Abstract

At the North Carolina State University Pamlico Aquaculture Field Laboratory in Aurora, North Carolina, white bass (Morone chrysops) and striped bass (Morone saxatilis) are bred for research in support of the commercial hybrid striped bass industry. Since 2007, staff biologists have observed a high mortality rate in the white bass during high stress periods, such as spawning events and transportation. All mortalities had pale gills on presentation. Full necropsies were performed on several affected fish. A severe trematode burden on the gills was found consisting of polyopisthocotylean monogenean trematodes. The trematodes were identified as Gamacallum macroura, belonging to the family Microcotylidae. Six white bass with pale gills and six apparently healthy fish were randomly selected and euthanized for post mortem examination. Venipuncture was performed from the caudal tail vein for a complete blood count. The first gill arch on both left and right sides of each fish was microscopically evaluated, and the number of trematodes present on the gills was recorded. Histopathology of the affected fish gill arches revealed mild branchitis, dilated hyperplastic lamellae sometimes occluded by fibrin, and occasional trematodes attached to the lamellae. Unaffected fish had similar lesions but at a much lower frequency. Initial results showed that white bass heavily infested with G. macroura are anemic and have marked gill pathology.

A treatment pilot study was performed. White bass with pale gills (n = 31) were divided into six groups and placed into isolated systems. Five different antiparasitic treatments (formalin 250 mg/L, and 400 mg/L, potassium permanganate 5 mg/L, Abate [temephos] 0.006 mg/L, Dylox [trichlorfon] 1 mg/L) were evaluated, and one group acted as a control and received no drug treatment. A one time application of the selected antiparasitic treatment was administered to each system. Fish were checked for any mortalities at 24, 48, and 72 hours. Mortalities occurring before the final 72 hour check were collected and the number of trematodes was counted microscopically from the 1st left and right gill arches. At the final 72 hour check, all remaining fish were euthanized and the previously prescribed procedure of counting trematodes was performed. Initial results indicate that treatment with formalin at 250 mg/L and 400 mg/L, and temephos at 0.006 mg/L, are effective in reducing G. macroura numbers in white bass.

Acknowledgements

The authors thank the Environmental Medicine Consortium faculty, College of Veterinary Medicine, North Carolina State University for their financial and technical assistance. This work was supported, in part, by a grant R/AF-49 to one of the authors (C.V.S.) from the North Carolina Sea Grant Program.