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In this paper, maximum sustainable yield (MSY) parameters for the Baltic Sea sprat (Sprattus sprattus) are estimated in relation to pressure from cod (Gadus morhua) predation and the influence of density dependence on sprat growth. This study is based on long-term deterministic and stochastic simulations in which sprat density-dependent growth and predation mortality are considered. The resultant model is a relatively simple tool that allows for streamlined analyses of problems typically approached using complex multispecies models. The analysis indicates that estimates of the MSY parameters (i.e., MSY and FMSY) and equilibrium biomass differ significantly between approaches that hold growth and natural mortality constant and those that allow for density-dependent growth and natural mortality. Based on the cod biomass observed in the 1980s, the MSY parameters estimated by a model that accounts for density-dependent growth and by a model assuming constant growth may differ by a factor of 2. As such, the MSY parameters decline (approximately linearly) with the size of the cod stock. Dans le present article, des parametres associes au rendement equilibre maximum (REM) pour les sprats (Sprattus sprattus) de la mer Baltique sont estimes au vu de la pression exercee par la predation des morues (Gadus morhua) et l'influence de la dependance sur la densite de la croissance des sprats. L'etude repose sur des simulations deterministes et stochastiques a long terme dans lesquelles sont integrees la croissance dependante de la densite et la mortalite par predation. Le modele qui en resulte est un outil relativement simple qui permet des analyses simplifiees de problemes habituellement abordes a l'aide de modeles multispecifiques complexes. L'analyse indique que les estimations des parametres du REM (c.-a-d. REM et FREM) et de la biomasse a l'equilibre obtenues par des approches ou la croissance et la mortalite naturelle sont maintenues constantes et des approches dans lesquelles la croissance et la mortalite naturelle dependent de la densite sont significativement differentes. Sur la base de la biomasse de morues observee dans les annees 1980, les parametres du REM estimes par un modele qui integre la croissance dependante de la densite et par un modele dans lequel la croissance est constante pourraient diverger par un facteur de 2. Aussi, les parametres du REM diminuent (de maniere a peu pres lineaire) parallelement a la taille du stock de morues. [Traduit par la Redaction], Introduction In 2002, the World Summit on Sustainable Development participants declared that all fish stocks should be exploited at an intensity that promotes maximum sustainable yield (MSY) (UN 2002). As [...]

Equations for equilibrium yield and biomass are presented and used to derive [F.sub.MSY] and alternative reference points (ARPs). ARPs are analogues of the traditional points based on yield-per-recruit (YPR) or spawning stock-per-recruit (SPR) ([F.sub.0.1], [F.sub.40%], [F.sub.50%]), but refer to the equilibrium total yield or biomass. The method combines YPR and SPR analysis with stock-recruitment relationships. The sensitivity of [F.sub.MSY] and ARPs to the range of available stock-recruitment data, recruitment variance, various steepness levels in the stock-recruitment models, assessment variance, and bias are tested. The analysis showed that in most cases, [F.sub.40%B] and [F.sub.50%B], defined by the equilibrium biomass (B), were the most robust relative to the different sources of uncertainty. However, in the case of the misspecification of the stock-recruitment relationship, [F.sub.MSY] showed superior performance. [F.sub.40%B] for the Beverton and Holt stock-recruitment model and [F.sub.50%B] for the Ricker recruitment model can be recommended as conservative fishing mortalities associated with high long-term yield. For the considered steepness, they produced yield up to 5%-10% lower than yield at [F.sub.MSY]. Des equations pour le calcul du rendement et de la biomasse equilibres sont presentees et utilisees pour calculer le taux de mortalite correspondant au rendement maximum durable ([F.sub.MSY]) et des points de reference alternatifs (ARP). Les ARP sont des analogues de points traditionnels bases sur le rendement par recrue (YPR) ou sur le stock reproducteur par recrue (SPR) ([F.sub.01], [F.sub.40%], [F.sub.50%]), qui sont toutefois calcules par rapport a la biomasse ou au rendement equilibre total. La methode jumelle l'analyse du YPR et du SPR aux relations stockrecrutement. Une evaluation est presentee de la sensibilite de [F.sub.MSY] et des ARP a la fourchette de donnees disponibles pour etablir les relations stock-recrutement, a la variance du recrutement, aux divers niveaux de declivite (<< steepness >>) dans les modeles de stock-recrutement, a la variance de l'evaluation et aux biais. L'analyse demontre que, dans la plupart des cas, [F.sub.40%B] et [F.sub.50%B] definis par la biomasse equilibree (B) sont les taux les plus robustes aux vues des differentes sources d' incertitude. Par contre, dans les cas d' erreur de specification de la relation stock-recrutement, [F.sub.MSY] donne de meilleurs resultats. A la lumiere des resultats obtenus, [F.sub.40%B] pour le modele stock-recrutement de Beverton et Holt et [F.sub.50%B] pour le modele de recrutement de Ricker constituent des estimations prudentes recommandables des taux de mortalite par peche associes a des rendements a long terme eleves. Pour la fourchette de declivites concernee, ils ont donne des rendements pouvant etre de 5% a 10% inferieurs au rendement pour [F.sub.MSY]. [Traduit par la Redaction], Introduction The maximum sustainable yield (MSY) concept has a long history in fisheries management. Its origin comes from the 1930s, and the concept gained popularity in the 1950s and 1960s [...]

Pretty good yield (PGY) is a sustainable fish yield corresponding to obtaining no less than a specified large percentage of the maximum sustainable yield (MSY). We investigated 19 European fish stocks to test the hypothesis that the 95% PGY yield range is inherently precautionary with respect to impairing recruitment. An FMSY range was calculated for each stock as the range of fishing mortalities (F) that lead to an average catch of at least 95% of MSY in long-term simulations. Further, a precautionary reference point for each stock (FP.05) was defined as the F resulting in a 5% probability of the spawning-stock biomass falling below an agreed biomass limit below which recruitment is impaired (Blim) in long-term simulations. For the majority of the stocks analysed, the upper bound of the FMSY range exceeded the estimated FP.05. However, larger fish species had higher precautionary limits to fishing mortality, and species with larger asymptotic length were less likely to have FMSY ranges impairing recruitment. Our study shows that fishing at FMSY generally is precautionary with respect to impairing recruitment for highly exploited teleost species in northern European waters, whereas the upper part of the range providing 95% of MSY is not necessarily precautionary for small- and medium-sized teleosts. [ABSTRACT FROM AUTHOR]