We conducted a PHABSIM study on Bingham Creek, Washington, by using validated habitat suitability criteria for the rearing of coho salmon Oncorhynchus kisutch. We compared the relationship between weighted usable area (WUA) and flow with a previously determined empirical relationship that showed increasing coho salmon smolt production with increasing summer low flow (). The relationship between juvenile coho salmon WUA and flow indicated that the greatest amount of habitat occurred at a flow that was lower than our low‐flow measurement, and the amount of habitat decreased with increasing flow. Thus, PHABSIM results were contrary to empirical measurement of coho salmon smolt production. Based on the relationship between summer flow and smolt production, production of smolts would decline if flow was reduced to the flow that maximizes WUA. The failure of PHABSIM to be consistent with empirical results may have be related to habitat suitability being influenced more by the numerous subdominant, schooling juvenile coho salmon and less by the dominant, territorial individuals, which have higher survival and prefer higher velocities.
The conservation of aquatic species with complex life histories requires the protection of habitat for each life stage. Coastal cutthroat trout (Oncorhynchus clarkii clarkii) exhibit both mobile and sessile stages and are of conservation concern in north‐western North America. Models that predict how habitat changes with flow can inform water management for the species but rely on habitat descriptors that change with flow and are not suited to sessile life stages. Spawning and incubation habitat availability and use for coastal cutthroat trout were assessed in mesohabitats and microhabitats. Primary channel units were tallied, dimensions were measured, redds were counted, and redd locations within channel units were measured. Primary channel units were subdivided based on relative residual depth (RRD, bed elevation in relation to maximum pool depth) and hydraulic control (HC, the lowest point on the highest ridge across the riffle that controls the water surface elevation in the upstream pool). Habitat suitability was calculated for depth (D), velocity (V), Froude number (Fr), substrate (S), RRD, and channel units. Redds in riffles and pools were close to hydraulic controls; suitability decreased upstream in pools and downstream in riffles. Maximum suitability values were: 21–24 cm for D, 0.05–0.10 for RRD, 61–64 cm s−1 for V, >0.80 for Fr, and 1.3 to <3.8 cm for S. Channel units, RRDs, and microhabitat better identified spawning habitat than microhabitat alone. The results suggest that RRD and stage of zero flow are good indicators of habitat suitability for spawning through fry emergence and could be adapted for use with other species. This work generates criteria for delineating flow‐independent channel units that can be used to set more protective instream flows, prioritize the acquisition of instream water or conservation easements to protect critical habitat, and inform restoration projects to optimize habitat availability.
During 2001–2003, we counted redds of Coastal Cutthroat Trout Oncorhynchus clarkii clarkii and measured microhabitat variables at fresh redds in Irely Creek upstream of Irely Lake in a largely undisturbed watershed within Olympic National Park, Washington. Redd numbers declined abruptly in 2003, the year after a dry summer caused the dewatering of Irely Lake; therefore, we continued the redd counts during 2004–2012 to determine whether pond dewatering was a factor in the decline and whether redd counts would track Irely Lake surface area. The estimated number of redds varied from a high of 51 (in 2002) to a low of 2 (in 2006, 2007, and 2010); increases in estimated redd number occurred after wetter summers, and decreases were observed after drier summers. This pattern was significant in chi‐square tests for redd number and the change in redd number with Irely Lake condition in the preceding year (dry, semi‐dry, or wet). Rather than indicating juvenile mortality, the lack of a lag in response suggests the mortality of adults that would otherwise have spawned during the subsequent spring. The clear impact of dry summers in one of the wettest regions of North America underscores the important influence of hydrology on fish, even in the absence of other disturbances. Received September 16, 2015; accepted March 15, 2016 Published online July 20, 2016
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