Density-dependent interactions in an Arctic char – brown trout system: competition, predation, or both?

In the study of mechanisms structuring fish communities, mixed competition–predation interactions where large predators feed on prey fish versus those in which small predators compete with prey fish for a shared prey have been the focus of substantial research. We used a long-term data set from a system inhabited by brown trout (Salmo trutta) (predator) and Arctic char (Salvelinus alpinus) (prey) to evaluate whether mixed interspecific interactions were present in this system as suggested in other studies focusing on this species pair. We found no evidence for a negative interspecific density dependence in individual performance in either Arctic char or brown trout. In contrast, a negative intraspecific density dependencewas present, especially in Arctic char. Furthermore, large brown trout condition showed a positive response to encounter rate with Arctic char (related to the density of small Arctic char). The most parsimonious interaction module to explain the Arctic char – brown trout interaction patterns in the studied system does therefore not need to include interspecific competition. We suggest that size-structured mixed competition–predation interactions in different systems are realized as being either mainly structured through interspecific predation or by competition depending on species life history characteristics and environmental