Causal correlation between recruitment and spawning stock size of central Baltic cod?

Abstract

A biologically sensible model for cod recruitment in the central Baltic is proposed. The model takes into account, in a simple way, (1) the spawning stock size of cod, (2) the annual spawning conditions in terms of salinity and oxygen content, and (3) the predation on cod eggs by sprat. The parameters are estimated using regression analysis methods. All three factors are shown to be statistically significant, although sprat predation is the least clear effect. The model residuals are not correlated (no auto-correlation). The model is successfully evaluated against independent data. According to the model, cod recruitment at age 0 is proportional to the spawning stock biomass (SSB) for realistic ranges of SSB, for a given set of spawning condition and sprat predation. It is further shown that it is important to use recruitment data from an MSVPA, where cod cannibalism is included, in stock recruitment analysis of central Baltic cod.