Search

The relationships between fecundity and size of Atlantic herring (Clupea harengus) were estimated within five different spawning areas off the coast of Nova Scotia in 2019 and 2020. Statistically significant differences in fecundity relative to body weight were observed among spawning areas and between years. Fecundity-at-length on the German Bank spawning ground was 29%-36% and 22%-28% lower than estimates from 2001 and 1970, respectively. Temporal changes in weightand relative fecundity-at-age resulted in a decrease in the number of eggs-per-recruit (in an equilibrium unfished state) by 50% and a decrease of 27% in the egg production per tonne of spawning stock biomass (SSB) in 2020 relative to 1970. Decreases in SSB-per-recruit and eggs-per-recruit over time resulted in proportional decreases in equilibrium SSB at maximum sustainable yield (MSY); however, the fishing mortality rate (F) at MSY remained relatively stable over time. Total egg production was shown to be disproportional to SSB. Equilibrium SSB at MSY was greater (and F at MSY lower) when estimated using eggs-per-recruit compared to SSB-per-recruit. Failing to account for fecundity and assuming that egg production is proportional to SSB resulted in an overestimate of stock status. Les relations entre la fecondite et la taille des harengs atlantiques (Clupea harengus) ont ete estimees pour cinq lieux de frai differents au large de la Nouvelle-Ecosse en 2019 et 2020. Des differences statistiquement significatives de la fecondite par rapport a la masse corporelle sont relevees entre les lieux de frai et d'une annee a l'autre. La fecondite selon la longueur sur le banc German et de 29-36 % et 22-28 % plus faible que les valeurs estimees pour 2001 et 1970, respectivement. Des changements dans le temps de la masse et de la fecondite relative selon l'age se traduisent par une diminution du nombre d'oeufs par recrue (dans un etat d'equilibre sans peche) de 50 % et une baisse de 27 % de la production d'oeufs par tonne de biomasse du stock reproducteur (BSR) en 2020 par rapport a 1970. Des baisses de la BSR-par-recrue et des oeufspar-recrue au fil de temps se traduisent par des diminutions proportionnelles de la BSR pour un rendement maximum durable (RMD); cependant, le taux de mortalite par peche (F) au RMD demeure relativement stable au fil du temps. Il est demontre que la production d'oeufs totale n'est pas proportionnelle a la BSR. La BSR a l'equilibre au RMD est plus grande (et F au RMD est plus faible) quand elle est estimee en utilisant les oeufs-par-recrue plutot que la BSR-par-recrue. Le fait de ne pas tenir compte de la fecondite et de presumer que la production d'oeufs est proportionnelle a la BSR se traduit par une surestimation de l'etat du stock. [Traduit par la Redaction], Introduction Estimation of life history parameters is important for understanding fish population dynamics and estimating biological reference points (BRPs) used for management decisions (Brooks et al. 2010; Zhou et al. [...]

Selectivity in fish is often size-dependent, which results in differential fishing mortality rates across fish of the same age, an effect known as 'Lee's Phenomenon'. We extend previous work on using length composition to estimate the spawning potential ratio (SPR) for data-limited stocks by developing a computationally efficient length-structured per- recruit model that splits the population into a number of subcohorts, or growth-type-groups, to account for size- dependent fishing mortality rates. Two simple recursive equations, using the life history ratio of the natural mortality rate to the von Bertalanffy growth parameter (M/K), were developed to generate length composition data, reducing the complexity of the previous approach. Using simulated and empirical data, we demonstrate that ignoring Lee's Phenomenon results in overestimates of fishing mortality and negatively biased estimates of SPR. We also explored the behaviour of the model under various scenarios, including alternative life history strategies and the presence of size-dependent natural mortality. The model developed in this paper may be a useful tool to estimate the SPR for data- limited stock where it is not possible to apply more conventional methods. La selectivite des poissons depend souvent de la taille, ce qui se traduit par differents taux de mortalite par peche pour des poissons du meme age, un effet appele << phenomene de Lee >>. Nous elargissons des travaux anterieurs sur l'utilisation de la distribution des longueurs pour estimer le rapport du potentiel de reproduction (RPR) de stocks pour lesquels les donnees sont limitees, en developpant un modele par recrue structure par la longueur efficace sur le plan informatique qui divise la population en un certain nombre de sous-cohortes, ou groupes de type de croissance, pour tenir compte des taux de mortalite par peche dependant de la taille. Deux equations recursives simples, qui utilisent le rapport associe au cycle biologique du taux de mortalite naturelle et du parametre de croissance de von Bertalanffy (M/K), ont ete etablies pour generer des donnees de distribution des longueurs, reduisant du coup la complexite de l'approche anterieure. En utilisant des donnees simulees et empiriques, nous demontrons que le fait de ne pas tenir compte du phenomene de Lee entraine une surestimation de la mortalite par peche et des estimations du RPR caracterisees par des erreurs systematiques negatives. Nous avons egalement explore le comportement du modele pour differents scenarios, dont differentes strategies de cycle biologiques et la presence d'une mortalite naturelle dependant de la taille. Le modele pourrait constituer un outil utile pour estimer le rapport du RPR pour des stocks pour lesquels les donnees sont limitees auxquels l'application de methodes plus traditionnelles n'est pas possible. [Traduit par la Redaction], Introduction To maintain the long-term stability of yield from capture fisheries, the dynamics of fish populations and the impact of harvesting these populations must be understood. While there have been [...]

Fisheries managers are in need of quantitative tools to inform decisions regarding selection of robust management practices, prioritising research gaps and stocks to focus on, particularly where there are limited resources or data. To support these decisions, the use of Management Strategy Evaluation (MSE), that is, closed loop simulation‐testing of management procedures, is widely regarded as best practice. However, applying MSE is time‐ and computationally intensive, and requires highly skilled expertise and processes for stakeholder input and peer review. For data‐ and capacity‐limited fisheries, MSE may be particularly challenging to implement. Yet, these are the contexts where it is most critical to test assumptions, evaluate the implications of all sources of uncertainty and identify the most informative data sources. To facilitate wider use of MSE, the Method Evaluation and Risk Assessment (MERA) framework was developed as an accessible online interface, with quick processing time, focused on generic data‐limited management procedures, but allowing progression to tailored and more data‐rich methods. The framework links a quantitative questionnaire and data input standard to a flexible operating model with optional customisation via command line access to the back‐end open‐source R libraries. Here, we illustrate a case study application of MERA for the bocinegro (Pagrus pagrus, Sparidae) fishery in the Gulf of Cadiz, where in conjunction with fishery stakeholders, a custom management procedure was developed and tested and key research gaps and data collection priorities were identified. We discuss implications for wider use of MSE in various contexts, including eco‐certification and fishery improvement projects. [ABSTRACT FROM AUTHOR]

A retrospective pattern within a stock assessment occurs when historical estimates systematically increase or decrease as data are removed and has been cited as a cause of persistent overfishing. For two case studies, Gulf of Maine cod and New England pollock, we demonstrated how closed-loop simulation can be used to evaluate the impacts of retrospective patterns with respect to management objectives. Operating models (OM) representing alternative states of nature were developed and various management procedures (MP) that account for retrospective patterns in the fitted assessment models were applied. From the cod example, downward adjustment of the catch advice based on Mohn's rho was more beneficial over model averaging (MA) to meeting biomass objectives from the cod example and avoiding stock crashes. For pollock, downward adjustment and MA were robust to meeting biomass objectives at the cost of foregone catch. The ability to discriminate OMs, using indicators generated from the simulated projections, varied by MP and time for cod, but was poorer overall for pollock. This framework could be used to identify if retrospective issues generate poor management outcomes and, in some cases, alleviate pressure to identify the single most credible state of nature. [ABSTRACT FROM AUTHOR]

The precautionary approach to fisheries management advocates for risk‐averse management strategies that include biological reference points and account for scientific uncertainty (i.e. process, model and observation uncertainty). In this regard, two approaches have been recommended: (a) biomass reference points to safeguard against low stock biomass, and (b) uncertainty buffers that reduce the catch limit as a function of the scientific uncertainty. This study compares the effectiveness of these two precautionary approaches in recovering over‐exploited fish stocks. We evaluate the performance of more than 80 harvest control rules (HCRs) within a stochastic management strategy evaluation (MSE) framework for three stocks with contrasting life‐history parameters and under various levels of scientific uncertainty. The results show that both approaches reduce the risk of overfishing at the expense of expected yield. This risk‐yield trade‐off strongly depends on the HCRs, life‐history parameters of the species, as well as the level of the scientific uncertainty. Nevertheless, some combinations of biomass threshold and limit reference points as well as uncertainty buffers lead to a more favourable risk‐yield trade‐off than other rules. This study elucidates the multiple factors affecting the effectiveness of management strategies and highlights key features of HCRs for precautionary fisheries management. [ABSTRACT FROM AUTHOR]

Stock assessments are often used to provide management advice, such as a total allowable catch (TAC), to fishery managers. Many stocks are not assessed annually, and the TAC from the previous assessment is often maintained in years between assessments. We developed two interim management procedures (MPs) that update the estimate of current vulnerable biomass from a surveyed index of abundance to adjust the TAC from a previous assessment. These MPs differ in how they handle uncertainty in observed indices. Using closed‐loop simulation, we evaluated the two interim MPs (with 10‐ and 5‐year assessment intervals) against several "status quo" approaches: (1) an annual assessment, and (2) a stock assessment every 5 or 10 years with (a) fixed TACs or (b) projections between assessments. We evaluated performance across three life‐history types and six operating model scenarios. The interim MPs performed similarly to annual assessments in terms of trends in biomass and yield, regardless of the assessment interval of the interim MPs. The interim MPs often produced more yield than the Fixed TAC MP with 10‐year assessment intervals, for example, in depleted scenarios. The Fixed TAC MP performed more similarly to interim MPs when the assessment interval for the Fixed TAC MP was decreased to five years. The interim MPs can also perform well when circumstances arise that are not accounted for in the Projection MP. Our results show that interim MPs should be considered for infrequently assessed stocks or rebuilding stocks, and highlight potential cost savings of interim MPs over annual assessments. [ABSTRACT FROM AUTHOR]

Common uncertainties in stock assessment relate to parameters or assumptions that strongly determine both the estimates of quantities of management interest (e.g. stock depletion) and related reference points (e.g. biomass at maximum sustainable yield). The risks associated with these uncertainties are often presented to managers in the form of decision tables. However, a formal evaluation of the risks from misspecifying an assessment model over time‐horizons spanning multiple assessment cycles requires closed‐loop simulation. There were two aims of this study: (a) develop an approach to identify and evaluate asymmetries in risk to yields and spawning biomass due to biases in key parameters and data sources in a stock assessment model, (b) quantify the relative importance of correctly specifying the various assessment attributes. A computationally efficient stock reduction analysis was evaluated using closed‐loop simulation to identify risks associated with a stock assessment with persistent positive and negative biases in the key parameters and inaccurate assumptions regarding data sources. Six types of assessment misspecification were examined, namely the assumed natural mortality rate, the assumed recruitment compensation ratio, the assumed age of maturity, a hyper‐stable or hyper‐deplete index of abundance, over‐ or under‐reporting of historical catch, and misspecification of the assumed shape of the selectivity curve. This study reveals large asymmetries in risk associated with common uncertainties in stock assessment processes. We highlight the value of reproducible and computationally efficient stock assessment models that can be investigated by closed‐loop simulation before being used for fisheries management. [ABSTRACT FROM AUTHOR]

Potential users of the model proposed by Froese et al. (2018) should be aware of several issues. First, the method to calculate equilibrium numbers-at-length is incomplete and leads to negatively biased estimates of fishing mortality. Second, inadequate simulation testing fails to reveal that the method is highly sensitive to assumptions of equilibrium conditions and that the population average asymptotic length (L∞) can be approximated by the largest observed size (L max). Finally, the Froese et al. (2018) model relies on the assumption that the ratio of natural mortality (M) to the von Bertalanffy growth parameter (K ; M / K) is typically around 1.5, which they argue is supported by the literature for most fish stocks. We believe that this conclusion is based on an insufficient reading of the literature and, in fact, there is strong evidence to support the claim that M / K is outside the narrow bounds of 1.2–1.8 for many exploited species. Potential users of the method are alerted to these issues and alternative approaches are recommended to avoid these biases. [ABSTRACT FROM AUTHOR]

A simulation‐based approach known as management strategy evaluation (MSE) is increasingly used by resource managers to identify management procedures that are robust to uncertainties in system dynamics.The majority of global fish populations are data limited and there is large uncertainty over their population and exploitation dynamics.The Data‐Limited Methods Toolkit (DLMtool) is an R package that allows for rapid and flexible MSE specification. The package consolidates a large number of existing data‐limited management procedures and allows for rapid MSE testing of new approaches.The DLMtool package has supported transparent and rigorous decision‐making for a number of data‐limited populations, identifying robust management procedures and revealing performance trade‐offs. [ABSTRACT FROM AUTHOR]

Qualitative risk assessment frameworks, such as the Productivity Susceptibility Analysis (PSA), have been developed to rapidly evaluate the risks of fishing to marine populations and prioritize management and research among species. Despite being applied to over 1,000 fish populations, and an ongoing debate about the most appropriate method to convert biological and fishery characteristics into an overall measure of risk, the assumptions and predictive capacity of these approaches have not been evaluated. Several interpretations of the PSA were mapped to a conventional age-structured fisheries dynamics model to evaluate the performance of the approach under a range of assumptions regarding exploitation rates and measures of biological risk. The results demonstrate that the underlying assumptions of these qualitative risk-based approaches are inappropriate, and the expected performance is poor for a wide range of conditions. The information required to score a fishery using a PSA-type approach is comparable to that required to populate an operating model and evaluating the population dynamics within a simulation framework. In addition to providing a more credible characterization of complex system dynamics, the operating model approach is transparent, reproducible and can evaluate alternative management strategies over a range of plausible hypotheses for the system. [ABSTRACT FROM AUTHOR]

Fisheries management rights (FMRs), such as territorial use rights in fisheries (TURFs), are a promising approach for fisheries management that, if implemented on larger geographic scales, may be able to reduce the risk of fisheries decline, particularly for small-scale fisheries (SSF). SSF are significant throughout Asia, Africa and South America for millions of fishers and supporting the livelihoods of coastal communities. This research evaluates the potential of TURFs as a management tool in Indonesia and develops this into a concept of contemporary FMRs. In Indonesia, the adat communities, i.e. those communities where customary tenurial claims are still practiced, acknowledged under law and respected by migrant communities, have been able to implement TURFs. In Maluku and West Papua regions, TURFs (≈customary marine tenure or petuanan laut) are possible because tenure rights are recognized and confer the essential TURFs elements of exclusivity and security. We investigate the potential for scaling up TURFs by transforming petuanan laut that has enabled the successful implementation of sasi laut , a traditional measure to regulate the utilization of marine resources, into modern FMRs, especially for SSF. Elements of policy reform, the use of science-based fisheries management principles, and data-limited stock assessment methods to inform adaptive management are also considered. We discuss the adoption of FMRs into relevant law to allow the scaling up of TURFs throughout Indonesia. The FMRs are conceived as a privilege granted by government to an entity or group that comes with resource stewardship responsibilities and should be revocable if they are breached. • Small-scale fisheries are significant for livelihoods of coastal communities but are difficult to manage. • Traditional marine tenure, petuanan laut in Indonesia, is the equivalent of Territorial Use Rights in Fisheries (TURFs). • Traditional resource management practice, sasi laut , temporarily prohibits harvest of certain resources in the sea. • Petuanan laut and sasi laut were evaluated to develop contemporary Fisheries Management Rights (FMRs) in Indonesia. • FMRs are revocable privileges granted by government to an entity with responsibilities to steward their resources. [ABSTRACT FROM AUTHOR]