FWS has decided against listing the Rio Grande cutthroat trout, despite a "reduction in resiliency" that makes it less likely the subspecies as a whole can continue to persist.
In a brief press release, FWS said it had "found that the Rio Grande cutthroat trout is not in danger of extinction throughout its range or in a significant portion of its range now, nor is it likely to become so in the foreseeable future. However, [we are] asking the public to submit any new information that becomes available concerning the status of the Rio Grande cutthroat trout at any time."
Evaluation and Finding (from FR)
"Our review found that there are currently 122 existing populations of the Rio Grande cutthroat trout in four GMUs. We consider each of these populations genetically pure enough to be Rio Grande cutthroat trout; that is, each population has 90 percent or more of the native Rio Grande cutthroat trout genes. To assess the current status of these populations, we sorted each of them into four categories to consider their current status, which was based on effective population size, occupied stream length, presence of competing nonnative trout, and presence of hybridizing nonnative trout. We categorized 55 of the populations (45 percent) as currently in the best or good condition of having no nonative trout, relatively large effective population sizes, and relatively long occupied stream lengths (Service 2014a, pp. 14–15). This current number of populations in the best or good condition existing across the subspecies’ range provides resiliency (45 percent of populations considered sufficiently large to withstand stochastic events), redundancy (55 populations spread across all four extant GMUs to withstand catastrophic events), and representation (multiple populations are persisting across the range of the subspecies to maintain ecological and genetic diversity).
"The Rio Grande cutthroat trout also historically occurred in a fifth GMU—the Caballo GMU. We only know of one historical population in this GMU, which was extirpated more than 30 years ago. With only one population, this area would not have significantly contributed to the resiliency and redundancy of the subspecies. However, it could have had some important genetic or ecological diversity that would have contributed to the adaptive capacity of the subspecies. Losing this population likely lowered the overall viability of the subspecies but would not be a substantial enough impact rangewide to meaningfully increase the overall risk of extinction of the Rio Grande cutthroat trout.
"To further consider the status of the Rio Grande cutthroat trout, we analyzed the condition of the subspecies over the next 10 years to evaluate its viability. In 2023, we projected an estimated range of between 104 and 131 populations will persist under worst case and best case scenarios, respectively. According to our forecasts, these populations would be distributed throughout the subspecies’ range, with multiple populations persisting in all four of the currently extant GMUs (see Service 2014a, pp. 44–45 for complete results). Therefore, because this worst case estimate of the number and distribution of populations provides resiliency, representation, and redundancy for the subspecies, we conclude the subspecies does not meet the definition of an endangered species under the Act. Although the subspecies has experienced substantial reduction from its historical distribution, the number of Rio Grande cutthroat trout populations currently persisting and expected to persist in the next 10 years across its range does not put the subspecies in danger of extinction."
More from the FR notice
"Currently the subspecies is distributed in 122 populations across the four [geographic management units] (ranging from 10 to 59 populations per GMU), and most of the populations are isolated from other populations. The total amount of currently occupied stream habitat is estimated to be about 11 percent of the historically occupied range. This large decline in distribution and abundance is primarily due to the impacts of the introduction of nonnative trout."
The GMUs are managed by the states of Colorado and New Mexico and other agencies "as separate units to maintain genetic and ecological diversity within the subspecies where it exists and to ensure representation of the subspecies across its historical range. GMUs were not created to necessarily reflect important differences in genetic variability, although fish in the Pecos and Canadian GMUs do exhibit some genetic differentiation from those in the Rio Grande basin GMUs. From a rangewide perspective, multiple Rio Grande cutthroat trout populations should be dispersed throughout the various GMUs to maintain subspecies viability and to reduce the likelihood of extinction."
"Rangewide, the resiliency of the subspecies has declined substantially due to the large decrease in overall distribution in the last 50 years. In addition, the remnant Rio Grande cutthroat trout populations are now mostly isolated to headwater streams due to the fragmentation that has resulted from the historical, widespread introduction of nonnative trout across the range of Rio Grande cutthroat trout. Therefore, if an extant population is extirpated due to a localized event, such as a wildfire and subsequent debris flow, there is little to no opportunity for natural recolonization of that population."