Egg cannibalism in Baltic sprat Sprattus sprattus

Throughout the last 2 decades a shift from a cod- to a sprat-dominated syst em occurred in the upper trophic levels of the Central Baltic Sea. This was caused by a decline in the cod stock, due to recruitment failure and high fishing intensity, resulting in a decrease in predation pressure on sprat. Concurrently with the lowest cod stock size on record, sprat reached biomas s values of above 2 x 10(6) t in 1992, being relatively stable afterwards. Besides predation mortality through cod and in recent years also an increas ing fishing pressure, cannibalism on eggs may be a compensatory process lim iting the reproductive success of sprat and hence contributing to the popul ation regulation in the Central Baltic. Based on sprat stomach sampling on 21 cruises between March 1988 and July 1996 cannibalism on sprat eggs was i nvestigated in the Bornholm Basin, one of the main spawning areas of Centra l Baltic sprat. Using a model of gastric evacuation to estimate daily food intake rates and a Virtual Population/Extended Survivor Analysis for comput ing predator population sizes, egg cannibalism rates were estimated. These were compared to egg abundance data from ichthyoplankton surveys and to pre liminary estimates of seasonal egg productions. The study revealed signific ant interannual differences in the intensity of sprat egg cannibalism with considerable predation in 1990 to 1992 (>15% of the egg abundance during pe ak spawning and >60% of the seasonal production) and a reduction in most re cent years (<16% of the corresponding abundance and production). As a possi ble reason for these differences a combination of changes in the vertical o verlap of predator/prey and variability in the food environment were identi fied. Shortcomings of the applied methods and the possible impact of cannib alism on the reproductive success and population development of sprat in th e Central Baltic Sea are discussed.