Size-selective mortality in plaice and cod eggs: a new method in the study of egg mortality

Abstract

In this paper a new method for studying egg mortality in pelagic fish eggs is proposed, based on the observed difference in egg-mortality rates between eggs of different sizes. It is suggested that the difference in mortality is due to size-selective predation. This hypothesis is explored by studying changes in size of plaice and cod eggs in plankton samples collected over the total spawning season in 1987 in the Southern Bight of the North Sea. Egg size increased during incubation in both plaice and cod. The size-selective mortality estimated from the change in egg size of plaice and cod separately appeared to be close to the size-selective mortality estimated from the difference in egg mortality between species. This suggests that one common process is operating. Implications of size-selective mortality in fish eggs are discussed for the tudy of the causes of egg mortality and the estimation of spawning-stock biomass from egg surveys.