Ecosystem Considerations

MSY is traditionally a single-species concept, and it conflicts with multispecies or ecosystem science and management. It is generally not possible to simultaneously achieve the MSY for multiple species that are caught in the same fishery. Even if the maximum combined yield of all species is obtained, the species that are most vulnerable will be overfished while those that are least vulnerable will be underfished relative to their single species MSY. Moreover, MSY does not consider interactions among species and the impact of removal of fish on ecosystem dynamics and functioning. The yields obtained by fishing each population at FMsY based on single-species analyses are likely to be much different than predicted from the single-species analyses.

Environmental and ecosystem variability can impact MSY. MSY is an equilibrium concept that has historically been viewed in a deterministic setting. However, even in the absence of fishing, populations vary over time. Therefore, MSY should be calculated as an average over time rather than using equilibrium deterministic calculations. Analyses have shown that average MSY in a stochastic setting is generally less than in a deterministic setting. In fact, fishing using a constant catch based on a deterministic MSY will cause a population to go extinct in the presence of population variability. Therefore, MSY-related quantities that use constant catch in a stochastic environment and define biological risk have been developed (e.g., the concept of maximum constant yield (MCY) used in fisheries management in New Zealand). There has been a recent trend toward reflecting environmental variability in MSY-related quantities. For example, rather than calculating a single equilibrium BMsY value, dynamic BMsY is calculated for each time period as the biomass under FMsY, which takes into consideration the variation in recruitment.

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