Selection for Growth Potential among Migratory Brown Trout (Salmo trutta) Fry Competing for Territories: Evidence from Otoliths

Otolith microstructure analysis was employed to determine differential size and growth characteristics between newly emerged migratory brown trout (Salmo trutta) fry which establish feeding territories and those which are forced to move downstream from the redd. Fry size at emergence was poorly predicted by otolith size, which precluded reliable back-calculation of fry size to investigate a possible size-selection mechanism. However, there was a clear-cut selection for individuals with significantly larger otoliths at emergence among fry which established territories in artificial stream sections. This selection pattern commenced with the onset of territorial aggression between fry. Fry which established territories to the end of the experiment had significantly higher initial otolith growth rates than emigrating fry, and their dry weights at the end of the experiment were well predicted by both otolith size at emergence and otolith size at the end of the experiment. Transparent zones in otoliths from downstream migrants suggested that these fry were starved; this was also supported by their declining dry weights over time. These results were explained as selection for greater growth potential as based on a proposed coupling between otolith microstructure formation and fish metabolism and on the behavioral ecology of migratory brown trout fry.