Using Habitat to Understand the Distribution of Apache Trout, a Rare Southwestern Salmonid Impacted By Non-Native Crayfish

Over the past century the majority (70%) of fishes endemic to the southwestern United States have been listed under the U.S.  Endangered Species Act (ESA). One species listed as threatened, Apache Trout Oncorhynchus gilae apache, a salmonid endemic to east-central Arizona, is thought to be affected by Virile Crayfish Orconectes virilis, a recently introduced invasive species.  No method has been found to effectively suppress crayfish in this environment.  We developed habitat suitability criteria for Apache Trout and Virile Crayfish to assess overlap in criteria and whether habitat management strategies could favor Apache Trout but suppress Virile Crayfish.   We sampled Apache Trout (snorkel survey) and Virile Crayfish (quadrat sampler) to identify occupied vs. unoccupied spots and measured habitat parameters (water velocity, depth, substrate, instream cover, overhead cover and temperature) at each spot. This data was used to identify habitat selected by each species. Virile Crayfish selected for substrate and temperature (P < 0.0001); whereas Apache Trout selected for all parameters tested (P < 0.01).  Apache Trout occupied areas with colder water and more instream and overhead cover than Virile Crayfish.  Knowledge of habitat preferences for each species may provide means to reduce Virile Crayfish abundance and distribution, but increase Apache Trout abundance and distribution.