Abstract
The recruitment processes of Baltic cod and sprat were analysed and critical periods were identified by addressing the major impact factors on individual early life history stages separately and relating observed abundance data between successive stages. For cod, recruitment appeared to be dependent on egg survival, with low oxygen concentration in dwelling depths and predation by clupeids as the major causes for egg mortality. Surviving egg production and larval abundance were weakly correlated, whereas larval abundance was significantly related to year class strength. This indicated that the period between the late egg and the early larval stage is critical for cod recruitment. A potential variable identified to affect this life stage was prey availability for larvae. For sprat, early and late egg stage production as well as late egg stage production and larval abundance were significantly related. However, year class strength was largely independent of larval abundance. Thus, the period between the late larval and early juvenile stage appeared to be critical for sprat recruitment. Potential variables identified to affect this life stage were ambient temperature and wind stress. Environmental factors showing statistically significant covariance with the survival of one of these critical life stages were incorporated into stock-recruitment models for individual spawning areas separately and for the Central Baltic combined.
Highlights
General approach and stock developmentThe present consensus in fisheries research is that the success of a year class is determined primarily by a series of density independent stochastic processes acting on the egg, larval and early juvenile stages
In the present study we examine the reproductive success in two fish species, cod and sprat, characterised by differing life history strategies in a biologically simple but hydrographically complex system, the Central Baltic Sea
The present paper presents results for Baltic cod and sprat, and summarises the information available or required to improve the stock-recruitment relationships for these two populations
Summary
The present consensus in fisheries research is that the success of a year class is determined primarily by a series of density independent stochastic processes acting on the egg, larval and early juvenile stages. These processes, such as co-occurrence with sub-optimal habitats, overlap with predators, reduce the abundance surviving to the juvenile stage. In the present study we examine the reproductive success in two fish species, cod and sprat, characterised by differing life history strategies in a biologically simple but hydrographically complex system, the Central Baltic Sea. The analyses follows the success of a year-class during ontogeny from spawner abundance through the egg and larval to the juvenile stage according to Paulik’s (1973) framework, considering various endogenous and external processes affecting reproductive success
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