Competition and Predation: Interactions between American eels (Anguilla rostrata) and brook trout (Salvelinus fontinalis) in Virginia mountain streams

Competition has played a large role in structuring natural communities, especially with regards to vulnerable organisms. Brook trout and American eel populations have declined in the Mid-Atlantic Region of the United States as a result of anthropogenic development around freshwater ecosystems, and thus, subsequent conservation efforts of both species have included habitat restoration. Conservation efforts have increased the co-occurrence of these predatory fish species that are known to require similar resources. The main objective of this research was to elucidate the potential for competition between brook trout and American eels through analysis of their preferred prey, diet overlap, and supplementary attributes of the respective fish and invertebrate populations. Over three sampling intervals in the summer of 2017, a reach of stream above Crabtree Falls in Nelson County, Virginia, acted as the control, where only brook trout are present, and a reach of stream below the same waterfall acted as the treatment, trout and eels present. Brook trout abundance was 140 fish per 100 m of stream above and 117 per 100 m below the falls, but size and body condition did not differ significantly between samples reaches, nor did size and body condition of the current year class. Both fish species had low rates of empty stomachs (7-8%) and there was zero observed predation on brook trout by eels. The efficiency of gastric lavage for American eels was determined to be 89% by number of prey items, but biased towards smaller prey size. Over the course of the study brook trout mostly preferred terrestrial invertebrates of the 12 available prey groups with no significant difference in diet where they co-occurred with eels, and eels preferred crayfish. Diet overlap between trout and eels below the falls was 73% overall; however, this does not directly indicate competition. Invertebrate communities had lower abundance below the falls, but both sample reaches had similar diversity, and there was no significant difference between invertebrate communities overall. As conservation efforts increase the co-occurrence of brook trout and American eels, there is limited potential for competition as demonstrated by the parameters examined in this regionally novel study