Federal researchers studied how coastal cutthroat trout use in-stream cover, both in Oregon streams and in an experimental study, where they controlled flow, depth, and cover availability to test multiple hypotheses about cover use. Trout showed behavioral plasticity, concealing under cover and emigrating as first options, followed by grouping, then habitat shifting. Trout selected boulders as the main cover in natural streams and disproportionately used cover near deeper water, selecting larger-sized cover in shallower water, and emigrating at higher levels when there was less cover available. Lack of feeding and growth suggested that perceived threat of predation was a more important driver of behavior than foraging. Collectively, findings indicate that cover can be considered a critical limiting resource for stream-living fish. 

Penaluna, B.E., Dunham, J.B., Andersen, H.V., 2020, Nowhere to hide: The importance of instream cover for stream-living Coastal Cutthroat Trout during seasonal low flow: Ecology of Freshwater Fish, https://doi.org/10.1111/eff.12581