Evaluating Coexistence of Fish Species with Coastal Cutthroat Trout in Low Order Streams of Western Oregon and Washington, USA

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.

UPRLIMET: UPstream Regional LiDAR Model for Extent of Trout in stream networks

“…Slope, the next variable of importance influencing the upper extent of fish, exerts control on physical habitats in streams, including channel morphology, hydraulics, sediment transport, substrate, and habitat 36 . Steep slopes drastically prevent trout from reaching areas above waterfalls or impassable chutes of over 25% slope, but trout can be found in streams channels without barriers at slopes as high as 28% 7,14,37 . Other fishes, such as Coho Salmon (O. kisutch) and steelhead (O. mykiss) are generally not found above 12% slope 38 .…”

“…Study area. The fish species found in the uppermost segments of streams across the Pacific coast of North America are commonly Coastal Cutthroat Trout 11,13,14,48 www.nature.com/scientificreports/ Trout are identified as the focus of monitoring for fish taxa at the upper extent and they are the species we consider here.…”

“…Fish species are distributed longitudinally within dendritic stream networks, and the upstream distribution boundary is driven by natural physical constraints. For example, the upper distribution of Coastal Cutthroat Trout (Oncorhynchus clarkii clarkii) is limited by a legacy of past anthropogenic activities including roads and culverts 7,8 in addition to stream size [9][10][11] , pool abundance 7 , channel slope [12][13][14][15][16][17][18] , and elevation 13 . High numbers of trout can be found in streams that are 1st-or 2nd-order 9 with wetted channel widths of less than 6 m 10,11 .…”

“…Availability of pool habitats can extend the upstream distribution of trout higher in the stream network 7 ; however, pools alone will not give an accurate indication of the uppermost presence of fish 12 . Channel slope or steepness can limit the upper extent of fish 12,13 , with trout found in portions of streams with steeper slopes than co-occurring fishes 14 . Slopes of 20% are recommended as the cutoff for the uppermost fish across various western states and provinces of the U.S. and Canada, e.g., [15][16][17][18] .…”