Some behavioural and ecological factors affecting distribution, biomass and production of juvenile Atlantic salmon

Observations were made in an experimental stream tank (total area 14.7 m(2)) on juvenile Atlantic salmon, Salmo salar L (parr), relating experimental observations to field observations, including the reported diurnal fasting behaviour of juvenile salmon at water temperatures <10 degrees C. Densities in the tank ranged from five to twenty parr, at water temperatures ranging from 4.6 degrees C to 15.8 degrees C. The wide channel of the stream tank, with mean water velocity of 18.8 cm center dot s(-1), was the preferred section, where territorial behaviour was observed. Biomass was regulated in the wide channel by territorial mosaics or by dominance hierarchies. Dominance hierarchies were reflected in coloration of the fish. Dominant salmon were generally in the wide channel. Densities of salmon parr (of mean fork length 10.2cm) in the channel ranged from 0.84 m(-2) to 1.73 m(-2), with an average biomass of 14.2 g center dot m(-2). Growth was least at the 5.9 degrees C temperature treatment. In experiments at temperatures below 10 degrees C, feeding, dominance hierarchy and territorial behaviour were observed in daylight hours, contrary to the published literature. Interactions with other species may affect behaviour. Some observations were made on a closely related species, brown trout (Salmo trutta L.), a commonly cohabiting species in many systems. Trout displaced salmon from their preferred locations in the tank and were more aggressive than the salmon, reducing agonistic behaviour by the salmon. The commonest agonistic act shown by salmon was 'charge' and that by the trout was 'approach'. Some field observations affecting behaviour and production are discussed.