Testing the assumptions of the ideal despotic distribution with an unpredictable food supply: experiments in juvenile salmon

Abstract

Summary Models linking the behaviours of individual animals, their positions within socially complex groups and spatio‐temporal variation in resource distribution offer a promising base for predicting population responses to changing environments. The ideal free and despotic distributions and their derivatives are particularly influential in this regard. Due to the difficulties of conducting work in the wild, for some groups of animals such models are often based on observations of animals in small‐scale systems under conditions that are well controlled, but unnaturally simple. Using an experimental system based on field observations of home range size and variation in food availability, the present study tested whether models derived using small‐scale laboratory observations are valid for juvenile Atlantic salmon in more natural conditions. Contrary to predictions, we found no differences in behaviour between the control fish (which experienced consistently rich feeding patches) and the experimental fish (which experienced unpredictable 10‐fold changes in patch quality). Also contrary to predictions, in the variable condition, salmon used high quality patches (which were an order of magnitude better than low quality patches) only marginally (5%) more than would be expected if they were to forage at random. There was significant variation in foraging strategies between individual fish, with 28% of the population making non‐random use of foraging patches. The only apparent systematic relationship between social rank and use of foraging patches was that fish that were both dominant and made many moves between feeding locations tended to leave rich patches less frequently than they left poor patches. Despite the low correlation between patch quality and movement, there was substantial movement of fish among patches. Forty‐four per cent of moves followed aggressive interactions and most others were spontaneous, with no obvious motivating factor apparent. The study exposes a discrepancy between expectations derived from the basic concepts of patch choice theory and the behaviour of Atlantic salmon in the conditions pertaining in the present study. It is suggested that this discrepancy may arise both from the fact that applicability of patch choice models may be very sensitive to the stability of differences in patch quality and from uncertainties about the costs of habitat sampling.