Externally driven mortality of cod early life stages in the central Baltic: hydrography vs. predation

Cod (Gadus morhua L.) recruitment success in the central Baltic Sea is influenced by various abiotic and biotic factors, which include ambient salinity and oxygen conditions as well as predation pressure on early life stages by planktivore clupeids, such as sprat (Sprattus sprattus) and herring (Clupea harengus). After a period of very low recruitment and stock size during the 1990s–early 2000s, the eastern Baltic cod stock exhibits signs of recovery, at least partly owing to several stronger year classes formed in recent years. In this paper we investigate whether or not changes in predation pressure by clupeids on the early life stages of cod could have enhanced cod recruitment in recent years. The analyses are based on a large dataset of stomach content of clupeids, cod egg abundances from ichthyoplankton surveys, and hydrographic measurements. We investigate temporal and spatial variability in predation pressure on cod eggs, both within and between years, and relate this to variability in hydrographic conditions. Preliminary results indicate lower predation pressure on cod eggs in the recent period compared with some earlier years