Infectious and parasitic diseases are suspected of limiting the recovery of Pacific herring in Prince William Sound (PWS). They are also likely to be the cause of rapid declines in the herring population as diseases have been associated with massive fish kills in herring and other forage fish. Paul Hershberger and Maureen Purcell (USGS) are leading a team of researchers in exploring the prevalence and intensity of diseases and how they are transmitted in PWS herring. Of particular interest are viral hemorrhagic septicemia virus (VHSV) and Ichthyophonus parasite.
VHS is extremely harmful to herring, often causing mortality within several days of exposure to the virus. Herring undergoing active VHS disease are often lethargic and demonstrate external hemorrhages around the eyes, mouth, and fins. 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 massive disease outbreaks in Atlantic herring populations and currently occurs in high prevalence among Pacific herring. Ichthyophonus can kill herring directly or cause chronic infections that result in fish with decreased condition, decrease swimming performance, and decreased ability to avoid predators. It has been shown that the prevalence of the parasite increases with age of the 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?
New techniques for detecting antibodies formed in response to exposure to VHS virus is providing us with a better ability to detect the effects of this disease that can spread rapidly through the herring population. Outbreaks of the disease may have been missed by the prevalence sampling in the past. We have also demonstrated pathways for exposure to Ichthyophonus. In 2018, overall Ichthyophonus infection prevalence in Prince William Sound was 13% (24/179) while in Sitka Sound prevalence was 21% (38/177). These numbers are consistent with previous years. Our continued work will focus on evaluating how the available disease information can be applied for determining the number of herring killed during disease events. We will also be examining the transmission of Ichthyophonus. The intent is to learn how best to incorporate the disease information in the numerical models used to predict herring populations.