Aggressive behavior as a mechanism of spatial differentiation of juvenile salmonid fishes (based on Black Sea brown trout Salmo trutta labrax (salmonidae))

Abstract

Aggressive behavior of hatchery-reared juvenile Black Sea brown trout Salmo trutta labrax at the age of 5.5–6.0 months is investigated. Shortage of suitable territory leads to the separation of the fish into two spatial groups: demersal and pelagic. During the process of spatial differentiation of the fish, the individuals that have not selected permanent habitats (demersal or pelagic) are characterized by the highest aggression level. The duration of the formation of stable spatial differentiation of the fish depends on the stocking density. At a low stocking density (10–45 fish/m2), spatial differentiation is established by the beginning of the second day after the transfer of the fish to new conditions; at a high stocking density (182 fish/m2), it is established approximately by the seventh day. Following the establishment of the (secondary) spatial groups, aggressive acts are registered mainly between the individuals from the same spatial group. A role of aggressive behavior in intrapopulation differentiation of brown trout is discussed.