Territorial and foraging behaviour of juvenile Mediterranean trout under changing conditions of food and competitors

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Territoriality is probably the most important ecological mechanism regulating densities in stream-living salmonids. Body size is typically regarded as the best predictor of territory size, but food abundance and competitor density may be key driving factors. However, a global analysis of literature data showed no clear patterns on the relative causal role of those factors on determining territory size in juvenile salmonids. Thus, in a factorial experiment, we estimated to what extent simultaneous variations of fish size, competitor density, and food abundance affected the size of foraging and defended areas of Mediterranean brown trout (Salmo trutta). In contrast with former studies, we found that foraging areas were larger than defended territories. Foraging and territorial behaviour changed significantly under varying density and feeding regimes. Foraging areas decreased with increasing competitor density and food availability, and there was a strong interaction between these two factors. Defended territories decreased with increasing density, irrespective of food abundance. Although our findings showed a significant allometric relationship between fish length and territory size, the data contained much unexplained variability. Our findings suggest that defended areas are relatively fixed for a given trout length. However, at extremely high population densities, defended areas decreased. Thus, under extreme competition, such as during critical periods right after emergence, trout may subdivide available habitat and thereby moderate density declines.
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