There is a critical period for survival (tc days after fry emergence; see 90L/03507). Density-dependent mechanisms responsible for population regulation occurred during, but not after, this period. Survivor density was strongly density-dependent on egg density during, but not after tc. Number of juveniles decreased markedly during tc to reach a fairly constant value for territorial fish and a negligible value for non-territorial fish by the end of tc. Selection intensity for territorial fish was highest during tc and was density-dependent on egg density. Territorial possession was linked to size-dependent selection for the holders. Territorial trout formed groups of 1-6 fish with a social hierarchy within each group. Number of groups and mean number of fish per group both decreased during tc to fairly constant values after tc. Territorial fish distribution between groups became more even as density-dependent selection intensity increased. The maximum area defended by each territorial fish was simply a function of fish size. Attack rates in defence of a territory were a function of territorial size at the start of tc and after tc, when trout without territories were absent, the relationship being well-described by the same power function for all year-classes. Attack rates greatly increased during tc, the increase being directly related to the number of trout without territories. Territorial possession and the size of territorial fish provided the link between density-dependent survival, selection intensity and size-dependent mortality. Territorial size and defence were chiefly related to fish size but as egg density increased, the number of trout without territories increased and time spent in territorial defence during tc increased. Increasing defence costs were probably responsible for elimination of larger juveniles at high egg densities, and for increasing selection intensity for an optimum size of trout during the critical period. -from Author
