Infectious and parasitic diseases are suspected of limiting the recovery of Pacific herring in Prince William Sound (PWS), as diseases have been associated with massive fish kills in herring and other forage fish. Paul Hershberger (USGS - Marrowstone Marine Field Station) is leading a team of researchers to explore the prevalence and intensity of diseases in PWS herring as part of the larger juvenile herring study.
Viral hemorrhagic septicemia, or VHS, is extremely harmful to herring, often causing mortality several days after exposure to the causative virus. Herring undergoing active VHS disease are often lethargic and demonstrate external hemorrhages around the eyes, mouth, and fins. Upon first exposure to the virus, herring are highly susceptible to VHS; however the fraction of individuals that survive the disease develop long-term resistance to subsequent disease outbreaks.
Ichthyophonus is a protist, or single-celled organism, that has caused recurring massive disease outbreaks in Atlantic herring populations and currently occurs in high prevalence among Pacific herring. Ichthyophonus can kill herring directly (often causing 85% mortality within 4 weeks post-exposure) or cause chronic infections that result in fish with decreased condition, decreased swimming performance, and decreased ability to avoid capture by predators. It has also been shown that prevalence of the parasite increases with age of herring.
Through field studies, researchers hope to better understand the prevalence of disease and infections in PWS herring, as well as in Sitka and Puget Sound. Lab studies will shed light on the relationships between the host, pathogens and environmental conditions. For example, can pre-exposure to VHS help anticipate future disease outbreaks? Does change in water temperature change the virulence, or harmfulness, of the pathogens?
Findings for this project will help test current disease forecasting tools as well as develop other tools that can help resource managers predict the timing of disease outbreaks and potentially mitigate the impacts of disease on wild herring populations.