Herring Disease Program

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Using an approach that involves a combination of field- and laboratory-based studies, we are investigating fish health factors that may be contributing to the failed recovery of Pacific herring populations in Prince William Sound, AK.  Field studies are providing infection and disease prevalence data to inform the population models, serological data that indicate the prior exposure history and future susceptibility of herring to VHS, and diet information that provides insights into the unusually high prevalence of Ichthyophonus that occurs in juvenile herring from Cordova Harbor.  Laboratory studies are validating a newly-developed plaque neutralization assay as a quantifiable measure of herd immunity, providing further understanding of disease cofactors including temperature and salinity, investigating the possibility of an invertebrate host for Ichthyophonus, and assessing the virulence of other endemic pathogens to Pacific herring.  Information from the field and laboratory studies are integrated into an age-structured assessment model, a novel ASA-type model that is based on the immune status of herring age cohorts, and a novel mixture-cure simulation model for VHS.

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Oil Tanker in Prince William Sound

Oil Tanker in Prince William Sound. Credit: Ashley MacKenzie, USGS - Western Fisheries Research Center. (Public domain)

The biomass of adult herring in Prince William Sound (PWS) collapsed from 111,000-121,000 mt in 1988-1989 to 30,000 mt in the early 1990’s; since then, the population has remained depressed, fluctuating between 10,800-32,500 mt. Consequently, the PWS herring population is currently classified as an “injured resource” that is “not recovering” and commercial herring fisheries have remained severely curtailed or closed. In addition to the human economic impacts of the population decline, the prolonged ecological impacts were devastating. 

Definitive cause(s) of the herring population decline and failed recovery in PWS remain undetermined; however, a leading hypothesis involves chronic and epizootic mortality that result from infectious and / or parasitic diseases. In 1993 only 20% of the anticipated adult herring biomass appeared in the known spawning areas. Returning fish were lethargic and demonstrated external hemorrhages consistent with viral hemorrhagic septicemia (VHS).  The etiological agent, VHS virus (VHSV), was later isolated from moribund individuals.  Ichthyophonus, and ENV also occur in PWS herring and have been associated with epizootics in populations of wild herring. Alternative and complementary hypotheses accounting for the herring population dynamics include competition with pink salmon for limited resources and predation on herring populations by humpback whales and other predators.

A better understanding of the epidemiological principles governing herring diseases in PWS is necessary for the development of adaptive management strategies intended to mitigate the effects of diseases to wild herring populations. In an effort to document changes in pathogen prevalence and severity within the PWS herring population, surveillance efforts were initiated in 1994 and heve continued as the Herring Disease Program from 2007 – present. The incorporation of laboratory-based manipulations and observations in the HDP has led to the realization that some of our prior assumptions of these diseases were incorrect.  For example, in a typical herring population, the prevalence of VHSV generally falls below the realistic detection threshold obtained from 60-fish subsamples of a population.  Even though the endemic prevalence is typically extremely low, an epizootic can occur very quickly as a result of changing host and environmental conditions.  As such, the incorporation of VHSV prevalence data into the ASA model as a forecaster of future disease potential is inconsequential from an epidemiological perspective.  For this reason, we have developed a serological assay that is capable of determining whether herring have survived previous exposure to VHSV.  This knowledge is extremely important from a disease forecasting perspective because survivors of prior VHSV exposure remain refractory to the disease for a very long time; presumably for life.  We are beginning to apply the novel PNT assay to the level of herring populations by proceeding with a series of validation experiments intended to determine the temporal and geographic scales of serological sampling that are required to assess population herd immunity against VHS.  It is anticipated that this approach can be used to assess the future potential for VHS impacts in wild herring populations - a critical piece of information that will be useful for assigning the amount of disease risk associated with opening certain herring fisheries.

These and other required updates and possible changes to our modeling approaches will be assessed by working closely with the ASA and other modelers to begin to develop disease-based models that are more built upon biologically-relevant disease principles.  For example, we are continuing to provide infection and disease data for the current ASA model, providing herring antibody results to for a novel VHSV-based ASA-type model, and providing other VHSV epizootiological data for a mixture cure-type simulation model for VHS in herring.

Although Ichthyophonus is one of the most significant parasites of wild marine fishes, causing recurring population-level impacts during the past century, very little in known about its natural life cycle.  From a disease forecasting perspective, the most important information gap involves unresolved routes of exposure and transmission to planktivorous fishes.  Laboratory studies indicate that the parasite is not readily transmitted from herring-to-herring via direct contact or through the water.  Recently, we have successfully established infections in herring by habituating them to the consumption of large quantities of infected fish tissues; however, the relevance of this exposure route to wild populations of Pacific herring remains questionable, as herring are generally considered planktivores.  These and other results have resulted in the elevation of a hypothesis that an invertebrate, intermediate host may be involved in completing the Ichthyophonus life cycle.  However, until recently, appropriate scientific tools did not exist for examining the possibility of an Ichthyophonus intermediate host.  Recent work performed in the Herring Disease Program was successful in developing novel tools (quantitative PCR and chromogenic in situ hybridization) that will be useful for assessing wild zooplankters as intermediate hosts for Ichthyophonus

Finally, we are continuing to advance our understanding of basic epizootiological principles that govern the primary diseases of herring by continuing to employ specific pathogen-free (SPF) laboratory animals in controlled laboratory experiments.  For example, we will further evaluate the importance of temperature as a VHS perpetuation cofactor factor and we will evaluate the susceptibility of herring to Vibrio spp. – likely the most prevalent bacterial pathogens of marine fishes in the world.

Partners:

The Herring Disease Program is a component of the Herring Research and Monitoring (HRM) Program, a multidisciplinary effort funded by the Exxon Valdez Oil Spill Trustee Council that is investigating possible factors impacting recovery of herring.  Partners within the HRM Program are evaluating the effects of food web disruptions, bioenergetic constraints, movement patterns, and predation on the failure of the herring populations to recover.