Predation by North Sea Herring Clupea harengus on Eggs of Plaice Pleuronectes platessa and Cod Gadus morhua

The stomach contents of 5,408 herring caught during North Sea trawl surveys in February 1980, 1982, and 1983 were analysed with a view of estimating predation mortalities exerted by the herring stock on plaice and cod eggs, based on the spatial distribution of size categories of herring and the numbers at age estimated from virtual population analysis. Predation on plaice and cod eggs was generally confined to the southern North Sea and only the younger age groups of herring (ages 2 and 3) consumed substantial numbers of fish eggs. Occasionally fish larvae (herring and plaice) were encountered. The estimated fraction consumed of the initial numbers of eggs produced varied from 0.7 to 1.9% for plaice and from 0.04 to 0.19% for cod. These estimates are necessarily crude and can only give a rough indication of the true predation mortalities. Although some effect of herring stock size on the level of recruitment of plaice might be expected, it is unlikely that the generally observed increase in recruitment to various demersal stocks in the 1970s could be entirely accounted for by a reduced predation on eggs by herring.