Developing Baltic cod recruitment models. II. Incorporation of environmental variability and species interaction

We investigate whether a process-oriented approach based on the results of field, laboratory, and modelling studies can be used to develop a stock-env ironment-recruitment model for Central Baltic cod (Gadus morhua). Based on exploratory statistical analysis, significant variables influencing surviva l of early life stages and varying systematically among spawning sites were incorporated into stock-recruitment models, first for major cod spawning s ites and then combined for the entire Central Baltic. Variables identified included potential egg production by the spawning stock, abiotic conditions affecting survival of eggs, predation by clupeids on eggs, larval transpor t, and cannibalism. Results showed that recruitment in the most important s pawning area, the Bornholm Basin, during 1976-1995 was related to egg produ ction; however, other factors affecting survival of the eggs (oxygen condit ions, predation) were also significant and when incorporated explained 69% of the variation in 0-group recruitment. In other spawning areas, variable hydrographic conditions did not allow for regular successful egg developmen t. Hence, relatively simple models proved sufficient to predict recruitment of 0-group cod in these areas, suggesting that key biotic and abiotic proc esses can be successfully incorporated into recruitment models.