A multispecies biomass dynamics model for investigating predator-prey interactions in the Bering Sea groundfish community

Concerns over long-term cumulative impacts of fishing on marine ecosystems and the status of many marine species have led to calls for ecosystem-based approaches to fishery management. Single-species population dynamics models traditionally used in fisheries stock assessments lack separate accounting of predation mortality and often underestimate natural mortality. Age-structured models used for stock assessments in the Alaska region are biologically realistic, but their applicability is limited to a relatively small number of commercially exploited species because they require accurate survey and fishery age composition data. The objective of this study was to develop relatively simple age-aggregated multispecies models that describe biomass dynamics of some commercially and ecologically important groundfish species in the eastern Bering Sea by accounting for their trophic interactions. We developed and contrasted two alternative model formulations: a multispecies biomass dynamics (MBD) model and a multispecies delay difference (MDD) model. Both MBD and MDD models were able to capture observed trends in survey biomass data over 1982-2009. Although the MDD model fit the survey biomass estimates better than the MBD model, the MDD model and its maximum likelihood parameters produced biologically unrealistic biomass projections at values of instantaneous fishing mortality (F)>0.14. Strong positive correlations among many of the predation parameters resulted in highly uncertain parameter estimates. Consistent with previous studies, both multispecies models predicted the multispecies B0, MSY, and FMSY to be lower than the sum of the estimates from single-species assessment models. Although the results of the current study should be taken with caution because of high estimation uncertainty, our multispecies models can be a useful addition to the current management process by providing estimates of natural mortality and tactical advice on the implications of single-species harvest strategies for the broader groundfish community in the eastern Bering Sea.