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 ichthyop lankton surveys conducted between 1987 and 1991 in seven different are as ranging from the Bristol Channel and western English Channel to the German Eight of the North Sea, encompassing central as well as periph eral 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 decre ased from 1.37 mm in central areas in the south-west to 1.13 mm in per ipheral areas in the northeast, corresponding to a decrease in egg vol ume of 44%. The difference in egg size between areas coincided with di fferences in the timing of spawning, but a significant small geographi cal 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 commo n process of ovarian development which is influenced by the photoperio d and temperature. Egg size increased between fertilization and hatchi ng, suggesting that the mortality process is size-selective with a hig her mortality rate for smaller eggs. A negative relationship between e gg mortality and egg size was observed among the seven sole stocks stu died and also in a study of literature data on egg mortality rate of v arious species spawning in the North Sea and adjacent areas. The ecolo gical implications of these results are discussed.