Cold Summer Temperature Limits Recruitment of Age‐0 Cutthroat Trout in High‐Elevation Colorado Streams

Translocation is a key strategy for conserving native subspecies of cutthroat trout Oncorhynchus clarkii that have declined markedly throughout their native range. Previous research showed that successful translocations of cutthroat trout in high‐elevation southern Rocky Mountain streams were more likely in streams with warm summer water temperature and led to the hypothesis that cold summer temperatures govern translocation success by limiting recruitment. We tested this by measuring the density and size of age‐0 cutthroat trout (greenback cutthroat trout O. c. stomias and Colorado River cutthroat trout O. c. pleuriticus) in six headwater streams in north‐central Colorado that varied in thermal characteristics. Surveys were conducted at peak emergence during 3 years in two widely spaced study reaches in each stream. Fry density increased with Celsius degree‐days accumulated during the growing season but did not vary significantly among years. We used laboratory data on the growth and survivorship of age‐0 cutthroat trout in three temperature regimes that were similar to those studied in the field to determine expected survivorship and size of fry at the start of winter, which occurred several weeks after peak emergence in our warmer reaches and coincided with emergence in colder reaches. Our results support the hypothesis that recruitment of native cutthroat trout in Colorado is limited by cold water temperatures that reduce growth and recruitment. High‐elevation streams, like those studied, that accumulate 900‐1,200 Celsius degree‐days during the growing season afford the best opportunity for cutthroat trout recruitment and translocation success. Streams that provide 800‐900 degree‐days probably sustain recruitment in some years, and those with less than 800 degree‐days are generally unsuitable for translocations because of a greater risk of recruitment failure and the smaller sizes attained by fry by the onset of winter.