PhD defence about clupeids' predation on cod eggs

Cod, sprat and herring are key species of the upper trophic levels in the Baltic Sea ecosystem and are strongly interlinked: Cod is the main predator on sprat and herring (clupeids), which in return feed, amongst others, on cod eggs. Egg predation by clupeids has been suggested as one of the factors limiting cod recruitment success in the Baltic Sea in the 1990s. Since then, changes have taken place in cod recruitment as well as in the factors influencing egg predation. The overall aim of this thesis was to elucidate possible changes in predation pressure on cod early life stages in the 2000s compared to the 1990s, as well as to enhance our understanding of the processes affecting egg predation and its implications for cod recruitment.

This thesis is based on data of stomach contents of sprat and herring, ambient environmental conditions, as well as predator (sprat and herring) and prey (fish egg) distribution and abundance for the 1990s and 2004-2008. Changes in diet composition of sprat and herring were investigated, including temporal and spatial variability in egg predation. The changes were driven by environmental conditions,  cod egg abundance, predator-prey overlap as well as abundance of alternative prey. Next, cod egg consumption by both clupeids was quantified and compared with revised estimates from the 1990s to elucidate potential changes in predation mortality of cod eggs. A major methodological focus in this investigation was related to obtaining realistic estimates of predator abundances in the area where cod eggs can be found. As a next step, predation pressure was quantified separately for different egg development stages, both for cod and sprat. Prey selection by clupeids on different fish egg species and development stages was investigated to improve our understanding of the mechanisms underlying egg predation. The results were reviewed in the context of other processes acting on early life stage survival and influencing cod recruitment in the Baltic Sea.

Overall diet composition of sprat and herring was generally similar in both periods. Differences occurred in the proportions of cod eggs in the diet in recent years, which were higher than in the 1990s. However the overall quantities of cod eggs consumed were generally lower in the 2000s. This suggests reduced predation on cod eggs in the 2000s and was confirmed in quantitative analyses of predation mortality of cod eggs. The reduced predation pressure in the 2000s was related to a combination of different drivers and was identified as one of the factors contributing to increased cod recruitment in this period. Predation affected mainly eggs at older development stages, suggesting a higher impact of predation on cod recruitment than formerly thought. In contrast, sprat egg mortality through predation was identified as less important for sprat recruitment. An earlier theory that high abundances of sprat or fish eggs in general trigger predation on cod eggs could not be confirmed.

This thesis provides new knowledge on clupeid foraging and on processes and mechanisms underlying fish egg consumption in the central Baltic. Further, the thesis contributes improved methodology for quantifying egg predation. Results also provide useful methodological input to investigations in other ecosystems, where predation on early life stages also contributes a crucial factor influencing fish recruitment.