The effects of trophic interaction between the Patagonian native Percichthys trucha and the invasive Oncorhynchus mykiss during the juvenile period

Abstract

Introduced salmonids have invaded almost all Patagonian freshwater environments, and yet the effect they have had on native fish populations is difficult to evaluate due to a lack of data prior to their introduction. In this study we focused on evaluating trophic interactions during the juvenile period of Oncorhynchus mykiss, and the ecologically similar native Percichthys trucha. For this purpose, we evaluated the diet and size range of juveniles of both species in the Caleufu River over one yearly cycle. The functional response (FR: the relationship between prey density and consumption rate) and functional response ratio (FRR = attack rate/handling time) were estimated for each species, in mono-specific and multi-specific trials. Comparison of the diet of these species by season showed high similarity for spring and summer. It was also found that juvenile sizes of these species overlap during early ontogeny. Additionally, both species presented a type II FR in mono-specific trials, with similar functional curves. In multi-specific experiments O. mykiss juveniles were dominant, leading to a reduction in P. trucha food intake. FRR was higher for O. mykiss than for P. trucha in mono-specific experiments, a difference which increased in multi-specific experiments, indicating the high potential ecological impact of O. mykiss. Our results show it is very likely that O. mykiss and native P. trucha compete in lotic environments, O. mykiss being competitively superior, highlighting the potential detrimental effect they may have during the juvenile period, especially in habitats and seasons where food resources are scarce.