The effect of group size on the foraging behaviour of juvenile coho salmon: reduction of predation risk or increased competition?

Animals often increase their apparent willingness to incur risk when foragi ng in groups, presumably because group membership reduces an individual's r isk of predation. As group size increases, however, competition for resourc es may also increase, resulting in a decrease in the quantity of resources available to each member of the group. When resources are scarce, individua ls might be expected to increase their foraging effort in an attempt to inc rease their share. Such increases in effort will often appear to increase a n individual's risk of predation. Thus, increased competition may contribut e to the frequently observed relationship between risk-taking behaviour and group size. To date, no experimental assessment of the relative importance of these Two mechanisms exists. We argue;that to differentiate between the hypotheses of 'risk reduction' and 'increased competition', it is necessar y to quantify the effect of predation risk on the form of the relationship between group size and risk-taking behaviour, and thus, to manipulate both group size and predation risk. We conducted an experiment to determine the relative importance of risk reduction and increased competition to the fora ging decisions of juvenile coho salmon, Oncorhynchus kisutch. We recorded t he foraging behaviour of 18 focal individuals in the presence and absence o f a predator, and in the company of zero, one and three conspecifics. As gr oup size increased from one to four, focal fish captured more prey items, v entured closer to the feeder (and predator) to intercept them, and decrease d their use of cover. Furthermore, although focal individuals captured fewe r prey items and intercepted them further from the feeder in the presence o f the predator than in its absence, the form of the relationship between ri sk-taking behaviour and group size was not affected by the level of predati on risk. The results of our experiment are consistent with the interpretati on that increases in risk-taking behaviour with group size occurred primari ly as a consequence of increased competition for scarce resources. (C) 1999 The Association for the Study of Animal Behaviour.