Resiliency of Apache Trout Habitats in a Warmer and Drier Climate Future

Objective: The Southwest has the hottest and driest climate in the United States, and projections show that it will only get hotter and drier into the 2100s. The Apache Trout Oncorhynchus apache is native to the Southwest and is currently listed as threatened under the U.S. Endangered Species Act. Our goals were to understand how climate factors influence the distribution of juvenile Apache Trout (<125 mm TL) and how climate change will influence the suitability of Apache Trout habitat into the 2080s.Methods: We used a species distribution model to evaluate how climatic and other factors influence the distribution of juvenile Apache Trout. We used predictions from the model to evaluate how climate change might impact the suitability of streams designated for recovery of the species into the 2080s.Result: Juvenile Apache Trout occurrence was predicted well by mean July stream temperature (C), mean annual precipitation (dm), stream slope (%), and the presence of nonnative trout (area under the receiver operating characteristic curve = 0.85). Standardized parameter estimates showed that Rainbow Trout O. mykiss presence and annual precipitation influenced occupancy the most. Model predictions for the 2080s showed suitable habitat (occurrence probability = 0.25) to increase for 11 (of 45) Apache Trout streams in the increased temperature (+3 degrees C) only scenario (scenario 1), as headwater reaches that are currently too cold warmed to become more suitable. When we also included projected declines in annual precipitation (-5%) for the 2080s (scenario 2), the amount of suitable habitat decreased for eight Apache Trout streams and remained unchanged in all other streams. Conclusion: Most Apache Trout populations are isolated upstream of barriers to nonnative trout in stream reaches that are currently thermally suitable with respect to mean July temperatures and would remain suitable into the 2080s. Cold headwater reaches are projected to warm, becoming more suitable in the 2080s. Thus, intentional isolation and the resultant truncated downstream distributions of Apache Trout populations in headwater streams explain the nominal effect of projected temperature increases due to climate change on this cold-adapted salmonid. Standardized model parameters suggest that future declines in precipitation, manifested through reduced snowpack and its influence on streamflows, will play a larger role than temperature in the suitability- and, thus, resiliency- of Apache Trout habitats at least into the 2080s.