Adaptive flexibility in the feeding behaviour of brown trout: optimal prey size

Abstract

Background: Brown trout, Salmo trutta Linnaeus, 1758, is a species of significant conservation and socio-economic importance. A consequence of this importance is the enormous amount of literature that has been published on the species in the last few decades. In general terms, brown trout has been considered as a size-selective predator, even though it is able to feed on a wide range of prey sizes. Nevertheless, there are still some gaps in our knowledge, for example the theoretical relationship between prey numbers and prey sizes eaten by the fish need to be addressed. This research aimed to study optimal prey size in the environment (benthos and drift) as well as the potential relationship between prey size and two other feeding variables (prey numbers and stomach fullness). Additionally, ontogenetic shifts in these variables were addressed. Results: Brown trout showed a clear preference for 4- to 6-mm-length prey, although the use of prey larger than 10-mm length may be feasible. The similarity of the prey size frequency distribution between the environment (benthos and drift) and the diet in some cases was considerable (from 57.7% to 95.9%). Moreover, the results revealed that the feeding strategy can be related to prey size and the numbers of prey eaten by the brown trout; as food size decreased, prey numbers increased. On the contrary, the correlation between the average prey size and fish length was positive but statistically nonsignificant. A significant ontogenetic shift, in terms of prey size sorted by age classes, was found in only two of eight studied populations. No clear relationship between prey size and stomach fullness was found. Conclusions: The feeding strategy of this species is flexible and clearly influenced by the size frequency distribution of potential prey: trout fed on either small numbers of large prey or large numbers of small, and theoretically low energy, prey. Our approach covers a general subject in trophic ecology and animal behaviour that may be applicable to other fish species to improve our understanding of predator feeding behaviour