Evaluation of alternative approaches to PHABSIM modeling of coastal cutthroat trout spawning habitat

In the face of a changing climate and increasing human demand for water, an understanding of habitat preference has become critical for managing wild fish populations and projecting potential changes in habitat and populations. Two approaches to Physical Habitat Simulation (PHABSIM) modeling of coastal cutthroat trout Oncorhynchus clarkii clarkii spawning habitat were compared by modeling a reach, consisting of eight transects covering a pool and upstream and downstream riffles, of a small Puget Sound stream with two sets of habitat variables. The reach contained a cluster of redds 2 years in a row, assumed to be an indication of preferred spawning habitat and was located in the area of maximum spawning in the watershed, based on multiple years of redd surveys. Two PHABSIM instream flow models of the study site, one based on standard microhabitat (depth [D], velocity [V], and substrate [S]) suitabilities and the other based on D, V, and channel unit (CU) suitabilities, were developed and compared for their relative ability to correctly predict coastal cutthroat trout spawning habitat selection at the redd cluster within the PHABSIM study site. One approach was the standard use of habitat suitability criteria (HSC) for D, V, and S to indicate spawning habitat quality. The alternate approach was to replace substrate HSC with CU index HSC that incorporated dominant substrate particle diameter, CU (riffle, deep and shallow pool tail, pool body, deep and shallow pool edge, cascade, waterfall, and terrestrial), where deep and shallow units were based on relative residual depth (RRD), size of CU relative to channel width, and position within CU. Spawning habitat quality was calculated for each transect as weighted usable width with the standard HSC metrics (WUW_s) as well as the modified CU index (WUW_m). WUW_m at the transects bracketing the redd cluster exceeded WUW_m at the remaining six transects and was outside the 95% confidence interval for WUW_m at the remaining transects. In contrast, WUW_s varied less between the redd cluster and the remainder of the transects, suggesting the CU index better reflected spawning habitat quality than substrate. Incorporation of elevation relative to SZF addressed vulnerability to declining flow during incubation. Both models resulted in maximum WUA within the range of discharges that coincided with the majority of fresh cutthroat trout redds in Skookum Creek.