Nonnative Virile Crayfish (Faxonius virilis) Distribution and Density in Secondary Channel and Island Backwaters of the San Juan River, NM, CO, and UT

Authors: Michaela Fishback, James Whitney, Blake Hansen, Amber Bell

Year: 2021

Abstract

The non-native Virile Crayfish (Faxonius virilis) has been observed throughout the Colorado River basin, where it has negative impacts on native fishes and macroinvertebrates. The San Juan River is a major tributary of the Colorado River where Virile Crayfish are present, although their distribution, abundance, and potential impacts in the San Juan River have yet to receive extensive study. The San Juan River hosts several imperiled fish species (e.g., Colorado Pikeminnow (Ptychocheilus lucius); Razorback Sucker (Xyrauchen texanus)) which may be impacted by Virile Crayfish, thus further information on this nonnative species is needed for effective conservation and management. The objective of our research was to quantify spatiotemporal variation in Virile Crayfish density in the San Juan River along an 89-river kilometer reach between Shiprock, New Mexico and Montezuma Creek, Utah. Within this reach we focused our sampling on secondary channel and island backwater habitats, as they are crucial nursery habitats for imperiled native fishes. We seined 50 m sites within 20 backwater habitats (10 secondary channel and 10 island), with each site sampled four times from July 2021 through September 2021, for a total of 80 sites. All captured individuals were counted, had their length measured from the rostrum to the telson, and then were returned to their capture habitat. We calculated density (#/100m2) for each site and sample period by dividing number of individuals captured by sample area. We collected a total of 457 Virile Crayfish at 40/74 (54%) sites; we did not sample all 80 sites because high and low flows prevented sampling at some sites during certain sample trips. Mean density of Virile Crayfish was 2.25/100m2 and ranged from 0.00 to 26.90/100m2. Mean density in secondary channel backwaters was 2.95/100m2 and ranged from 0.00 to 26.90/100m2, whereas in island backwaters mean density was 1.42/100m2 and ranged from 0.00 to 18.24/100m2. We found that nonnative Virile Crayfish attain higher densities in secondary channel backwaters than in island backwaters, possibly due to secondary channels exhibiting lower water velocity and connectivity with the mainstem compared to island backwaters. The higher densities of Virile Crayfish in secondary channel backwaters could result in competition with and predation on the early life stages of imperiled fishes that prefer these habitats as nursery areas. For these reasons, further study concerning how nonnative Virile Crayfish are impacting native fishes in the San Juan River is warranted.