Effect of rainbow trout introductions on food webs in lakes of the arid Patagonia

Abstract

The exotic rainbow trout (Oncorhynchus mykiss) has been stocked in naturally fishless lakes on the reproductive area (Southern Patagonia) of the critically endangered Hooded grebe (Podiceps gallardoi), an aquatic bird endemic of this region. The effects of trout introduction were proposed as one of the potential causes of their critical situation. Trout could modify the trophic structure of the aquatic community, altering the trophic relationships of stocked lakes and could generate changes in the spatial distribution and/or diet of its main prey, which is strongly domined by amphipods in this aquatic system, affecting their trophic position, and inducing the use of alternative carbon sources. We compared the food webs between two fishless and two stocked lakes using stable isotope analysis, based on Layman´s community metrics. Results provided evidence that the presence of fish exerts a marked influence on the food web structure of the studied communities, as well as on the limnological characteristics. Lakes stocked with trout showed enriched values of δ15N and higher values of total nitrogen, conductivity, dissolved organic and inorganic carbon compared to fishless lakes. Community metrics estimation evidenced an increase in trophic redundancy in stocked shallow lakes, which could be explained by the high degree of trout omnivory. Additionally, the study evidenced changes in use of carbon sources in amphipods between stocked and fishless shallow lakes. The spatial distribution of the amphipods towards littoral zone of the stocked shallow lakes, as an anti-predatory response, could has impacts on the Hooded grebe.