SIZE-SELECTIVE MORTALITY IN AN EXPLOITED PERCH POPULATION AND THE RECONSTRUCTION OF POTENTIAL GROWTH

This study assesses the extent of size-selective mortality exerted on perch cohorts recruited to the commercial gillnet fishery in Lake IJss el. The size distribution of the 1985 cohort was surveyed with bottom trawling before and after the winter commercial gillnet fishery, from 1985 till 1990. Ageing and reconstruction of individual growth was don e, using opercular banes from fish collected during the surveys and sc ales sampled from the commercial gillnet catch. The reconstructed pote ntial average size of the 1985 cohort showed that the size reached aft er five summers could have been 26.9 cm, whereas average length observ ed in the population was 25.3 cm. Correlations between population size and back-calculated size were highest for back-calculations for the p revious year. When earlier back calculations were considered, the corr elations became poor or even insignificant. These findings stress the large individual differences in growth history within a cohort and sho w that the size selective mortality of perch can only be proven over r elatively short growth trajectories of less than one year.