Social dominance, prior residence and the acquisition of profitable feeding sites in juvenile Atlantic salmon

Abstract

Twelve groups of 10 Atlantic salmon Salmo salar parr were screened for dominance in small tanks with a single localized food source using a serial removal method and the top, middle and bottom two fish in each group were classified as high, medium and low social status, respectively. These 72 ranked fish were weighed and formed into four groups or waves, each consisting of six fish of each status category. The fish were given dye marks according to status and wave and were then introduced into an artificial stream in four waves over a 12‐day period. Patterns of settlement/emigration were recorded over a 2‐month period, at the end of which the fish were removed from the stream, identified and reweighed, and daily specific growth rates calculated. Fifty‐one fish established feeding stations in the flume, the remainder emigrating via a downstream trap. The average growth rate over the 2‐month period was 0·87% day−1. Probability of settlement and growth rates following settlement were significantly higher in fish from the earlier waves. In contrast, salmon that were dominant in the initial trial were less likely than subordinate fish to settle in the experimental stream and showed lower postsettlement growth rates. The possibility that fish have individually consistent behavioural profiles that promote success in different competitive environments is discussed with reference to published literature for other groups. A small‐scale trial at the end of the main experiment confirmed the prior residence effect and demonstrated that 1 day was sufficient for such an effect to be evident.