Territorial Behaviour of Juvenile Atlantic Salmon (Salmo Salar L.)

Abstract

1. The territorial behaviour of young Atlantic salmon is described from observations made in the field and the laboratory. Six agonistic activities occur during the defence of territories; these are charging, nipping, chasing, frontal display, lateral display and fleeing. Certain colour changes of the fish are associated with extremes of aggression and submission. 2. The causation of the six agonistic acts is discussed. Charging, nipping and chasing result from high attack tendencies; fleeing is a result of high escape tendencies. Frontal and lateral displays occur as a result of conflict between attack and escape and are most common during fights between two aggressive fish when conflict is presumed to be high. Frontal display indicates relatively high levels of the tendency to attack; lateral display of the tendency to escape. 3. Agonistic behaviour among aquarium-held fish fluctuates with several factors. Territories are not defended actively until after a period of adaptation to new surroundings, particularly if the fish have been kept in crowded holding tanks for some time. Agonistic encounters increase with more frequent feeding, due probably to better condition and greater activity of the fish, resulting in more frequent infringements of territories. Gradual increase in numbers of fish in an aquarium leads first to an increase, then to a falling off in agonistic activity, as the non-territory-holding fish, which are moderately active with small numbers of fish present, form a closely-knit, stabilized group as the aquarium becomes crowded. 4. The territories of young salmon appear to be primarily feeding territories. The behaviour associated with their maintenance is important for optimum growth and survival and for maintaining position for long periods in fast flowing streams. The fate of hatchery-reared fish planted in rivers may be related to their ability to secure and maintain territories in competition with wild fish.