The role of predation on early life stages of cod in the Baltic [review]

Only scarce information is available that can be used to assess the importance of predation on the developmental success of early life stages of cod (Gadus morhua) in the Baltic Sea. Among all possible predators, the two commercially utilized pelagic species, herring (Clupea harengus) and sprat (Sprattus sprattus), appear to have the highest potential as predators of the early pelagic stages of cod in the Central Baltic, and their effect on the cod stock may be important with regard to the fishery management in this area. In recent years, successful reproduction of cod in the Baltic proper has been restricted by the hydrographic conditions to the Bornholm Basin. Concentrating on this area, the present report summarizes new results from stomach content analysis for herring and sprat during the cod spawning seasons 1997-1991. The results are used to obtain preliminary estimates on the consumption of early life stages of cod in this area. Larvae and 0-group cod appear not to be substantially affected by predation, whereas the eggs were found to be more heavily preyed upon, especially by sprat in spring. The daily egg consumption, calculated for the entire sprat population in the central Bornholm Basin, ranged from 90% to well above 100% of the standing stock of cod eggs during spring dates. In summer, consumption estimates remained well below corresponding standing stock or production values. Herring appeared to be generally less important as a predator due to a smaller population size in this area. The importance of predation on eggs for the recruitment success of cod in the Central Baltic is discussed in relation to the hydrographic conditions. Implications for future research activities are highlighted in connection to process orientated recruitment studies and multispecies modelling.