Abstract
When multiple species of fish coexist there are a host of potential ways through which they may interact, yet there is often a strong focus on studies of single species without considering these interactions. For example, many studies of forestry–stream interactions in the Pacific Northwest have focused solely on the most prevalent species: Coastal cutthroat trout. To examine the potential for interactions of other fishes with coastal cutthroat trout, we conducted an analysis of 281 sites in low order streams located on Washington’s Olympic Peninsula and along the central Oregon coast. Coastal cutthroat trout and juvenile coho salmon were the most commonly found salmonid species within these streams and exhibited positive associations with each other for both presence and density. Steelhead were negatively associated with the presence of coastal cutthroat trout as well as with coho salmon and sculpins (Cottidae). Coastal cutthroat trout most frequently shared streams with juvenile coho salmon. For densities of these co-occurring species, associations between these two species were relatively weak compared to the strong influences of physical stream conditions (size and gradient), suggesting that physical conditions may have more of an influence on density than species interactions. Collectively, our analysis, along with a review of findings from prior field and laboratory studies, suggests that the net effect of interactions between coastal cutthroat trout and coho salmon do not appear to inhibit their presence or densities in small streams along the Pacific Northwest.
Highlights
Many studies of stream fishes involve consideration of just a single species or just a small fraction of the total species assemblage, though it is well-known that interactions within these assemblages can be important [1,2]
Coastal cutthroat trout are more prevalent in smaller streams in the Pacific Northwest, they share these habitats with a host of other species, including congeneric salmonids, especially steelhead (O. mykiss), Chinook salmon (O. tshawytscha), and coho salmon (O. kisutch), as well as sculpins (Cottus, sp.), which are often numerically dominant when present [9]
It is possible that patterns of presence or density of a given species may be more tied to environmental gradients than to interspecific interactions [13,14], and potentially contextspecific interactions [12]
Summary
Many studies of stream fishes involve consideration of just a single species or just a small fraction of the total species assemblage, though it is well-known that interactions within these assemblages can be important [1,2]. Coastal cutthroat trout are more prevalent in smaller streams in the Pacific Northwest, they share these habitats with a host of other species, including congeneric salmonids, especially steelhead (O. mykiss), Chinook salmon (O. tshawytscha), and coho salmon (O. kisutch), as well as sculpins (Cottus, sp.), which are often numerically dominant when present [9]. These species can interact ecologically in numerous ways throughout the life cycle, and in the case of steelhead, hybridization is a possible interaction [11]. Our intent is not to delve deeply into single or multiple sets of processes explaining species coexistence, but rather to evaluate associations as a first approximation of the net effect of these processes on realized occurrence and density of coastal cutthroat trout
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