Epizootiology of viral hemorrhagic septicemia virus in Pacific herring from the spawn-on-kelp fishery in Prince William Sound, Alaska, USA

Both the prevalence and tissue titer of viral hemorrhagic septicemia virus (VHSV) increased in Pacific herring Clupea pallasi following their introduc tion into net pens (pounds) used in the closed pound spawn-on-kelp (SOK) fi shery in Prince William Sound, Alaska. VHSV was also found in water samples from inside and outside the SOK pounds after herring had been confined for several days; however, water samples taken near wild free-ranging, spawnin g herring either failed to test positive or tested weakly positive for viru s. Little or no virus was found in tissue samples from free-ranging, spawni ng herring captured from the vicinity of the pounds, nor did the prevalence of VHSV increase following spawning as it did in impounded herring. The da ta indicated that increased prevalences of VHSV were correlated with confin ement of herring for the closed pound SOK fishery and that infection was sp read within the pounds through waterborne exposure to virus particles origi nating from impounded fish. In addition, pounds containing predominantly yo ung fish had higher prevalences of VHSV, suggesting that older fish may be partially immune, perhaps as a result of previous infection with the virus. Operation of SOK pounds during spawning seasons in which young herring pre dominate may amplify the disease and possibly exacerbate the population flu ctuations observed in wild herring stocks.