THE ECOLOGICAL SIGNIFICANCE OF GEOGRAPHICAL AND SEASONAL DIFFERENCES IN EGG SIZE IN SOLE SOLEA-SOLEA (L)

Egg size of the common sole Solea solea (L.) was studied from ichthyoplankton surveys conducted between 1987 and 1991 in seven different areas ranging from the Bristol Channel and western English Channel to the German Eight of the North Sea, encompassing central as well as peripheral parts of the distribution range of the species. Egg size showed a seasonal decline of 0.00179 mm d(-1). Egg size at peak spawning decreased from 1.37 mm in central areas in the south-west to 1.13 mm in peripheral areas in the northeast, corresponding to a decrease in egg volume of 44%. The difference in egg size between areas coincided with differences in the timing of spawning, but a significant small geographical difference in egg size remained after removing the seasonal effect. Egg size was largest in the southwestern areas and decreased to the northeast, The close relationship between spawning time, egg size and egg number (fecundity) suggest that these are three aspects of a common process of ovarian development which is influenced by the photoperiod and temperature. Egg size increased between fertilization and hatching, suggesting that the mortality process is size-selective with a higher mortality rate for smaller eggs. A negative relationship between egg mortality and egg size was observed among the seven sole stocks studied and also in a study of literature data on egg mortality rate of various species spawning in the North Sea and adjacent areas. The ecological implications of these results are discussed.