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

Abstract

Rainbow trout have been stocked in naturally fishless lakes in the reproductive area of the endangered Hooded Grebe, an endemic aquatic bird of Southern Patagonia. 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 by altering the trophic relationships of stocked lakes. Changes in the spatial distribution and/or diet of the Grebe’s main prey, which is dominated by amphipods, could affect their trophic position, and introduce the use of alternative carbon sources. We compared the food webs between fishless and stocked lakes using stable isotope analysis and associated community metrics. Results provided evidence that the presence of fish influences the food web structure of the studied communities. Stocked lakes showed higher values of δ15N compared to fishless ones. Community metrics estimation described an increase in trophic redundancy in stocked lakes, which could be explained by the high degree of trout omnivory. Additionally, the study found changes in use of carbon sources in amphipods between stocked and fishless lakes. The spatial distribution of the amphipods toward littoral zone of the stocked lakes, as an anti-predatory response, could impact the Hooded Grebe.