Parasites and Fish Health in a Desert Stream, and Management Implications for Two Endangered Fishes

Abstract

Reintroductions of the endangered razorback sucker Xyrauchen texanus and Colorado squawfish Ptychocheilus lucius into the Verde River, Arizona, have failed to establish self-sustaining populations. The failure of these reintroductions is often attributed to predation and competition by nonnative fishes and habitat loss, but parasite infestations (particularly a copepod, Lernaea cyprinacea) also have been implicated. We assessed the relationships among fish health, eukaryotic parasites, and bacterial and viral infections of these endangered fishes collected from two locations (Perkinsville and Childs) on the Verde River, Arizona, during February and June 1996. We also examined surrogate nonendangered native fishes (Sonora sucker Catostomus insignis, desert sucker Catostomus clarki, and roundtail chub Gila robusta) and nonnative species (common carp Cyprinus carpio and smallmouth bass Micropterus dolomieu) to determine if there were any systemwide patterns in parasite infestation and health of fishes. All tests for viruses were negative. We identified 19 bacteria (7 known fish pathogens) and 14 parasites from examined fishes. Six of the seven dominant (>10% prevalence) parasite taxa were more prevalent at Perkinsville than at Childs; Lernaea occurrence was not related to site. Time of year did not have a consistent effect on parasite prevalence across fish species, and Lernaea occurrence was not related to time of year. Lernaea cyprinacea were present on 17.3% of the 313 fish examined; 67% of the parasitized fish were infected with a single L. cyprinacea. Regression analyses on the calculated health assessment index, hematocrit, and leukocrit indicated that month, site, Trichodina, Ichthyophthirius (ich), Ornithodiplostomum and Posthodiplostomum (white grubs), and the cestode Isoglaridacris hexacotyle affected the health of fishes. However, all fishes examined were considered “healthy,” suggesting that parasites were not seriously affecting Verde River fishes. A suite of factors is likely responsible for the failure of these endangered species to reestablish.

Notes

¹ Not known or identified as a fish pathogen.