MECHANISMS RESPONSIBLE FOR POPULATION REGULATION IN YOUNG MIGRATORY TROUT, SALMO-TRUTTA .2. FISH GROWTH AND SIZE VARIATION

There is a critical period (tc days after fry emergence) for survival of young migratory trout; survival is density-dependent during but not after this period, and the length of the critical period is inversely density-dependent on egg density. Water temperature and fry size, but not availability of food or egg density, were chiefly responsible for differences in mean size and grown rates between year-classes in Black Brows Beck, English Lake District. There was no critical size corresponding to tc; as the length of the critical period increased, fish size at the end of the critical period increased. Variation in individual fish size increased in the first few days of growth but then remained fairly constant within each year-class; CV was inversely density-dependent on egg density and inversely related to mortality rate during, but not after, the critical period. Natural (stabilizing) selection was probably the basic mechanism responsible for between year-class variation in individual size. Selection intensity was density-dependent and strongest during the critical period; as selection intensity increased, size-dependent mortality increased, affecting only small trout at low egg densities but both small and large trout at high egg densities. Selection and survival are closely linked. Spatial and behavioural heterogeneity may be the chief mechanism responsible for unequal energy intake among individuals in the population, and hence also for density-dependent variations in their survival and size. -from Author