Demographic and Genetic Attributes of Small, Isolated Populations of Gila Trout: Prospects for Persistence Under a Shifting Climate Regime

By the time Gila trout Oncorhynchus gilae was described in the mid-twentieth century, it persisted only in a few disconnected small headwater streams in the upper Gila River catchment of southwest USA. Isolation of remnant populations eliminated the possibility of recolonization after local population extirpation following catastrophic disturbance. Small population size and demographic isolation suggested that Gila trout should be sensitive to press disturbances associated with lowered precipitation and increased drought, together with catastrophic disturbances like wildfire and associated debris flows. Using data collected over a 35-year period, we explored the relationships between population abundance and age-structure (remnant and replicated), population fitness (estimated as relative condition factor), and genetic diversity measured as allelic richness and heterozygosity. Of particular interest were temporal trends in population response to pulse (wildfire) and press disturbances (extended drought) over the time series. There was little evidence that within-population genetic diversity was related to population fitness. Despite being genetically depauperate, we found that Gila trout populations were resilient to the pressures of extended decreasing precipitation and drought, but not to multiple mega wildfires that the upper Gila River catchment experienced over the past 35 years. Gila trout has survived with human assistance, but it is a fragile persistence made additionally so because of its reduced genetic diversity. A robust wildfire rescue plan, repatriation to historic habitat, restoration of gene flow across extant populations in dendritically complex drainages, and genetic rescue are necessary actions to ensure the demographic and genetic survival of the species.