As part of a propagation effort to supplement diminishing returns of an endangered species, the Bodega Marine Laboratory (University of California) maintains captive broodstock of the winter run Chinook salmon (Oncorhynchus tsharwytscha), in cooperation with the Coleman National Fish Hatchery and several state and federal agencies. A systemic protozoan parasite has been isolated from multiple year classes of the captive broodstock and mortality associated with the organism has occurred in both sub adult and adult fish held at the facility. Gross pathology associated with this disease includes multifocal granuloma formation in the visceral organs and internal hemorrhage; glomerulonephritis is detected microscopically. The disease may be exacerbated by concurrent infections with Renibacterium salmoninarum, the causative agent of bacterial kidney disease. To date, the organism has not been detected in any of the four runs of salmonids spawned at the hatchery, nor in any inhabitants of the Sacramento River drainage. Detection of this parasite has created concern over the potential introduction of an exotic pathogen into the Sacramento River system via movement of exposed fish or their gametes. Therefore, a thorough examination of the transmission of the parasite, its detection and control, are critical to the continuation of the winter run Chinook captive broodstock propagation program as well as to the protection of this endangered species.

A similar organism, termed the rosette agent, was reported as the cause of a systemic infection in net pen reared Chinook salmon in Washington (Elston et al. 1986, Harrell et al. 1986). This parasite has been cultured by in vitro techniques (Elston et al. 1986), and a portion of its genomic DNA encoding the small subunit ribosomal RNA (16s-like rRNA) has been sequenced (Kerk et al. in press). Sequence data suggests that this organism is related to protists of the Choanoflagellidae. Another parasite, termed "Dermocystidiumlike", has been reported from cultured Atlantic salmon in Santa Cruz, California (Hedrick et al. 1989). The precise relationship between the systemic protozoan detected in the winter run Chinook salmon captive broodstock and either the rosette agent or "Dermocystidium-like" parasite is not known. However, results from fluorescent antibody test comparisons and electron microscopic comparisons suggest relationships between all three isolates. We have successfully cultivated the agent from the captive broodstock by in vitro methods. Using sequence information for the rosette agent reported from Washington, we have recently amplified a parasite-specific product from the agent isolated from infected winter run Chinook by the polymerase chain reaction (PCR). We are now evaluating the PCR test for use as a method of detection for the presence of the parasite in potential spawning candidates. To date, detection of infection has been limited primarily to histological examination, a relatively insensitive method requiring lethal sampling. Use of the PCR test may allow earlier detection, and minimize the potential impacts of the agent and the disease it causes. In this way, the maximum number of fish representing the best genetic diversity can contribute to the progeny expected from the captive winter run Chinook salmon broodstock project.

References

1.  Elston, R.A., Harrell, L., and M.T. Wilkinson. 1986. Isolation and in vitro characteristics of Chinook salmon (Oncorhynchus tshawytscha) rosette agent. Aquaculture 56:1-21.

2.  Harrell, L.W., Elston, R.A., Scott, T.M. and M.T. Wilkinson. 1986. A significant new systemic disease old net-pen Chinook salmon (Oncorhynchus tshawytscha) brood stock. Aquaculture 55:249-262.

3.  Hedrick, R.P., Friedman, C.S., and J. Modin. 1989. Systemic infection in Atlantic salmon Salmo solar with a Dermocystidium-like species. Diseases of Aquatic Organisms 7:171-177.

4.  Kerk, D., Wainwright, P., Gee, A., Drum, A.S., Standish, M.J., Elston, R.A., and M.L. Sogin. In press. The rosette agent of Chinook salmon (Oncorhynchus tshawytscha) is closely related to Choanoflagellates, as determined by the phylogenetic analyses of its small ribosomal subunit RNA. Marine Biology.