Developing Baltic cod recruitment models. I. Resolving spatial and temporal dynamics of spawning stock and recruitment for cod, herring, and sprat

The Baltic Sea comprises a heterogeneous oceanographic environment influenc ing the spatial and temporal potential for reproductive success of cod (Gad us morhua) and sprat (Sprattus sprattus) in the different spawning basins. Hence, to quantify stock and recruitment dynamics, it is necessary to resol ve species-specific regional reproductive success in relation to size, stru cture, and distribution of the spawning stock. Furthermore, as species and fisheries interactions vary between areas, it is necessary to include these interactions on an area-specific basis. Therefore, area-disaggregated mult ispecies virtual population analyses (MSVPA) were performed for interacting species cod, herring (Clupea harengus), and sprat in the different subdivi sions of the Central Baltic. The MSVPA runs revealed distinct spatial trend s in population abundance, spawning biomass, recruitment, and predation-ind uced mortality. Results, when evaluated with respect to trends in populatio n sizes from research surveys, were similar for the cod and sprat stocks bu t different for herring. Horizontal distributions from MSVPA runs and resea rch surveys indicate that cod and sprat undergo migrations between basins d uring different life stages. This is an observation potentially influencing estimates for the different stock components but not affecting the overall stock sizes.