DAO 35:23-29 (1999)  -  doi:10.3354/dao035023

Pathogenicity of Ichthyophonus hoferi for laboratory-reared Pacific herring Clupea pallasi and its early appearance in wild Puget Sound herring

R. M. Kocan1,*, P. Hershberger1, T. Mehl1, N. Elder2, M. Bradley2, D. Wildermuth3, K. Stick3

1School of Fisheries, University of Washington, Box 355100, Seattle, Washington 98195, USA
2USGS-BRD, Marrowstone Island Marine Station, 616 Marrowstone Pt. Rd, Nordland, Washington 98358, USA
3Washington Dept of Fish and Wildlife, 1702-4 Anderson Rd, Mt. Vernon, Washington 98273, USA

ABSTRACT: Laboratory-reared pathogen-free Pacific herring were exposed to pure cultures of Ichthyophonus hoferi, and reproduced the disease seen in naturally infected fish--thus fulfilling Koch's Postulates. Pathogen-free herring used in this study were reared from artificially spawned eggs incubated in filtered, UV-sterilized seawater, eliminating the variables associated with multiple infections, which are common in wild herring. Wild free-ranging herring were captured monthly from June through October by dip net from 'herring balls' located in the northern Puget Sound. I. hoferi infections were identified in these fish soon after metamorphoses, about 4 mo post-hatch. The prevalence increased from 5 to 6% in 0-yr fish to 24% in 1-yr-old fish to 50 to 70% in fish over 2 yr old, with no associated increase in mortality. The route of natural transmission to wild herring was not determined, but carnivorous fish became infected and died when they were experimentally fed tissues infected with the organism. In vitro culture of tissues was the most sensitive method for identifying both clinical and subclinical infections.

KEY WORDS: Ichthyophonus · Pacific herring · Clupea pallasi · Laboratory-reared herring · In vitro

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