Fish egg predation by Baltic sprat and herring: do species characteristics and development stage matter?

Abstract

Predation of eggs by clupeids has been identified as a major factor contributing to early life stage mortality of Baltic cod (Gadus morhua). We used data from ichthyoplankton sampling and clupeid stomach analyses to investigate whether eggs of other fish species are to a similar extent subject to predation and how predation pressure differs between egg development stages. Cod, sprat (Sprattus sprattus), and rockling (Enchelyopus cimbrius) eggs dominated in the ichthyoplankton fraction in herring (Clupea harengus) and sprat diet, whereas flounder (Platichthys flesus) and dab (Limanda limanda) eggs occurred only occasionally. In spring, cod eggs at advanced development stages were positively selected and sprat eggs generally negatively selected by both predators, while fish eggs were nonselectively consumed in summer. Predation is suggested to account for a large fraction of mortality of cod eggs at older stages, i.e., those eggs that have survived the often detrimentally low oxygen concentration in and below the permanent halocline. The consumption rates of sprat eggs at all development stages relative to production rates were considerably lower compared with cod eggs, suggesting that egg predation is of lesser importance for sprat recruitment.