Virile crayfish in the Lower Colorado River basin: detection, distribution, and interactions with native fishes

Authors: Matthew Troia

Year: 2024

Abstract

Virile crayfish (Faxonius virilis; hereafter crayfish) established invasive populations in the Lower Colorado River basin (hereafter LCRB) in the 1960s, are now abundant and widespread throughout the basin, and ostensibly negatively impact native fishes and herpatofauna. Still, spatiotemporally comprehensive field studies and quantitative analyses are lacking which limits understanding of population structure, distribution, and interactions with native species. Here, we highlight findings from our recently completed US Fish and Wildlife Service Science Applications Project for which we surveyed crayfish from 120 wadeable sites in the LCRB in May-June of 2021, 2022, and 2023. We also deployed in situ data loggers to monitor wet-dry status and water temperature over 24 months. We detected crayfish in 71% of sites at which mean abundance was 3.4 individuals per square meter (range 1.0-37.2). We also detected 10 native and 14 non-native fish species. Our first analytical objective was to map the distribution of perennial streams and suitable crayfish habitat using ensembles of statistical models and geospatial data layers. Predictive accuracy of ensemble runs were higher for wet-dry status (AUC = 0.87) than for crayfish habitat suitability (0.67). Of 130,847 stream km in the LCRB, we estimate that 22.2% have wetted channels during the May-June dry season and 50.3% of these perennial streams provide suitable habitat. Next, we quantified negative interactions between crayfish and fishes using our field-based co-occurrence data, species traits, and null model analysis. Although co-occurrence between crayfish and individual fish species depends on shared habitat similarity, the probability of co-occurrence for native fishes is significantly lower compared to non-native fishes. This suggests negative biotic interactions—predation and/or competition—impact native fishes. Lastly, we measured sex ratios, body size, and morphology of crayfish at 45 sites in 2022 and 2023, and collected eDNA samples from all 120 sites in 2023. A brief overview of these data will be provided. Overall, this project provides maps and species vulnerability rankings to support management actions and our aim is disseminate these tools to 2024 DFC attendees.