Predicting Persistence of Rio Grande Cutthroat Trout Populations in an Uncertain Future

Abstract

The Rio Grande Cutthroat Trout Oncorhynchus clarkii virginalis (RGCT) occupies just 12% of its ancestral range. As the southernmost subspecies of Cutthroat Trout, we expect a warming climate to bring additional stressors to RGCT populations, such as increased stream temperatures, reduced streamflows, and increased incidence of wildfire. We developed a Bayesian network (BN) model using site‐specific data, empirical research, and expert knowledge to estimate the probability of persistence for each of the 121 remaining RGCT conservation populations and to rank the severity of the threats they face. These inputs quantified the genetic risks (e.g., inbreeding risk and hybridization risk), population demographics (disease risk, habitat suitability, and survival), and probability of stochastic disturbances (stream drying risk and wildfire risk) in an uncertain future. We also created stream temperature and base flow discharge models coupled with regionally downscaled climate projections to predict future abiotic conditions at short‐term (2040s) and long‐term (2080s) time horizons. In the absence of active management, we predicted a decrease in the average probability of population persistence from 0.53 (current) to 0.31 (2040s) and 0.26 (2080s). Only 11% of these populations were predicted to have a greater than 75% chance of persisting to the 2080s. Threat of invasion by nonnative trout had the strongest effect on population persistence. Of the 78 populations that are already invaded or lacking complete barriers, 60% were estimated to be extirpated by 2080 and the remainder averaged only a 10% chance of persistence. In contrast, the effects of increased stream temperatures were predicted to affect the future persistence of only 9% of the 121 RGCT populations remaining, as most have been restricted to high‐elevation habitats that are cold enough to buffer against some stream warming. Our BN model provides a framework for evaluating threats and will be useful to guide management actions that are likely to provide the most benefit for long‐term conservation.

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

“…We found evidence for a strong negative density effect on the rate at which immature RGCT transitioned into a mature state, contrary to expectations for species like RGCT that are at the periphery of all cutthroat trout distributions (Guo, Taper, Schoenberger, & Brandle, 2005; Zeigler et al., 2019). Similarly, Huntsman and Petty (2014) found peripheral brook trout ( Salvelinus fontinalis ) populations within an Appalachian watershed were regulated and argued that this was likely an indirect effect of a regulated source of immigrants rather than density dependence on local demographic performance.…”

“…Although we have evidence for a brown trout effect on RGCT survival, a direct test of these effects would require a greater number of invaded streams be studied to confirm such an effect. Regardless, non‐native competitors have been identified by numerous researchers as among the most serious threats to persistence of native fishes, with concerns only heightened by the threat of a changing climate (Benjamin & Baxter, 2010; Muhlfeld et al., 2014, 2017; Roberts et al., 2017; Wenger et al., 2011; Zeigler et al., 2019).…”

“…Whether anadromy or potamadromy, migratory movements are commonly presented as examples of the various life‐history traits expressed by cutthroat trout (Gresswell, Liss, & Larson, 1994; Homel, Gresswell, & Kershner, 2015). However, many populations of cutthroat trout persist above natural and man‐made barriers, environments that preclude a migratory life‐history trait (Harig & Fausch, 2002; Peterson, Rieman, Dunham, Fausch, & Young, 2008; Zeigler, Rogers, Roberts, Todd, & Fausch, 2019). Interestingly, these trout populations persist by adapting to the demographic pressures within isolation, favouring life‐history traits that often reflect a faster pace of life ( e.g .…”

“…Rio Grande cutthroat trout ( O. c. virginalis , RGCT) is native to the Rio Grande, Canadian and Pecos Rivers in New Mexico and Colorado (Figure 1) at the southern periphery of all cutthroat trout distributions and thus harbingers of large‐scale ecological disturbances such as climate change (Bakevich, Paggen, & Felt, 2019; Behnke, 2002; Hampe & Petit, 2005). Haak, Williams, Neville, Dauwalter, and Colyer (2010) demonstrated edge populations of cutthroat trout have experienced the greatest loss in occupied habitat and RGCT is no exception, where they currently occupy less than 12% of their historical range due to invasive species, climate change, disease and habitat loss (Bakevich et al., 2019; Zeigler et al., 2019). Nearly all contemporary populations occur within isolated high‐elevation stream segments (Bakevich et al., 2019), a potential fate for many trout species (particularly cutthroat trout) in the conterminous United States in response to warming temperatures and altered stream flows due to a changing climate (Isaak et al., 2016; Isaak, Young, Nagel, Horan, & Groce, 2015; Roberts, Fausch, Peterson, & Hooten, 2013; Wenger et al., 2011).…”

“…These streams also commonly exceeded thermal limits during much of the summer (upper incipient limit of 21.7°C; Zeigler et al., 2013; Zeigler et al., 2012) which, along with drought, are among the most common environmental constraints directly linked to demographic performance of cutthroat trout populations (Kovach et al., 2016). However, many RGCT populations are isolated within high‐elevation streams that are likely buffered against a warming climate (sensu Isaak et al., 2016; Isaak et al., 2015), and a recent study indicates temperature and flow have a relatively minor impact on RGCT persistence relative to other pressing concerns (namely, non‐native species; Zeigler et al., 2019). Furthermore, cutthroat trout recruitment is limited by cool temperatures in high‐elevation streams (Coleman & Fausch, 2007), which may partially explain the quadratic relationship between RGCT abundance and temperature found by Huntsman et al.…”