The parasite Ichthyophonus sp. in Pacific herring from the coastal NE Pacific

Abstract

The protistan parasite Ichthyophonus occurred in populations of Pacific herring Clupea pallasii Valenciennes throughout coastal areas of the NE Pacific, ranging from Puget Sound, WA north to the Gulf of Alaska, AK. Infection prevalence in local Pacific herring stocks varied seasonally and annually, and a general pattern of increasing prevalence with host size and/or age persisted throughout the NE Pacific. An exception to this zoographic pattern occurred among a group of juvenile, age 1+ year Pacific herring from Cordova Harbor, AK in June 2010, which demonstrated an unusually high infection prevalence of 35%. Reasons for this anomaly were hypothesized to involve anthropogenic influences that resulted in locally elevated infection pressures. Interannual declines in infection prevalence from some populations (e.g. Lower Cook Inlet, AK; from 20-32% in 2007 to 0-3% during 2009-13) or from the largest size cohorts of other populations (e.g. Sitka Sound, AK; from 62.5% in 2007 to 19.6% in 2013) were likely a reflection of selective mortality among the infected cohorts. All available information for Ichthyophonus in the NE Pacific, including broad geographic range, low host specificity and presence in archived Pacific herring tissue samples dating to the 1980s, indicate a long-standing host-pathogen relationship.

Evaluating signals of oil spill impacts, climate, and species interactions in Pacific herring and Pacific salmon populations in Prince William Sound and Copper River, Alaska

“…For example, we did not include covariates that only existed for portions of the time series, such as disease. Disease has been proposed as one mechanism for explaining declines in herring abundance in PWS [98–100]. The PWS herring disease data (1994–present) starts after EVOS and other climatic perturbations and therefore cannot be used to assess the decline of herring during 1992–93.…”

Infected Donor Biomass and Active Feeding Increase Waterborne Transmission of Ichthyophonus sp. to Rainbow Trout Sentinels

“…Because demersal piscivorous species have limited latitudinal migrations, are generally long-lived, and once infected do not readily clear their infection (van Duijn 1956;Hershberger et al 2016), they would be capable of harboring and releasing an infectious stage of Ichthyophonus into the water, as we demonstrated here, thus making them potential reservoirs of infection for susceptible planktivores repopulating an area. Observations of Atlantic Herring C. harengus support this hypothesis; i.e., "[n]egative findings (i.e.…”

“…First, when present in a population of schooling planktivores, Ichthyophonus utilizes a dispersal mechanism characterized by the release of infectious cells into the water column, which are then consumed by susceptible individuals. As the population ages, infection prevalence increases (Rahimian and Thulin 1996;Hershberger et al 2002Hershberger et al , 2016, thus producing a pool of older fish serving as a source of infection for new recruits; as older fish die off the younger fish become the source of new infections within the population. This mechanism enables the parasite to be continuously maintained within a schooling population for multiple generations.…”

“…The relationship between infected biomass and risk of infection in susceptible populations offers insight into the possible cause of fluctuating infection prevalence and periodic epizootics in wild populations. Most of the available data on epizootics in planktivores comes from herring surveys, most of which show that Ichthyophonus infection prevalence is low, generally <5%, in age-1 herring, and that as the population ages, prevalence increases to >70%, then dramatically drops off in the oldest age-classes (Sindermann and Chenoweth 1993;Rahimian and Thulin 1996;Marty et al 1998;Kocan et al 1999;Hershberger et al 2002Hershberger et al , 2016. Sindermann and Chenoweth (1993) first proposed that an increase in infection prevalence and the periodic case-related mass die-offs of herring could result from an increase in infected reservoir biomass, resulting in the release of increasing numbers of infective cells into the surrounding water.…”

“…Although planktivores such as herring often exhibit high prevalences of infection (Fish 1934;Sindermann and Scattergood 1954;Kocan et al 1999;Hershberger et al 2002Hershberger et al , 2016Marty et al 2003), conclusive evidence for the mechanism of transmission has been elusive. Some authors have proposed that waterborne infective cells released from epidermal ulcers present on infected herring are responsible (Daniel 1933;Sindermann and Scattergood 1954).…”