Maturation of male and female North Sea plaice (Pleuronectes platessa L.)

Abstract

The maturation of North Sea plaice is studied on the basis of two research-vessel surveys and market-sampling data from the commercial fisheries. In female plaice the juvenile-adult transition occurred between May and September. Development of the female gonad took place between July and December, although the gonad did not gain weight before October. Gonad weight reached a maximum of circa 17% of the gutted body weight between January and March. The average female plaice was in spawning condition for about five weeks from late January to early March. Male plaice were in spawning condition for at least 11 weeks from the second half of December to the second half of March. In females the start of spawning was earlier in older fish, but the end of spawning did not differ between age groups. In males spawning ended later in older fish. The time that mature plaice were in spawning condition increased with the age of the fish in males and females. During the spawning season mature plaice hardly fed. Spent fish resumed feeding in January. The cessation of feeding in mature plaice was not related to the absence of food. Male plaice became sexually mature as II- and Ill-group, females as IV- and V-group. Length at 50% maturity (L 50 ) was 22 cm and 34 cm for males and females respectively. L 50 and age at 50% maturity (A 50 ) differed between geographical areas, increasing from south to north. Annual differences were observed in L 50 and A 50 which were related to annual differences in growth rate. Slower-growing plaice reached maturity at a smaller length but higher age than faster-growing plaice. Comparison with published data shows that at present both male and female plaice mature at a younger age and at a smaller size than at the beginning of this century.