Exploited Fish Stocks Research Articles

Using High‐Resolution Fisheries Data to Identify Spatial Patterns in Retained Catch Compositions for Mixed Fisheries

ABSTRACTMixed fisheries exploit fish stocks that are heterogeneously distributed in space using gears that are not species selective. This poses a challenge for management as catch limits for less productive stocks constrain catches of more productive stocks leading to losses in yield and economic value. Decoupling catches of stocks caught together in mixed fisheries could increase potential yields and may be achieved through changes in spatial fishing patterns. This study identifies fine‐scale spatial patterns of retained catches for the UK otter trawlers in UK waters by combining vessel monitoring system positional information and logbook data on retained catches. Spatially contiguous units were identified through a combination of Principal Components Analysis and spatial clustering. Our results show a complex spatial structure in the fish assemblage which differs across UK sea areas, with greater similarity between the Northern North Sea and West of Scotland compared to other sea areas. Through simulation, we highlight how fine‐scale spatially resolved fisheries data can identify areas where choke risks from catches of low‐quota, low‐productivity species associated with target species can be reduced. This underscores the value of fine‐scale data for enhancing efficiency and sustainability in mixed fisheries. Wider benefits from the use of fine‐scale data include the ability to identify consistent spatial métier definitions for use in modelling technical interactions. Ultimately, our study informs strategies and approaches that decouple catches of low‐quota, low‐productivity species caught together in mixed fisheries, improving sustainability and the conservation of living resources under complex management challenges.

Estimates of life-history and growth parameters of exploited fish species in lakes Edward and George: Implications on exploitation status, population dynamics, management, and conservation of native species.

Adequate knowledge is essential for responsible inland fisheries. However, many inland fisheries lack monitoring, and therefore, decision-making for fisheries management is not reliable. In this paper, we used data from surveys and literature to estimate the life-history and growth parameters of 16 exploited fish stocks in the Ugandan part of Lake Edward and Lake George (East Africa). The estimated parameters are pivotal indicators of fish stock status, particularly in data-poor fisheries. The estimated parameters included maximum length (Lmax) and mean length (Lmean) as indicators of size structure in experimental and commercial catches, coefficients of length-weight relationships, length at 50% maturity (Lm50), fecundity, von Bertalanffy parameters, total mortality (Z), and natural mortality (M). These parameters were estimated using empirical formulae, statistical methods, and analyses of length frequencies. Only two stocks of semutundu Bagrus docmak exhibited significant and increasing trends in Lmax (Lake Edward) and Lmean (Lake George). The estimates for the remaining parameters were consistent with those in FishBase and other literature resources, either for the same species or related species. This consistency indicates their reliability for application in decision-making and further assessments. Some parameters showed evidence of unsustainable fishing. For example, estimates of Lm50 for four of the assessed stocks belonging to two species (Nile tilapia Oreochromis niloticus and marbled lungfish Protopterus aethiopicus) were lower than baseline estimates in the studied waterbodies. Furthermore, the Lmean in catches for all the stocks were less than the optimum lengths (Lopt), which maximize catches with a minimal impact on biomass and size structure. No significant changes in Lmean, length-frequency distributions, and size at maturity could be attributed to the management changes implemented in 2018, probably because it is too early to observe changes in these parameters. However, there are positive signs attributable to the changes in management as shown by a high proportion of mature individuals in commercial catches for most of the stocks for which the proportion was calculated, and an increase in Lmean and Lmax for some stocks, such as B. docmak, in commercial or experimental catches. New estimates from this study will enhance decision-making and further assessments of fisheries. Routine monitoring is recommended to update and improve the estimates.

Exploitation of mesopelagic fish stocks can impair the biological pump and food web dynamics in the ocean

The demand for marine living resources is increasing at an unprecedented scale because of the need for continuous food provision to the world’s population. The potential of already exploited fish stocks to meet this demand is limited. Therefore, mesopelagic fish have recently become attractive potential targets for fisheries because of their vast conjectured biomass. However, the role of mesopelagic fish in marine ecosystems is poorly understood. Before developing commercial exploitation plans, the relationship between mesopelagic fish and other groups in the marine food web and biogeochemical cycles should be analyzed quantitatively. In this study, we coupled a one-dimensional biogeochemical model (North Atlantic Generic Ecosystem Model) with a higher-trophic-level food web model (Ecopath with Ecosim) for the Sargasso Sea in the North Atlantic to investigate changes in carbon export and trophodynamics under two mesopelagic fish harvesting scenarios. The coupled model represented the marine food web from plankton to fish and mammals, vertical carbon export dynamics, and their interaction with fisheries. The results showed that when mesopelagic fish were not harvested, they contributed approximately 6% of the total carbon export in the surface waters, but up to 40% of the total carbon export below 400 m. Harvesting mesopelagic fish altered the energy transfers within the food web as well as to fisheries. The ecological footprint of fisheries increased significantly. Due to declining competition in the food web, epipelagic fish increased to exert elevated grazing pressure on phytoplankton; hence, phytoplankton-mediated carbon export decreased. The total carbon export decreased by 14% due to the decreases in mesopelagic fish- and phytoplankton-mediated carbon exports. The simulated increase in zooplankton- and non-mesopelagic fish-mediated carbon exports (up to 92% and 96%, respectively) did not compensate for the total decrease in carbon exports under harvesting scenarios. The findings of this study highlighted that mesopelagic fish not only have a direct control on carbon dynamics by their metabolic releases and diel vertical migration, but also strong indirect controls through prey-predator interactions within the food web. Therefore, the implications of harvesting mesopelagic fish should be carefully considered from a holistic perspective.

Projecting contributions of marine protected areas to rebuild fish stocks under climate change

No-take marine protected areas (No-take MPAs) are considered as a major tool for conserving marine biodiversity and ecosystem services. No-take MPAs can also contribute to climate adaptation for exploited fish stocks. Meanwhile, many fish stocks in the world are overfished and management institutions are developing plans to rebuild them. Understanding the potential effects of no-take MPAs on fish stocks under climate change can help develop strategies for climate-resilient stock rebuilding. Here, using a linked climate-fish-fishing model, we undertake simulation experiments to examine the effects of no-take MPAs on biomass and potential catches of 231 exploited fish and invertebrate species in eight marine ecoregions in the Northeast Atlantic under climate change. The simulations include different levels of fishing, no-take MPAs coverage, atmospheric global warming levels, and account for the expected displacement of fishing to the area around the no-take MPAs. Average individual stock biomass is projected to decrease by 5–15% per degree Celsius atmospheric warming. Having 30% of the distribution of over-exploited fish stocks under no-take MPAs together with conservation-focused fisheries management of these stocks are projected to offset the negative impacts on their biomass under 2.6–2.9 °C global warming. Meanwhile, potential catches increase when a portion of the over-exploited fish stocks is protected from fishing as higher biomass in the no-take MPAs spills-over to the surrounding areas. Our findings highlight that no-take MPAs, combined with reducing fishing intensity, can help rebuild over-exploited fish biomass and benefit their dependent fisheries in the Northeast Atlantic under projected climate change in the 21st century.

Spatiotemporal variation in the size structure of the chub mackerel, Scomber japonicus, over half a century in the East Asian Marginal Seas

The size and age structure of a population are crucial aspects for understanding population dynamics. Many marine fish species have experienced long-term reductions in size and age owing to overfishing and/or global warming; however, these aspects remain unexplored. The present study elucidated the spatiotemporal variations in chub mackerel size and age structure based on the operational reports of large- and medium-sized Japanese purse seine fisheries for nearly half a century. Our results revealed size- and age-dependent segregation at a fine spatial scale, which may indicate ontogenetic niche shifts in response to thermal and feeding conditions or intercohort competition. On a long-term temporal scale, a significant reduction in size and age was observed in the East China Sea, whereas a contrasting increase in size and age was observed in the northern Sea of Japan. This may indicate that large fish have greater sensitivity to global warming and shift their distribution more rapidly northward than do small fish. In addition, the reduction in size and age during the spawning season in the East China Sea may result in a bias towards younger spawners, potentially rendering population dynamics more unstable. This study highlights the importance of investigating the spatiotemporal variation in the size structure of exploited fish stocks.

Bio‐ecological studies on two cichlids in the Brimsu reservoir of Ghana

AbstractAlthough small‐scale fisheries and aquaculture provide opportunities to increase fish yield to supplement rural food and income, they remain undeveloped in Ghana. To contribute towards the development of the sectors, this study sought to assess the biology and ecology of Oreochromis niloticus and Sarotherodon melanotheron in the Brimsu reservoir to provide the requisite information for their management. From the results, mean temperature, dissolved oxygen, pH and transparency of the reservoir were 30.03 ± 0.27°C, 3.78 ± 0.38 mg/L, 8.30 ± 0.21 and 69.79 ± 1.76 cm, respectively. S. melanotheron had a sex ratio of 1:1.36; with mean condition factors of 2.177 ± 0.089 and 2.06 ± 0.029 (p = 0.08) for males and females, respectively. The sex ratio of O. niloticus was 1.45: 1; with mean condition factors of 2.208 ± 0.012 and 2.257 ± 0.014 (p = 0.07) for males and females, respectively. Both species exhibited positive allometric growth with no significant differences between the condition factors for males and females. In addition, there was no significant difference between the sex ratios for S. melanotheron but the number of males of O. niloticus was significantly higher than the females (p ˂ 0.05). Size at sexual maturity (Lm), form factor (a3.0), growth coefficient (K), asymptotic length (L∞) and natural mortality (M) of S. melanotheron were 18.00 cm, 0.020, 0.42 year−1, 30.32 cm and 0.64 year−1, respectively. For O. niloticus Lm, a3.0, K, L∞ and M were 18.57 cm, 0.0223, 0.26 year−1, 38.42 cm and 0.41 year−1, respectively. However, the catch per unit effort for O. niloticus was higher than that of S. melanotheron throughout the study period. The conditions of the reservoir were favourable for the growth of these cichlids. It is recommended that measures should be put in place for the sustainable management and exploitation of fish stocks.

Including older fish in fisheries management: A new age‐based indicator and reference point for exploited fish stocks

AbstractLarger and older fish contribute disproportionately to spawning and play an important role in the replenishment of exploited stocks. Fishing often removes specific size‐ and age‐classes, with direct impacts on stock productivity and population resilience. Despite this, fisheries advice is commonly based on estimates of spawning stock biomass (SSB) and fishing mortality (F) and makes little reference to the importance of size and/or age structure. Consequently, there is a need for indicators of size and/or age structure to better inform fisheries management and help assess global sustainability goals. Here, we introduce a new age‐based indicator ABIMSY that monitors age structure relative to the equilibrium age structure at FMSY. We apply this new indicator to 72 commercially important stocks in the Northeast Atlantic, covering 26 species, which collectively contributed 86% of all commercial catches in the region in 2019. We estimate that 62% (45 stocks) currently have proportionally fewer older fish relative to FMSY conditions, whereas 38% (27 stocks) have proportionally more older fish; we also note patterns with respect to geographic area and taxonomic family. Simulation testing demonstrated that ABIMSY is responsive to overfishing and generally tracks (with high sensitivity and specificity) a common measure of stock depletion, SSB relative to BMSY. Throughout, we show that ABIMSY provides information on the age structure of exploited stocks that is complementary to conventional reference points for SSB and F. Further, the framework used to estimate ABIMSY make it well placed for integration into current advisory frameworks on fisheries management.

Catches, bycatches and stock indicators of fisheries targeting cyprinids along the Swedish Baltic Sea coast

Decreasing abundance of many traditionally exploited fish stocks in the Baltic Sea force small-scale fisheries to find new ways to make a living. In line with Swedish national strategies on food supply there is an interest to develop commercial cyprinid fisheries. In the Bothnian Bay in the northern part of the Baltic Sea, annual catches have increased from zero-catches 2018–30 tonnes 2021. To aid a sustainable development of these cyprinid fisheries that target mainly bream (Abramis brama) and ide (Leuciscus idus), we study catch efficiency of target species and bycatch in different gears and seasons using logbook data from the Bothnian Bay. Using cameras, we also assessed bycatch rates. To assist the sustainability of the fishery we develop potential stock indicators. Our results suggests that larger gear (pound-nets) are more effective in catching bream, and that the proportion of bycatch decreased with gear size, being < 10% in the largest gear, which is similar or lower than many other Baltic Sea fisheries. By-catches of salmon is of concern in the Bothnian Bay, but the camera study indicates that salmon bycatches are sporadic. Catch per unit effort (CPUE) of bream was highest in spring and fall, and we conclude that site specific median CPUE is the most suitable stock abundance indicator. The size indicator L90, the 90th percentile of the length distribution, was similar among areas and we propose it as a suitable indicator of the demographic structure of the targeted bream stocks. Our results provide reference points for relatively unfished conditions, but as the study was based on mainly fishery dependent data, it is important to also include fishery independent data to assess ecosystem effects of a future and intensified cyprinid fishery.

The interactions between fishing mortality, age, condition and recruitment in exploited fish populations in the North Sea

A quantity of studies have documented how greater condition and older age of individual fish enhances reproductive success via a variety of processes from egg production to larval survival. If these relations translate into relationships between stock demography and recruitment, maintaining a “healthy” age composition may be a key step towards ensuring stock productivity. We examined the correlation between indicators of the age and size composition and the residual-variation from the stock-recruitment relationship (i.e. the variation not explained by the spawning stock biomass) in eleven commercially exploited fish stocks in the North Sea. We also analyzed to what extent spawner age and size composition responded to variations in fishing mortality. Before detrending to accommodate any temporal patterns, recruitment residuals correlated negatively with the relative age composition in all eleven stocks (plaice, whiting, and saithe being significant). However, after detrending the recruitment residuals, the correlations weakened and only nine out of eleven stocks correlated negatively. Lastly, we also conducted a delay difference analysis, which yielded a more or less even distribution between negative and positive correlations, and in addition two stocks (cod and Norway pout) came out showing significant positive correlations between recruitment residuals and age composition. In contrast, eight of the eleven stocks showed positive correlations between spawner weight and recruitment residuals and correlations were significant for plaice, cod, and saithe. After detrending the data, the significant correlations disappeared, but the overall tendency towards positive correlations between spawner weight and recruitment residuals prevailed. Overall, these results suggest that fish condition is more likely to show positive relationships with recruitment success than age composition. Lastly, it was found that the majority of stocks showed a strong link between fishing mortality and the two indicators of age composition, and when looking across stocks, the age composition appears to have returned to the same level as in the 1960 s. Conversely, spawner weight at age have not recovered, and considering the indications of a positive link between spawner weight and recruitment, recruitment success of the North Sea stocks may remain low in spite of the apparently successful rebuilding of the age composition.

A review of an emerging tool to estimate population parameters: the close-kin mark-recapture method

Knowing the number of individuals in a population is fundamental for the sustainable management of exploited marine resources but estimating this parameter is often extremely challenging, especially in large, highly mobile and dispersed populations. Abundance estimation traditionally relies on multiple data types that include the relationship between fishery catches and effort (Catch Per Unit Effort or CPUE), scientific research surveys and demographic models that are developed to estimate past and current stock dynamics, but uncertainty is often high. Close-kin mark-recapture (CKMR) is an alternative method for estimating abundance and other demographic parameters (e.g. population trend, survival rates, connectivity), using kinship relationships determined from genetic samples. This methodology is based on a simple concept - the larger the population the less likely to find relatives and vice versa - and was proposed two decades ago although regained considerable attention recently. Refinements in the statistical methodology and advances in high throughput sequencing methods have boosted the efficiency of genomic analysis, promising to revolutionize the field of fisheries stock assessments. In theory, it can be applied to almost any species, provided that there is sufficient information about the life-history/biology of the organism and that the populations are not so small as to be almost extinct or so large that finding relatives becomes extremely difficult. Thus, it has the potential to provide baseline data for the many exploited fish stocks that remain largely unassessed and to reduce uncertainty in those that are regularly evaluated. Here, we provide an overview of the method in the context of fisheries assessments, analyze the advances and synthetize the field studies published in the last five years. Moreover, we evaluate the readiness, viability and maturity of the method to infer demographic parameters in species spanning diverse life histories. We evaluate technical considerations and requirements for a successful application and analyze the main challenges and limitations preventing a broader implementation.

The potential use of genomic methods in bottom trawl surveys to improve stock assessments in Europe

In Europe, research surveys carried out by state governments provide the great majority of fishery-independent data. Member States (MS) in the European Union (EU) regularly conduct research surveys to provide the necessary data to assess the status of exploited fish stocks and to monitor the general condition of the marine ecosystem. In the surveys, samplings are carried out throughout the distribution range of the targeted fish species using standardized gears (e.g., trawls and seines) and other methods (e.g., hydroacoustics and underwater cameras). In the context of fish stock assessment, survey data are important because they provide indices that help tuning the stock assessment models (e.g., the index of fish abundance) and key information about the size and age distributions of the stock, the size-age relationships, the proportion of fish mature at each age, and information on reproductive performance of the stocks. However, research surveys have a number of shortcomings that include, for example, a high economic cost coupled with complex logistics and a long time required for processing the collected data. In addition, some of the parameters that are needed in stock assessment cannot be estimated from survey data for certain commercially important species. For instance, age is usually determined using hard structures (such as otoliths) in fish target species. However, for European hake, age cannot be determined accurately because there are many difficulties in interpreting the ring patterns of the otoliths. This highlights the need to look for alternative methodologies such as genomics, that have the potential of improving the data obtained from research surveys and hence, improve fish stock assessments. Considering this, we carried out a review of the bottom trawl research surveys in the EU with the purpose of: 1) identifying the current approaches for monitoring fishery resources and the ecosystem and 2) determining how genomic techniques can be used to improve survey data, taking into account the needs of current and future stock assessment in Europe.

Phenotypic Stock Evaluation of Plagioscion magdalenae (Steindachner, 1878): A Species in the Dique Channel in Colombia

Inland fishing is an essential activity for the livelihood and food security the Colombian population. The knowledge and evaluation of exploited fish stocks is a priority to develop sustainable management and conservation strategies of the fisheries. To optimize the management processes of fishery resources and conservation of species, it is necessary to evaluate the population structure and identification of stocks. Geometric morphometrics analysis have shown useful in the evaluation of fish stocks. This study focuses on the species Plagioscion magdalenae, commonly called “Pacora”, corvinata, or river croaker, which belongs to the family Sciaenidae, a family characterized as an important fishery resource. With the aim of generating a baseline about the state of the P. magdalenae population structure, samples were collected along the marshy complex of the Dique channel, Colombia, between December 2020 and October 2021. In this study, the existence of morphometric variability between individuals of Plagioscion magdalenae was found across sampling sites, Ciénaga de Capote and Ciénaga del Jobo; shape differences between location suggest the action of environmental pressures and the existence of anthropogenic pressures, such as unsustainable artisanal fishing.

Potential solutions to environmental conflict on exploitation of fish stocks in palk strait among fishermen of India and Sri Lanka

Illegal catching of fish stocks beyond the borders of India and Sri Lanka by violating the International Maritime Boundary Line (IMBL) by the fishing communities of both countries has become a major international direct environmental conflict between both nations. Overexploitation of natural resources is a keystone environmental problem. It has been reported that Indian fishermen are stealing fish catch worth many millions of US dollars annually from Sri Lankan coastal waters. The bilateral agreements of 1974 and 1976 are frequently violated. The heavy use of trawlers that are not permitted to fish on coastal seas and the use of internationally banned bottom sea nets by Indian poachers are major threats to the coastal resources of north Sri Lanka. Trawlers catch fish flocks unselectively; the catch may include several non-targeted species and juvenile stages of fish, which is leading to species extinction. Arresting and imprisoning of fishermen by coastal guards of both countries (and collecting penalties) continue, and this is an unsustainable way of handling the issue. Sri Lanka has banned bottom trawling since the 6th of July 2017, which is a sustainable solution and beseeched possible greener solutions such as creating awareness, sensitizing fishermen, and promoting the use of GPS navigation, and also provoked conventional solutions such as monitoring or patrolling, which often lead to imprisonment and penalties for trespassing and seizing of fishing trawlers. Restorative solutions such as coral restoration, mangrove restoration, implantation of artificial reefs, and establishing mutually beneficial ‘no catch zone’ along the IMBL and marine reserves (or marine protected areas) by both nations may facilitate preventing the deterioration of fish stocks and ensuring their revival in the region. Above all, if this illegal catching continues, the availability of fish stocks in the Palk Strait region in the next ten years will be in question.

Exploring reconfiguration of trends in CPUE to model sustainable TAC options for the pivotal YSLME anchovy fishery

Sustainable utilization of exploited fish stocks generally relies on characterizing key aspects of their population dynamics using mathematical models to evaluate their stock status. In this study, the Anchovy population (Engraulis japonicus) in the Yellow Sea large marine ecosystem (YSLME) was evaluated by using the ‘Just Another Bayesian Biomass Assessment’ (JABBA) model. Catch and the anchovy scientific survey biomass index data from 1989 to 2021 in its nominal or raw form, plus three adjusted CPUE trends using data acquired from the China Fishery Statistical Yearbook were each used to fit JABBA to explore the performance of reconfigured CPUE for setting a total allowable catch (TAC) quota to sustain the YSLME commercial anchovy fishery. Results showed contrasting trends in estimated biomass from the fishery independent surveys, and nominal and reconfigured commercial fishery CPUE, with the latter producing the most plausible results. The JABBA output based on the scientific survey biomass index indicated a 56.7% probability that the anchovy stock is currently healthy with 1.04B2021/BMSYand 0.51F2021/FMSYratios, whereas JABBA estimates based on the nominal commercial CPUE-based were overly optimistic (100% probability stock was healthy). JABBA outputs based on the first two reconfigured CPUEs (FacpueyandFECcpuey) countered this overoptimism, with the survey biomass index-based assessment results lying between the extremes. This improvement in the modelled assessment was achieved because the reconfigured CPUE time series accommodated technological progress in fishing vessels and gear performance. Therefore, in instances where survey biomass index data are unavailable, we recommend using the two improved fishery CPUEs and combining the results of the two assessments to guide fishery management. Management reference points, however, should preferably be based on the more conservative of the two reconfigured CPUEs as a precautionary approach for setting a total allowable catch (TAC) whilst concomitantly considering the role of anchovy as prey for other economically important fish in the system.We encourage researchers to extensively review and improve the quality and availability of fisheries statistical data worldwide in the shared endeavour towards obtaining more realistic stock assessments.

An approach to assess exploited fish stocks compliant to the requirements of the Marine Strategy Framework Directive (MSFD) including criterion D3C3

The Marine Strategy Framework Directive (MSFD) complements European fish stock assessments under the Common Fisheries Policy (CFP). CFP assessments separately assess fishing mortality (F) and spawning stock biomass (SSB). Contrary, within its third descriptor (D3) of good environmental status (GES) the MSFD requires to assess exploited fish stocks against three criteria, which are F (D3C1), SSB (D3C2) and the age or size structure (D3C3) within the stock. Further, the MSFD requires to integrate the status of all three criteria to determine whether a stock has achieved GES. The full implementation of MSFD compliant stock assessments has been impaired by the lack of operational indicators for age/size structure. This study presents an approach to assess D3C3 by analysing two indicators i.e. recruitment (R) and SSB/R using time series-based assessments. R and SSB/R reflect the small and large components of a stock and are used as proxy indicators for stock productivity and realised growth potential. Using stock assessment data from 20 North East Atlantic fish stocks, a RandomForest model validated the sensitivity of SSB/R against F and two selectivity indicators. Further, this study introduces an approach to integrate the assessment outcomes of the three D3-criteria by stock. The here demonstrated approach of assessing fish stocks according to D3 of the MSFD relies only on data available from stock assessment summary sheets and is thereby easy to implement for all stocks with analytical stock assessments. The implications of and future direction for assessing D3C3 are discussed.

Can price feedbacks cause human behavior-induced tipping points in exploited fish stocks? An extension of the bioeconomic Gordon-Schaefer model

The rarity of a good can increase its price. This so-called price feedback can motivate harvesters to exploit a fish stock even at low stock sizes, possibly contributing to collapse. To systematically examine the impact of price feedbacks on the bioeconomic equilibrium expected in a commercial fishery under regulated open access, we extend the standard Gordon–Schaefer harvesting model by incorporating a negative dependence of the price per fish on aggregate fish landings produced locally by the collective of fishers. We analytically show that both under Cournot competition (in which each fisher has market power in actively shaping the market price) and perfect competition (in which each fisher is a passive price taker) alternative stable steady states and a tipping point may emerge thanks to dynamic interactions between the variable market price of fish, fish population growth, and decisions of fishers in response to expected revenue. One of the bioeconomic steady states is found to be unstable, thus constituting a tipping point, where harvesters can abruptly drive fish stocks to low-abundance states and ultimately collapse. Mathematically, we show that under the assumption of a price feedback, the necessary condition for the existence of alternative steady states requires the ratio of catchability (or more generally the benefit of fishing) to marginal fishing cost to be sufficiently high. High prices at low stock sizes, subsidies, hyperstable catch rates, low opportunity costs of fishing (i.e., due to part-time fishing) or high benefits associated with non-catch aspects of fishing (e.g., in recreational fishing) all contribute to keeping the benefit-cost ratio high and motivate the maintenance of fishing pressure also at low stock sizes. We discuss these and other conditions that predispose a tipping point under regulated open access, which is particularly likely for small-scale fisheries with local markets and low substitutability of a given fish product, such as lagoon pike (Esox lucius) and eel (Anguilla anguilla) fisheries of the southern Baltic Sea. By contrast, the suggested price effect is much less likely or not present at all in global markets, such as the one for cod (Gadus morhua) in the Baltic Sea, where local supply is unlikely to affect market price. The sufficient condition for a tipping point requires that the number of fishers is within a specific intermediate range: if the number of fishers is small relative to stock productivity, there is only one stable steady state associated with a high fish population size, while if the number of fishers is high relative to stock production, again only one stable steady state associated with a low fish population is predicted. Therefore and on first sight counterintuitively, reorganization of a fishery from high to intermediate fisher numbers, e.g. as a preventive measures to avoid too high fishing pressures in response to lasting and productivity-harming environmental changes, or new entrants into a fishery may trigger alternative stable steady states, tipping points and destabilization of a fishery.

Applying life history traits to eco-labelling schemes for fisheries management: A case study in Taiwan

Eco-labelling scheme is an emerging management strategy for promoting the sustainable development of marine resources and fisheries. The recommendation lists for seafood that reflects the status of exploited fish stocks, based on scientific evidences and categorize species accordingly is a commonly used eco-labelling scheme. Nevertheless, the status of fish stocks may be influenced by abiotic factors and biotic factors which may affect the categorization of fish species in recommendation lists. In this study, four life history traits (maximum length, length at maturity, age at maturity and growth coefficient) and two ecological parameters (trophic level and unit price) of 35 marine fishes in the waters around Taiwan were examined through multivariate statistical analysis and compared against the Seafood Guide Taiwan. Two main factors were identified through principal component analysis. The first factor was correlated with maximum length, length at maturity and growth coefficient, and the second factor was correlated with age at maturity and trophic level. Two fish groups were identified according to the PCA bi-plot. The Best Choices and Good Alternatives categories had higher growth coefficients, and the Avoid group had a smaller age at maturity. The results indicated that the life history traits of fish stocks may serve as complementary information to seafood guides, and may have substantial implications for the future planning of eco-labelling schemes. Nevertheless, when developing a framework for a domestic eco-labelling scheme, the decision-makers should consider the applicability of framework components in the global market as well as research on fish stock status and management strategies.

Addressing the dichotomy of fishing and climate in fishery management with the FishClim model

The relative influence of fishing and Climate-Induced Environmental Change (CIEC) on long-term fluctuations in exploited fish stocks has been controversial1–3 because separating their contributions is difficult for two reasons. Firstly, there is in general, no estimation of CIEC for a pre-fishing period and secondly, the assessment of the effects of fishing on stocks has taken place at the same time as CIEC4. Here, we describe a new model we have called FishClim that we apply to North Sea cod from 1963 to 2019 to estimate how fishing and CIEC interact and how they both may affect stocks in the future (2020-2100) using CMIP6 scenarios5. The FishClim model shows that both fishing and CIEC are intertwined and can either act synergistically (e.g. the 2000-2007 collapse) or antagonistically (e.g. second phase of the gadoid outburst). Failure to monitor CIEC, so that fisheries management immediately adjusts fishing effort in response to environmentally-driven shifts in stock productivity, will therefore create a deleterious response lag that may cause the stock to collapse. We found that during 1963-2019, although the effect of fishing and CIEC drivers fluctuated annually, the pooled influence of fishing and CIEC on the North Sea cod stock was nearly equal at ~55 and ~45%, respectively. Consequently, the application of FishClim, which quantifies precisely the respective influence of fishing and climate, will help to develop better strategies for sustainable, long-term, fish stock management.

Fisheries Reference Points under Varying Stock Productivity and Discounting: European Anchovy as a Case Study

European anchovy (Engraulis encrasicolus) is the main commercially exploited fish stock in the Black Sea region, providing a vital source of livelihood and revenue for local communities and national economies. In recent decades, the Black Sea anchovy stock has faced many human-induced threats, including overfishing, eutrophication, invasive species, and climate change while these threats have raised concerns about the status and long-term productivity of the stock. To ensure sustainable levels of exploitation under potential future changes in stock productivity, we here estimate and compare a suite of biological and economic reference points under different levels of stock productivity and discount rates using an age-structured bioeconomic model setup. Our model simulations showed that optimal fishing mortalities achieving maximum sustainable yield (FMSY) and maximum economic yield (FMEY) increase at higher stock productivity but are always lower than the historically high mean levels of exploitation. Furthermore, we illustrate that the stock biomass at maximum economic yield (BMEY) is larger than the stock biomass at maximum sustainable yield (BMSY) at all stock productivities and discount rates, except at low stock productivity under high levels of discounting (i.e., 10%, 20%). By illustrating the ecological and economic benefits of reducing exploitation rates, we expect that our estimated reference points can add value to the decision-making process for the management of the European anchovy fishery and ensure long-term sustainable management even under future climate-driven changes in stock productivity.

Assessing the patchiness of early life stage of a fish stock (Gadus morhua) and its contribution to the stock recruitment

Patchiness, defined as spatial heterogeneity in distribution of organisms, is a common phenomenon in zooplankton including ichtyoplankton. In heterogeneous landscapes, depending on the scale of prey and predatory distributions, individuals in patches may experience distinct differences in the survival rate compared to individuals distributed more homogeneously outside patches. In this study, we focused on drifting eggs and larvae of Northeast Arctic (NEA) cod, one of the largest exploited fish stock in the world. The eggs and larvae are largely distributed along the north-western coast of Norway and northern Russia. We ask to what degree individuals are located in patches contribute to the species recruitment. For this purpose, we developed a patch recognition method to detect the existence of patches in particle tracking simulations using a connected-component labeling algorithm. We then assessed the contribution of individuals in detected patches to the total recruitment. Our results showed that depending on year, day of year, and resolution scale for detection of patches, recruits present in patches can vary between 0.6% and 38.7% with an average of 20.4% of total recruitment. The percentage decreased with increasing day of year in the drifting season but increased with decreasing patch resolution scale, down to the finest investigated scale of 8 km. On the basis of these results, we advise field recruitment studies of NEA cod to at least resolve an 8-km spatial scale to capture effects of spatial heterogeneity in the survival rate on the species recruitment.