Prior residency advantage for Atlantic salmon in the wild: effects of habitat quality.

Prior residency advantages have been explained by an asymmetry between the 'owner' and the 'intruder' in fighting ability (resource-holding potential) or motivation (value asymmetry (VA)). Here, we tested for the extent of prior residence effects in individually tagged Atlantic salmon juveniles being released in two bouts (4 days apart) during spring along a natural stream, and recaptured 3 months later. A prior residency advantage was detected both in terms of body growth, energy density and male gonad size. As we controlled for effects of initial body size, which correlates with dominance, these findings are in accordance with the VA hypothesis. The growth advantage of first arrivals also increased with local shelter abundance in the stream, which can be expected if a higher resource value of the habitat results in a higher defence motivation. We also found a prior residence effect on spatial distribution, with the second arrivals within each release site being recaptured further downstream. No effect on apparent survival rates was found. The observed reduced growth and energy density may have fitness consequences for the second arrivals, both in terms of lower winter survival rates and later age at maturity. For mature male parr, both decreased body and gonad growth may give an additional disadvantage through reduced fertilization rates during breeding. These costs may help to explain the tendency for stationary behaviour of stream salmonids, as the potential benefits of moving into less crowded areas would be reduced by the risk of becoming an intruder. Prior residence effects may therefore have influenced the evolution of movement behaviour in these organisms.