Foraging behaviour of brown trout in wild populations: can population density cause behaviourally-mediated foraging specializations?

Abstract

Brown trout is considered as a territorial fish, in which negative density effects on growth and survival rates can be mediated through competition mechanisms. Here, in order to examine whether competition mechanisms can affect the foraging behaviour of wildSalmo truttawith respect to active-bottom, benthic-drift or surface-drift foraging, three neighbouring populations under different levels of fish density (high, intermediate and low) were studied. We analysed the foraging behaviour of each population according to niche breadth, prey preferences, the modified Costello graphical method and prey trait analysis. The results revealed a remarkable similarity in the feeding behaviour among these feral fish populations, suggesting a foraging behaviour convergence in response to site-specific prey accessibility. A generalist foraging behaviour was the prevailing feeding strategy, independent of fish density. Hence, this study offered evidence for the occurrence of density-independent individual foraging behaviour when food is abundant and available; however, density-dependent foraging behaviour might occur when resource limitation exists. Studies under natural conditions like the present study are needed to increase ecological realism, and indeed this study opens promising research directions for future feeding studies in territorial fish species.