Changes in fecundity of female North Sea plaice (Pleuronectes platessa L.) between three periods since 1900

Variations in size-specific fecundity were studied over a period of seven years between 1977 and 1985 and fecundity-body length relationships were compared between three periods: 1900–1910, 1947–1949, and 1977–1985. Significant differences were observed between years and areas. The average fecundity for a 40-cm female was 12% less in the German Bight than in the southern North Sea, but the annual variability was higher; 15% and 7% respectively. Length-specific fecundity showed a significant positive correlation with the pre-spawning condition factor, but not with the somatic growth in the preceding year. Fecundity appears to have changed since the early 1900s and 1947–1949. In 30-cm females the present fecundity was about 30–100% higher compared to the first two periods, depending on the area. Fecundity of larger females (50 cm) was similar to that in 1900, but was 30–60% higher than in 1947–1949. The substantial reduction in fecundity in the period 1947–1949 contrasted with the much smaller change in ovary weight, suggesting not a change in the energy allocation over reproduction and somatic growth, but in the energy allocation over a small number of large eggs versus a large number of small eggs. Both length- and weight-specific fecundity decreased with age. The effect was largest in young fish. Weight-specific fecundity decreased by 3.9% when age increased from 4 to 5 years, by 1.7% when age increased from 10 to 11 years, and by 0.8% when age increased from 20 to 21 years. Significant differences in length-specific ovary weight were also observed between years and geographical areas, but the variability between years was less than in fecundity. Ovary growth was not synchronous between age groups, the younger age groups lagging behind by about one month. Egg weight calculated from ovary weight and fecundity was lower than egg weight measured from ripe running females, suggesting that ovary growth continues after the start of spawning in an individual plaice. The differences in fecundity and ovary weight between the three time periods are discussed in relation to the question whether these are a phenotypic response due to changes in the conditions for growth or to a change in the genetical composition of the population.