Eastern Baltic cod: Perspectives from existing data on processes affecting growth and survival of eggs and larvae

We review eastern Baltic cod literature to identify areas for future research involving egg and larval stages. Egg and larval abundances have been estimated for several decades in all of the major spawning areas. Using a subset of the available data we show that the timing of peak egg abundance in one of these areas (Bornholm Basin) varies seasonally by at least 2 mo. Trends in egg and larval abundance over time are not obvious due to differences in gear type, sampling intensity, and survey timing relative to spawning dates. Interannual differences in larval transport away from spawning areas may also contribute to variability in abundance estimates. The results of broad-scale zooplankton surveys suggest that the abundance and types of potential prey in the Baltic Sea are similar to those in other regions and perhaps suitable for moderate-to-fast larval growth. However, few systematic surveys have described food concentrations at appropriate scales for cod larvae, and the species composition of larval diets is unknown. Growth rates for Baltic cod larvae have not been measured and cannot be compared with rates in other areas or to variations in biotic and abiotic factors. Large gaps exist in our knowledge of processes affecting egg and larval growth and survival, but some promising areas of research are indicated. In particular the seasonality of spawning, deep water oxygen concentrations, predation on eggs, and larval food production require further investigation.