Epizootiology of viral hemorrhagic septicemia in confined Pacific herring

Abstract

Little research has been directed towards understanding disease outbreaks among wild fish and little attention has been placed on fishery-related activities which may be involved in activation and perpetuation of overt disease, among associated fish. Experiments reported in this dissertation were designed to describe the epizootics of viral hemorrhagic septicemia virus (VHSV) which often occur among wild Pacific herring confined in containment structures for the herring spawn-on-kelp fishery. Results indicated that wild herring confined in net pens and/or laboratory tanks underwent increased infections of VHSV, with both viral prevalences and titers increasing after confinement. The temporal occurrence, duration, and magnitude of infection were dependent on numerous factors including age and immune status of confined herring, crowding density, source of viral infection, and viral shedding intensity among VHSV positive herring. Additionally, waterborne virus was found in schools of wild, unimpounded herring displaying low-level infections. A mechanism to explain the epizootics which occurred among confined herring is proposed. This begins with impoundment and subsequent crowding of herring carrying low-level infections. Increased density and stress among the confined fish results in viral shedding by infected animals and increased exposure of other susceptible herring to unnaturally high levels of waterborne VHSV with a subsequent increase in VHSV prevalence among impounded herring. These findings have direct implications for management of the closed pound herring spawn-on-kelp fishery and alternative management options are discussed.