Coastal Cutthroat Trout Redd Counts Impacted by Natural Water Supply Variations

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