respectively. None of the reefs outside MPAs was “above BMMSY”. About 86% were “below BMMSY” and the rest of the 14% of the sites outside MPAs were “within BMMSY” (14%). The mean (±S.E.) fish biomass levels on reefs inside and outside MPAs were only about 20.4 ± 2.2% and 10.9 ± 1.3%, respectively, of the TRNMP level. Neither size nor age of MPAs was significantly associated with fish biomass. Overall, our study showed that the current locally managed MPAs are not effective enough for coral reef fisheries management but, nonetheless, better than having no MPA at all.

Marine reserves stabilize fish populations and fisheries yields in disturbed coral reef systems.

Marine reserve networks are increasingly implemented to conserve biodiversity and enhance the persistence and resilience of exploited species and ecosystems. However, the efficacy of marine reserve networks in frequently disturbed systems, such as coral reefs, has rarely been evaluated. Here we analyze a well-mixed larval pool model and a spatially explicit model based on a well-documented coral trout (Plectropomus spp.) metapopulation in the Great Barrier Reef Marine Park, Australia, to determine the effects of marine reserve coverage and placement (in relation to larval connectivity and disturbance heterogeneity) on the temporal stability of fisheries yields and population biomass in environmentally disturbed systems. We show that marine reserves can contribute to stabilizing fishery yield while increasing metapopulation persistence, irrespective of whether reserves enhance or diminish average fishery yields. However, reserve placement and the level of larval connectivity among subpopulations were important factors affecting the stability and sustainability of fisheries and fish metapopulations. Protecting a mix of disturbed and non-disturbed reefs, rather than focusing on the least-disturbed habitats, was the most consistently beneficial approach across a range of dispersal and reserve coverage scenarios. Placing reserves only in non-disturbed areas was the most beneficial for biomass enhancement, but had variable results for fisheries and could potentially destabilize yields in systems with well-mixed larval or those that are moderately fished. We also found that focusing protection on highly disturbed areas could actually increase variability in yields and biomass, especially when degraded reef reserves were distant and poorly connected to the meta-population. Our findings have implications for the design and implementation of reserve networks in the presence of stochastic, patchy environmental disturbances.

Trends in Effort and Yield of Trawl Fisheries: A Case Study From the Mediterranean Sea

The rationale applied for monitoring and managing fisheries is based on the implicit assumption that yield and stocks status is essentially determined by fisheries. Moreover, the fisheries yield is quantified and analysed in terms of landings with respect to official management area of registration of vessels. In this way, the real area of activity of each fleet is not considered and this prevent an effective spatial analysis of the factors affecting fisheries yield and stocks status. This paper firstly presents a VMS-based reconstruction of the fishing effort and of the area of activity of the Italian trawlers in the Mediterranean Sea. The fishing area of each fleet is then used as a spatial reference to estimate primary productivity rate and gross primary production and to investigate, by using General Additive Models, the effects of trawling effort, primary production and time on fisheries yield, fisheries productivity and overexploitation rate for some key demersal species. The results evidence that the usage of satellite-based information of fishing activities and of primary production, when combined at the real spatial scale of fishing activities, could effectively improve our ability to analyse the response of the ecosystems to these driving forces and allow capturing the main trends of yield, productivity and overexploitation rate of demersal stocks.

Optimum yield of demersal fisheries in Japan's coastal and offshore waters

Optimum yield of demersal fisheries in Japan's coastal and offshore waters

Assessing the potential for competition between Pacific Halibut (Hippoglossus stenolepis) and Arrowtooth Flounder (Atheresthes stomias) in the Gulf of Alaska

Pacific Halibut (Hippoglossus stenolepis) support culturally and economically important fisheries in the Gulf of Alaska, though recent decreases in mean size-at-age have substantially reduced fishery yields, generating concerns among stakeholders and resource managers. Among the prevailing hypotheses for reduced size-at-age is intensified competition with Arrowtooth Flounder (Atheresthes stomias), a groundfish predator that exhibited nearly five-fold increases in biomass between the 1960s and mid-2010s. To assess the potential for competition between Pacific Halibut and Arrowtooth Flounder, we evaluated their degree of spatiotemporal and dietary overlap in the Gulf of Alaska using bottom trawl survey and food habits data provided by the Alaska Fisheries Science Center (NOAA; 1990 to 2017). We restricted analyses to fish measuring 30 to 69 cm fork length and used a delta modeling approach to quantify species-specific presence-absence and catch-per-unit-effort as a function of survey year, tow location, depth, and bottom temperature. We then calculated an index of spatial overlap across a uniform grid by multiplying standardized predictions of species’ abundance. Dietary overlap was calculated across the same uniform grid using Schoener’s similarity index. Finally, we assessed the relationship between spatial and dietary overlap as a measure of resource partitioning. We found increases in spatial overlap, moving from east to west in the Gulf of Alaska (eastern: 0.13 ± 0.20; central: 0.21 ± 0.11; western: 0.31 ± 0.13 SD). Dietary overlap was low throughout the study area (0.13 ± 0.20 SD). There was no correlation between spatial and dietary overlap, suggesting an absence of resource partitioning along the niche dimensions examined. This finding provides little indication that competition with Arrowtooth Flounder was responsible for changes in Pacific Halibut alHHsize-at-age in the Gulf of Alaska; however, it does not rule out competitive interactions that may have affected resource use prior to standardized data collection or at different spatiotemporal scales.

Model-based on fishery management systems with selective harvest policies

Recently, marine reserves have become a widely advocated approach to marine conservation. However, these studies beg the question as to how the selection imposed by harvesting and the dispersal rates between two patches influence stock abundance and the maximum fishery yield from the perspective of mathematical model. In this paper, a prey–predator model with selective harvesting by incorporating two discrete state delays in age and size for both the species in unreserve area has been established. Considering the resources exploitation and ecological observation, a hybrid optimal problem with Lagrange function and state delays together with characteristic times is formulated. We then develop an efficient numerical method by the lights of optimal state delay control method and Pontryagin-type minimum (maximum) principle. Finally, we discuss the impacts of selection for harvesting ages and efforts as well as migration coefficients on the fishery abundance and maximal yield by a series of simulations. Our results manifest: (1) after establishing protected areas, the oscillating amplitudes of prey and predator in unprotected areas are lessened and the stocks of predator in both areas are greatly enhanced with increasing of dispersal rates. (2) selective harvest tactics is appropriate for the strong intensity harvest in order to increase the abundance of all populations; (3) at the specific ages, optimal harvest efforts can enhance greatly fishery yield for the harvested population. However, the too late harvest for populations can reduce fishery sustainable yield, and this undesirable effects should be considered alongside harvest efforts when developing hunting regulations or policies; (4) the appropriate age selections will bring about the higher yield when the over-exploitation is implemented; (5) creating reserved areas only for preys greatly enhances fishery stocks and the sustainable yield than the other two patterns. However, setting reserved areas merely for the predator population is undesirable for increasing the fishery stock and yield.

ANALYSIS OF FISHERY BIOECONOMIC IN THE EAST COASTAL OF BINTAN ISLAND

The objective of this study is to measure the potential economic value of fisheries in the area of conservation of seagrass meadows on the eastern coast of Bintan island. The result of this study could became a policy input for managing sustaibale fisheries resources. The presence of seagrass ecosystem could support carrying capacity of the surrounding area of seagrass for growing related biotas that associated with the seagrass ecosystem. The number of benefit of seagrass ecosystem could be measured by production surplus approach. The result of analysis shows that the economic value of fisheries of seagrass ecosystem is equal to the fisheries economic IDR 92.73 billion or IDR 558.28 million hectare-1 year-1 in 2016. The maximum economic yield (MEY) of this artisanal fisheries is 5032 ton year-1 with the number of effort should be maintained at 5281 trip of net fishing gear. A sustainable fisheries management should be maintained in the level of harvest and effort in MEY condition in order to give a maximum benefit to the fishermen and sustaible to the fisheries. Keywords: seagrass ecoystem, seagrass fisheries, bioeconomic of seagrass fisheries, eastern coast of Bintan island, maximum economic yield

INVESTIGAÇÃO SOBRE fishing down marine food webs NO SUL DO BRASIL: IMPLICAÇÕES FINANCEIRAS E PARA A SUSTENTABILIDADE

O crescente desembarque pesqueiro de espécies de nível trófico baixo leva ao efeito fishing down food webs. O presente trabalho investiga se há sinais do efeito na pesca marinha no sul do Brasil entre os anos 1990 e 2007, e discute implicações financeiras e para a sustentabilidade da pesca. Consideraram-se peixes piscívoros os recursos de nível trófico ‘alto’ (P), e os de nível trófico ‘baixo’ os não piscívoros (NP). Compilaram-se informações sobre desembarques e valor de primeiro comércio. Constatou-se que na década de 1990 os desembarques de P eram permanentemente superiores apenas no Rio Grande do Sul, mas na década seguinte passaram a sê-lo também em São Paulo e Santa Catarina, contrário ao fishing down. Apesar de recursos P terem valor de comércio superior a NP, o valor financeiro dos desembarques (VDF) em dois estados caiu mais de 50%: em São Paulo, pela diminuição na safra de NP, como sardinha-verdadeira; e no Rio Grande do Sul, de P, como pescada-olhuda. Em Santa Catarina o VFD cresceu, por aumento nos desembarques de P, como bonito-listrado. Nessa pescaria o fishing down manifesta-se de forma indireta, preocupante, pelo significativo contingente de sardinha verdadeira capturada para uso como isca-viva.

Toothed whale and shark depredation indicators: A case study from the Reunion Island and Seychelles pelagic longline fisheries.

Depredation in marine ecosystems is defined as the damage or removal of fish or bait from fishing gear by predators. Depredation raises concerns about the conservation of species involved, fisheries yield and profitability, and reference points based on stock assessment of depredated species. Therefore, the development of accurate indicators to assess the impact of depredation is needed. Both the Reunion Island and the Seychelles archipelago pelagic longline fisheries targeting swordfish (Xiphias gladius) and tuna (Thunnus spp.) are affected by depredation from toothed whales and pelagic sharks. In this study, we used fishery data collected between 2004 and 2015 to propose depredation indicators and to assess depredation levels in both fisheries. For both fisheries, the interaction rate (depredation occurrence) was significantly higher for shark compared to toothed whale depredation. However, when depredation occurred, toothed whale depredation impact was significantly higher than shark depredation impact, with higher depredation per unit effort (number of fish depredated per 1000 hooks) and damage rate (proportion of fish depredated per depredated set). The gross depredation rate in the Seychelles was 18.3%. A slight increase of the gross depredation rate was observed for the Reunion Island longline fleet from 2011 (4.1% in 2007–2010 and 4.4% in 2011–2015). Economic losses due to depredation were estimated by using these indicators and published official statistics. A loss of 0.09 EUR/hook due to depredation was estimated for the Reunion Island longline fleet, and 0.86 EUR/hook for the Seychelles. These results suggest a southward decreasing toothed whale and shark depredation gradient in the southwest Indian Ocean. Seychelles depredation levels are among the highest observed in the world revealing this area as a “hotspot” of interaction between pelagic longline fisheries and toothed whales. This study also highlights the need for a set of depredation indicators to allow for a global comparison of depredation rates among various fishing grounds worldwide.

Quantifying the benefits of spatial fisheries management – An ecological-economic optimization approach

Improving fisheries management is a key challenge in addressing the United Nations Sustainable Development Goal 2 (Zero Hunger) and support Goals 1 (No Poverty) and 14 (Life Below Water). However, sustaining the ocean’s living resources has important dimensions beyond food security, such as cultural values, which might be of equal importance in some settings. Fisheries management faces special challenges when there is a mismatch between biological units and management units, e.g., when ecological spatial structures are not reflected in how catch limits are set. This might result in overexploitation and even the loss of sub-stocks. We use a spatially structured ecological-economic model parameterized for a pelagic schooling fish to examine how the benefits of implementing spatially differentiated fisheries management depend on biological parameters. We focus on a subset of socio-ecological variables, i.e., fisheries yield, present value of economic surplus, and loss of spawning sites (which might be linked to loss of cultural values) to demonstrate that, in theory, ideally differentiated spatial management can be implemented without exact information about recruitment behavior. For imperfectly differentiated spatial management, however, knowledge about recruitment behavior becomes key to avoiding economic losses and to sustaining stock structure, especially when there is large spatial heterogeneity in biological parameters. Knowledge about variability in site-specific productivity determines the expectation of achievable sustainable harvest levels. Further research on such ecological issues is therefore warranted, both for ecological as well as economic reasons.

The Potential for Cryptic Population Structure to Sustain a Heavily Exploited Marine Flatfish Stock

AbstractComplex spatial structure is widely viewed as an attribute that can contribute to stability in fish populations. Depending on aspects of stock demography and productivity, the existence of complete or partial spatial refugia can enable population persistence when faced with high rates of exploitation. The Southern Flounder Paralichthys lethostigma has been harvested extensively by inshore fisheries in North Carolina for more than three decades, with estimates of fishing mortality (F) surpassing 2.0 year−1 in several years and systems. Potential hypotheses to explain population persistence under heavy exploitation include high levels of recruitment at low stock sizes (high steepness) and/or the maintenance of adult biomass in offshore habitats that remain cryptic to the fishery. We constructed and applied an age‐structured matrix model to evaluate the potential for cryptic stock structure, combined with varying levels of steepness in the stock–recruit relationship, to sustain the Southern Flounder stock across a range of plausible exploitation scenarios. Model simulations predicted that both high steepness in the stock–recruit relationship and large fractions of cryptic biomass were necessary to maintain even modest levels (~25%) of unfished biomass at the high estuarine harvest rates that have likely occurred since the inception of the fishery. When the estuarine fishery exploited the stock at an elevated rate (F > 1.0), high steepness in the stock–recruit relationship alone was insufficient to maintain population biomass at sustainable levels. Only maintaining a large fraction of the adult stock within the spatial refuge afforded by offshore habitats could offset high rates of estuarine exploitation and maintain more conservative levels (40% of unfished biomass) of population biomass. Future efforts should be focused on the identification and preservation of spatial structure within the Southern Flounder stock to maintain both age structure and fishery yield.