Mechanisms Responsible for Population Regulation in Young Migratory Trout, Salmo trutta. III. The Role of Territorial Behaviour

Abstract

(1) In parts I and II, frequent sampling revealed a critical period for survival (tc days after fry emergence); density-dependent mechanisms responsible for population regulation occurred during, but not after, this period and appeared to be linked to territorial behavior which is now examined in four year-classes. (2) Survivor density was strongly density-dependent on egg density during, but not after, tc. The number of juveniles decreased markedly during tc to reach a fairly constant value for territorial fish and a neglibigle value for non-territorial fish by the end of tc. Density-dependent survival was therefore linked to territorial possession. (3) Length-frequency distributions for territorial trout observed directly and for all trout taken by electrofishing were very similar at the start of tc and after tc, but not during tc. Selection intensity for territorial fish was highest during tc and was density-dependent on egg density; the size distribution of the territorial fish was trimodal, bimodal and unimodal at low to medium, relatively high and very high egg densities, respectively. Territorial possession was therefore linked to size-dependent selection for the holders. (4) Territorial trout formed groups of 1-6 fish with a social hierarchy within each group. The number of groups and mean number of fish per group both decreased during tc to fairly constant values after tc. Variation in the number of fish per group (measured by C.V.) increased during tc to a fairly constant value after tc, this latter value being inversely density-dependent of egg density. Territorial fish distribution between groups therefore became more even as density-dependent selection intensity increased. (5) The maximum area defended by each territorial fish was not density-dependent but was simply a function of fish size, the relationship being well-described by a power function with the parameter estimates changing at a fish length of 40 mm. (6) 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. (7) Territorial possesion 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 the time spent in territorial defence during tc increased. Increasing defence costs were probably responsible for the elimination of larger juveniles at high egg densities, and for increasing selection intensity for an optimum size of trout during the criticial period.