Intrinsic and extrinsic drivers of life‐history variability for a south‐western cutthroat trout

Abstract

The impacts of climate change on cold‐water fishes will likely negatively manifest in populations at the trailing edge of their distributions. Rio Grande cutthroat trout (Oncorhynchus clarkii virginalis, RGCT) occupy arid south‐western U.S. streams at the southern‐most edge of all cutthroat trout distributions, making RGCT particularly vulnerable to the anticipated warming and drying in this region. We hypothesised that RGCT possess a portfolio of life‐history traits that aid in their persistence within streams of varying temperature and stream drying conditions. We used otolith and multistate capture–mark–recapture data to determine how these environmental constraints influence life‐history trait expression (length‐ and age‐at‐maturity) and demography in RGCT populations from northern New Mexico, United States. We found evidence that RGCT reached maturity fastest at sites with warm stream temperatures and low densities. We did not find a strong relationship between discharge and any demographic rate, although apparent survival of mature RGCT decreased as stream temperature increased. Our study suggests plasticity in trait expression may be a life‐history characteristic which can assist trailing edge populations like RGCT persist in a changing climate.

Interacting Effects of Density‐Dependent and Density‐Independent Factors on Growth Rates in Southwestern Cutthroat Trout Populations

“…We classified sites as warm if they risked exceeding the 30-d ultimate upper incipient lethal temperature for juvenile RGCT (21.7°C;Zeigler et al 2013a). We included streams with and without historic records of intermittency to increase our chances of sampling a range of low-and high-flow conditions (see Huntsman et al 2021 for more details). We established all sample reaches above physical barriers to ensure that we would encounter sufficient numbers of RGCT for markrecapture analysis, since the presence of nonnative trout was expected to significantly reduce Cutthroat Trout production (Benjamin and Baxter 2012).…”

“…where μ isf is the expected SGR for observation i from stream s for identified fish f. A fish was classified as either immature (TL < 141 mm from cool sites; TL < 121 from warm sites) or mature based on size-atmaturity estimates generated for these RGCT populations during a previous study (Huntsman et al 2021).…”

“…where 5 represents the minimum temperature at which growth typically occurs in Cutthroat Trout (5°C; USFWS 1998; Coleman and Fausch 2007;Ward et al 2017). Degree-days have been found to directly correlate with developmental rates (Huntsman et al 2021) and affect the growth dynamics of RGCT (Huntsman et al 2018) and other salmonids (Ward et al 2017).…”

“…Huntsman et al (2018) also showed that stream temperatures have a significant effect on RGCT growth dynamics, with little evidence of DD effects. However, a recent study found that both DD and DI effects influenced maturation rates when study sites spanned a broad range of temperature and density gradients (Huntsman et al 2021), suggesting that further study on DD and DI effects is warranted.…”

“…The Rio Grande Cutthroat Trout O. clarkii virginalis (RGCT) is the southernmost subspecies of Cutthroat Trout, and current evidence suggests that RGCT are primarily limited by DI effects, such as low streamflow, intermittency, and warming temperatures, within much of their native distribution in the Rio Grande and Pecos River of Colorado and New Mexico (Zeigler et al 2012(Zeigler et al , 2019. Apparent survival of RGCT populations decreases with warming temperatures, and there is no evidence of DD effects on this demographic process (Huntsman et al 2021). Huntsman et al (2018) also showed that stream temperatures have a significant effect on RGCT growth dynamics, with little evidence of DD effects.…”