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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 [...]

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]

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]