Influence of Stream Woody Debris on Eight Populations of Rio Grande Cutthroat Trout in Northern New Mexico

The impacts of climate change on cold water species will likely manifest in populations at the trailing edge of their distribution. Rio Grande cutthroat trout (Oncorhynchus clarkii virginalis, RGCT) occupy arid southwestern U.S.A. streams at the southern-most edge of all cutthroat trout distributions; thus making RGCT particularly vulnerable to the anticipated warming and drying in this region. However, RGCT may possess a portfolio of life-history traits that aide in their persistence, attributes commonly observed in trailing edge populations. We used otolith and multistate capture-mark-recapture data collected along a temperature and stream drying gradient to determine how these environmental constraints influence life-history trait expression (length- and age-at-maturity), demography, and extirpation risk in RGCT populations from northern New Mexico, U.S.A. We found the rate at which RGCT reached maturity was highest at warm to intermediate stream temperatures, which was the demographic trait most strongly linked to RGCT persistence. Interestingly, older life-stages contributed more to population growth as temperatures decreased, providing further evidence of strong temperature effects controlling life-history trait expression in RGCT. Precipitation, however, had little effect on RGCT population dynamics and was likely influenced by the uncharacteristically wet years (2016-2017) during this study. Regardless, our results suggest that RGCT persistence depends on temperature-influenced diversity in life-history trait expression (e.g., longevity, age-at-maturity). Plasticity in trait expression is an important life-history characteristic of coldwater species, and may be vital for trailing edge populations to persist in a changing climate.