Trophic Interactions in the Baltic Sea: Predation on cod eggs by clupeids

Cod (Gadus morhua), sprat (Sprattus sprattus) and herring (Clupea harengus) are key species of the upper trophic levels in the Baltic Sea ecosystem and are strongly interlinked: The piscivore cod is the main predator on the planktivores sprat and herring, which feed, amongst others, on cod eggs. Egg predation by sprat and herring has earlier 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 ecological factors potentially influencing egg predation. The overall aim of this thesis is 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 impacting on egg predation and its implications for cod recruitment.

The investigations of this thesis are based on extensive datasets on stomach contents of sprat and herring, ambient hydrographic conditions, ichthyoplankton distribution and abundance as well as predator distribution and abundance from hydroacoustic data 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 ambient hydrographic conditions, cod egg abundance, predator-prey overlap as well as abundance of alternative prey (Paper I). Next, cod egg consumption by herring and sprat was quantified and compared with revised estimates from the 1990s to elucidate potential changes in predation mortality of cod eggs (Paper II).

A major methodological focus in this investigation was related to resolving the spatial distribution of sprat and herring to obtain realistic estimates of predator abundances in the area overlapping with cod eggs. As a next step, predation pressure was quantified separately for egg development stages, both for cod and sprat (Paper III).

Furthermore, ichthyoplankton prey selection by clupeids was investigated, with specific focus of predation on different fish egg species and development stages, to improve our understanding of the mechanisms underlying egg predation. Finally, the results on predation pressure on cod eggs were reviewed in the context of other processes acting on early life stage survival and influencing cod recruitment in the Baltic Sea (Paper IV).

The results showed that diet composition of both sprat and herring in terms of major taxonomic groups was generally similar in the 1990s and 2000s. Although higher proportions of cod eggs occurred in the diet in the 2000s, the overall quantities of cod eggs in the diet were generally lower in the 2000s compared to the 1990s (Paper I).

This suggests reduced predation on cod eggs in latter period, which was further confirmed in quantitative analyses of predation mortality on cod eggs (Paper II). The lower predation pressure on cod eggs in the 2000s compared to the 1990s was related to a combination of reduced predator abundance and lower daily rations by individual predators.

Reduced predation pressure was identified as one of the factors contributing to relatively higher year-classes of cod recruitment in the 2000s (Paper IV). Predation was found to affect mainly eggs at older development stages (mainly stage III), i.e. those eggs which have survived two out of three critical development phases in the often detrimental hydrographic conditions in and below the permanent halocline (Paper III).

This suggests a higher impact of predation on cod recruitment than formerly thought. In contrast, investigations on sprat egg mortality (Paper III) found that consumption rates of sprat eggs at all development stages relative to production rates were considerably lower compared to cod, suggesting egg predation to be of a lesser importance for sprat recruitment.

The results of this thesis provide new knowledge on clupeid foraging, including identifying processes and mechanisms behind fish egg consumption in the central Baltic. Further, the thesis contributes improved methodology for quantifying cod and sprat egg predation by egg development stages. Both in combination can be considered as a major advancement in this field of research, as predation pressure on early life stages is generally extremely difficult to quantify and comparable studies are scarce.

Hence, the thesis provides useful ecological and methodological input to other ecosystems and investigations, where predation on early life stages is an important factor in influencing fish recruitment