A comparison of parasite assemblages in native and introduced species in the Columbia River estuary: an examination of competition and trophic interactions

The Columbia River estuary, an important habitat for juvenile pacific salmon (Oncorhynchus spp.), is the site of many fish introductions. Often, introduced species have fewer parasites than natives, which may allow for successful colonization and competition, a process described as the enemy escape hypothesis. Parasite communities, acquired through consumption of infected prey items, of freshwater estuarine fishes, that share habitat and resources with juvenile salmon, have not been studied. I compared the parasite communities of two native species, three spined stickleback (Gasterosteus aculeatus) and peamouth (Mylocheilus caurinus), to the invasive banded killifish (Fundulus diaphanous) to determine if the introduced killifish has fewer parasites than native stickleback and peamouth as predicted by the enemy escape hypothesis. To date, I have necropsied 39 of 90 samples: three spined sticklebacks (n=13), banded killifish (n=15), and peamouths (n=10). In general, the banded killifish has a lower species richness and parasite prevalence than both the peamouth and three spined stickleback. For example, prevalence of the cestode Proteocephalus sp. was higher in the native peamouth (80.0%) than in the introduced banded killifish (0%) (Fisher’s Exact Test, p<0.001). In addition, prevalence of Hysterothylacium sp. (Nematoda) was higher in the native three spined stickleback (80.0%) than in banded killifish (30.0%) (p=0.030). Thus far, these data support the enemy escape hypothesis where the banded killifish has fewer parasites than native fishes, but the presence of parasites suggests potential for competition between native and introduced species in the Columbia River estuary.