Instream Habitat Refugia for Trout

Two coastal cutthroat trout take refuge from prey in the lee of instream cover. USDA Forest Service photo by Brooke Penaluna.

Research Description

To discover how trout choose and use physical cover in streams, including when preyed upon by birds, we conducted a manipulative experiment using outdoor streams. To understand which habitat characteristics mediate the effects of timber harvest and climate change, we used a trout population model. Both experimental designs approximated conditions experienced by coastal cutthroat trout (Oncorhynchus clarkii clarkii) during low-flow conditions in small Pacific Northwest streams. We conducted a realistically scaled experiment in a seminatural setting using trout where available food, space, and other potentially confounding covariates associated with cover such as water depth, turbidity, and velocity were held constant. The model mirrored conditions in actual streams to understand how trout respond to modeled changes from forest management and climate change.

Key Findings 

• Trout are restricted by available habitat during seasonal low flow. Streams with deeper pools have higher trout biomass, highlighting the importance of local habitat conditions. Hence, pools play a key role to trout populations, particularly with future projections of stream drying across portions of their range.
• Instream cover improves trout survival by providing a place to hide from bird predation. Trout survive better in places with more shade, suggesting that shade is important where instream cover is limited, especially during seasonal low flow.
• Local variability among streams—such as water depth and available habitat—mediates the effects of timber harvest and climate change on trout. Climate change affects trout by triggering early emergence, reducing numbers of older trout, and increasing numbers of younger trout.
• Timber harvest produces fewer and less consistent responses across trout populations and, in some cases, may reduce or counter the effects of climate change through increased summer flow.

A transect of Rock Creek in the Trask River watershed of western Oregon during seasonal low and high flows; more instream cover from predators is available to fish during higher flows.  Photo by Brooke Penaluna, USDA Forest Service.

Management Implications

• Although coastal cutthroat trout populations are declining, they continue to occupy headwater streams, precluding them from federal protected conservation status. Information about local habitat conditions that allow trout to persist in headwater streams can be used to develop conservation strategies.
• Habitat variability is the key to protecting trout because habitat diversity decouples the effect of human land use and climate change on trout. Management that encourages a mix of habitat conditions over time may help diversify responses in trout populations.
• Seasonal low flow (late summer or autumn) represents a biological crunch time for trout. Increasing the number of pools in streams will increase trout biomass by increasing their survival and body size.
• Availability of instream cover increases trout survival by mediating the effect of predation by birds. Maintaining or improving instream cover in areas with high natural predation or in areas where shade has been reduced after timber harvest will likely benefit coastal cutthroat trout populations.

Coastal cutthroat trout netted in Mack Creek at the H.J. Andrews Experimental Forest in the Western Cascades of Oregon. Photo by Ivan Arismendi, Oregon State University.