Many stream studies in the Pacific Northwest have focused solely on coastal cutthroat trout, the most prevalent species, though it is well-known that interactions within the total species assemblage can be important.
Researchers studied low order streams in western Washington and Oregon to evaluate the importance of other co-occurring fish in predicting the presence and density of coastal cutthroat trout. Steelhead were negatively associated with the presence of coastal cutthroat trout. Coastal cutthroat trout most frequently shared streams with juvenile coho salmon. Researchers found that physical stream conditions such as size and gradient may have more of an influence on fish density than species interactions - densities of juvenile coho and coastal cutthroat were positively, yet weakly associated. Their findings suggest that the net effect of interactions between coastal cutthroat trout and coho salmon cannot be easily described in simple terms, such as casting these species as “competitors” or “predators,” and that possible interactions could be highly variable.
Martens, K.D., Dunham, J.B., 2020, Evaluating coexistence of fish species with Coastal Cutthroat Trout in low order streams of western Oregon and Washington, USA: Fishes, v. 6, no. 4, https://doi.org/10.3390/fishes6010004