Powdermill Run in the Laurel Highlands is one of very few streams in Pennsylvania where stable, naturally-reproducing populations of native Brook trout (Salvelinus fontinalis) coexist with both introduced Brown trout (Salmo trutta) and Rainbow trout (Oncorhynchus mykiss) populations. With similarity in body shape and potential overlap in habitat distribution and insectivorous diet, these three species of trout would seem to violate the principle of competitive exclusion. The purpose of this study is to assess potential niche differentiation in size, spatial distribution, and diet that might explain their sympatry. Electrofishing surveys were conducted in three distinct areas ranging from upper headwaters to lower stream areas to compare frequency and size, and provide specimens for molecular dietary analysis. Trout were anesthetized and gastric lavage was performed to expel stomach contents into filters. Total DNA was extracted from the filters and arthropod-specific primers were used to amplify a small but variable region of the mitochondrial cytochrome c oxidase subunit 1 (COI) region. The amplicons were sequenced using Illumina’s MiSeq, producing over 150,000 sequence reads. The COI barcodes were compared to the Barcode of Life Database (BOLD) sequence reference library to identify the presence of arthropod prey species in the diets of these fish.
Results/Conclusions
In this study, we describe a molecular method for elucidating dietary information and produce evidence for dietary niche partitioning among trout species. Our preliminary findings indicate that two species in the order Ephemeroptera are shared among all three trout species. These two prey items are the only species observed in the Rainbow trout dietary analysis, while Brown trout and Brook trout are observed supplementing their diets with two orders of stream insect, orders Plecoptera and Megaloptera, and two orders of arachnid, respectively. On top of these arachnid orders, Brook trout are the only trout species observed to utilize a species from the order Trichoptera as a prey item. This molecular dietary analysis suggests Brown and Rainbow trout solely utilize autochthonous stream insects as prey, while Brook trout may rely on allochthonous terrestrial prey items to reduce dietary competition. These subtle dietary differences, combined with stream distribution preferences and temporal variation in spawning may help to reduce competition among these three salmonid species. These results highlight the potential of these types of molecular barcode analyses to address trophic competition among similar sympatric species.