Comparative fecundity of North Sea plaice ( Pleuronectes platessa L.)

Abstract

Stage IV ovaries of plaice were obtained at sea from five spawning areas of the North Sea and English Channel (Eastern English Channel, Southern Bight, Central Southern North Sea, German Bight and Flamborough) in the years 1977, 1979 and 1980. Additional samples were provided from Dutch landings at IJmuiden for 1972. The numbers of eggs were counted from 790 fish. After a logarithmic transformation, analysis-of-covariance techniques were used to describe the relation between fecundity, given by the number of eggs, and fish length and age, and to examine fecundity differences between spawning area and year. After removing the effects of length and age, significant differences in fecundity were found between years. In the German Bight these were as much as 1.44 times. The data indicate that fecundities at a standard length and age were larger in 1980 than in 1979 but that the extent of the difference varied with spawning location. A gradual decrease in fecundity at a given length and age is shown from the eastern English Channel northwards to the German Bight. A significant decrease in fecundity with age, at a given length, is shown for all locations. Data from the Southern Bight and Flamborough were compared with those collected over the period 1947-9. The recent data from Flamborough show an increase in fecundity at length and age of 1.36 times and from the Southern Bight an increase of 1.6 times is found. The difference in the Southern Bight is reflected in the greater relative weight of the ovaries to body weight in 1980 compared with that in the 1940s. It is suggested that the modulation of fecundity could provide a potentially significant contribution to the natural regulation of the plaice population.