Cold Summer Temperature Regimes Cause a Recruitment Bottleneck in Age-0 Colorado River Cutthroat Trout Reared in Laboratory Streams

Abstract

Native salmonids are increasingly restricted to upstream habitats that may be too cold to sustain recruitment, and recruitment limitation owing to cold temperatures is a main hypothesis to explain translocation failures of native cutthroat trout Oncorhynchus clarkii in high-elevation streams in the southern Rocky Mountains. We subjected Colorado River cutthroat trout O. c. pleuriticus fry to one of three temperature regimes (cold, intermediate, or warm), which averaged 7.0, 8.5, and 10.0°C during the warmest summer month, in each of 2 years. The regimes mimicked those of natural streams where translocated fish had died out or produced populations of cutthroat trout of low or high abundance. The mean fry survival rate from hatching through swim-up was high during both experiments, ranging from 97% in the warm regime to 85% in the cold. After swim-up, fry in the warm regime grew more than 60% more on average than those in the cold regime by the onset of winter in 2003 during a 22-week period. Survival rates through midwinter were also higher in the warm regime (76%) than in the intermediate (62%) and cold (29%) regimes. A similar pattern of temperature-related growth and mortality was apparent in the 2004 experiment (12 weeks), in which survival rates to early winter ranged from 44% in the warm regime to 10% in the cold. Most mortality in all three treatments occurred during a recruitment bottleneck encompassing a 4-6-week period following swim-up. Analyses of size and percent dry weight indicated that the energy content of fry after swim-up was lower in colder regimes and declined during the recruitment bottleneck in all three regimes. Our findings indicate a strong recruitment bottleneck after swim-up when temperature-related energy deficits apparently cause significant mortality. Managers may increase the viability of these translocated populations by selecting sites that accumulate more than 900 degree-days during summer.