Behavioural responses of juvenile Atlantic salmon (Salmo salar) to presence of boulders

Abstract

The influence of boulder presence on the behaviour of juvenile Atlantic salmon (Salmo salar) was investigated experimentally in an indoor flume fed by a 161s(-1) flow of natural river water. The flume was divided into 16 arenas (each 1 m2) that were landscaped with river gravel and standardized boulders, to represent relative 'complex' or 'simple' habitats. Each arena housed three wild-caught fish. In a three-week trial, the effects of landscape on aspects of individual behaviour were recorded. Food intake was highest in the simple landscape and directly related to social status and time spent in the water column. The fish in the complex chambers actively maintained station in the water column significantly more than fish in simple landscapes and therefore partially compensated for reduced foraging rates associated with complexity. Fish in simple chambers spent more time orientated upstream than those in complex landscapes. There was no evidence that habitat complexity influenced levels of aggression, average aggressive distance, 'constrained' territory size, or dominance. Activity and space use varied with social status. Overall, this study illustrates that addition of boulders can result in costs to Atlantic salmon parr, which can be expected to offset to some extent benefits brought about by increased stream complexity. Work is now needed to evaluate the balance of costs and benefits at different life stages and under various environmental conditions.