Prior residence, territory quality and life-history strategies in juvenileAtlantic salmon (Salmo salar L.)

Three groups of juvenile salmon were introduced sequentially into an artifi cial stream to investigate the effects of prior residence on behaviour and territory choice. Almost half of the first group obtained and defended dist inct territories, the other half being constrained to an area approximately the size of one large territory. All of the fish in the subsequent groups, bar one, were also constrained to the same site. Since the fish were of si milar size, prior residence alone seemed to influence which individuals obt ained territories. However, within the first group, the fish that obtained territories were larger and more aggressive. The territorial fish did not a ppear to choose the most profitable territories, although they had the grea test opportunity to do so. Since juvenile salmon emerge from their gravel n ests fairly synchronously, a time constraint on site sampling is hypothesis ed: there may be a risk in taking time to sample sites, since these same si tes may become occupied with conspecifics. However, fish with territories f ed at faster rates than non-territorial fish, possibly because of reduced c ompetition for prey items. Consequently, fish from the first group (contain ing most of the territorial fish) grew faster than the other two groups. Mo reover, most of the territory holders, but only one of the non-territorial fish, reached the threshold size that increases their probability of smelti ng the following year. This suggests that ability to obtain a defensible te rritory, primarily through prior residence, influences the age at which juv enile salmon can migrate to sea. (C) 1999 The Fisheries Society of the Brit ish Isles.