Effects of drift supplementation and altered channel complexity on the foraging distance of coastal cutthroat trout (Oncorhynchus clarkii clarkii)

We studied the foraging distances of resident coastal cutthroat trout (Oncorhynchus clarkii clarkii) and their relationship to invertebrate drift and channel complexity. Our study tested the hypothesis that invertebrate drift is homogenous by capturing and analyzing drift across different velocities. Our experiment had three distinct trials (1) existing conditions, the control group; (2) supplemented invertebrate drift by 550%, and (3) increased channel complexity. The novelty of our study is the in situ approach we took to experimentally quantify O. c. clarkii foraging behavior, unlike previous studies, which have typically taken place in hydraulic flumes or been observational based (Fausch 1984; Hughes et al. 2003). We observed that invertebrate drift was not homogeneous across different velocities (p = 0.031). There was a noticeable decrease in foraging distance when invertebrate drift was supplemented. The decline in foraging distance was likely observed because higher prey density allowed individuals to travel shorter distances while feeding. It is likely that foraging distance was greater with increased channel complexity because salmonids were able to forage in lower velocities. Our study provides the framework for future in situ experimental studies attempting to quantify differences in salmonid foraging behavior. Our study and similar investigations will likely be applicable in future habitat conservation and remediation efforts. Introduction Resident coastal cutthroat trout (Oncorhynchus clarkii clarkii) are drift-feeding salmonids, dependent on downstream movement of invertebrates for energy gains. Salmonids’ primary energy expenditure is from maintaining position or a focal point within the stream (Fausch 1984). It was proposed that salmonids inhabit lower velocity areas, holding focal points, and move into fast-moving currents to forage in order to maximize energy gains (Fausch and White 1981). Salmonids exhibited an inverse relationship between foraging distance and water velocity, and were more selective in their foraging attempts within high velocity areas (Godin and Rangeley 1989; Piccolo et al. 2008; O’Brien and Showalter 1993). Differences in velocity across streams develop from channel complexity and formulate the basis for salmonid habitat. Within the Pacific Northwest, historic removal of large woody debris simplified stream channels, decreasing habitat suitability (Swanson and Lienkaemper 1978). Remediation efforts include additions of large wood, which can restore fish habitat via increasing channel complexity (Beechie et al.