Dynamics of a Warmwater-Coldwater Fish Assemblage in a Wildfire-Prone Landscape

Gila trout Oncorhynchus gilae historically occupied high-elevation (>2000 m) headwater streams of the Gila River in southwest New Mexico and central Arizona, USA. By the mid-twentieth century, human activities such as resource extraction and nonnative species introductions caused habitat degradation and loss, thereby restricting the trout to a few small remote streams in the upper Gila River catchment in New Mexico. Since 1989, conservation efforts have been hampered by numerous wildfires that caused elimination of some populations and greatly reduced others. But wildfires also eliminated nonnative fishes from streams thereby making them available for Gila trout repatriation. Elimination of all fishes, including nonnative trout, from upper Black Canyon (ca. 17 km), an East Fork Gila River tributary, by wildfire and associated sediment-laden flows in 1995 and subsequent colonization by native warmwater species (longfin dace Agosia chrysogaster, speckled dace Rhinichthys osculus, Sonora sucker Catostomus insignis, and desert sucker Pantosteus clarkii) provided the opportunity to study the dynamics of a mixed warm-coldwater fish assemblage. Although upper Black Canyon was thermally compromised (summer maxima >25 °C), age-0 hatchery-produced Gila trout (≈1000–2000) were released annually from 1998 through 2012. Establishment of a nonnative piscivore, brown trout Salmo trutta, added complexity to the study. By 2012, native warmwater species had thriving populations but despite limited reproduction annual stocking of Gila trout likely enabled it to persist in the stream. It was not possible to determine the effects of brown trout predation on the Gila trout population or negative interactions, if any, between native warmwater fishes and Gila trout.  Regardless, warmwater fish and brown trout abundance was greatest in downstream reaches and that of Gila trout was greatest in the most upstream reach. Inter-annual differences in somatic metrics (condition and size structure) of the Black Canyon Gila trout population were as great as it were between it and reference populations. Unassisted re-population of upper Black Canyon by native warmwater species succinctly illustrated the importance connectivity within a drainage and the rapidity in which this was accomplished is testimony to the resilience of each species to major disturbance. In 2013, wildfire again eliminated all fishes from upper Black Canyon. Hatchery-produced Gila trout were annually stocked from 2013 through 2018 and sampling in 2018 yielded a large number of age-0 fish, suggesting that a Gila trout population might be established in upper Black Canyon, but in the absence of native warmwater fishes and piscivorous brown trout.