Effects of Nanophyetus salmincola on the Health and Survival of Puget Sound Steelhead

Science Center Objects

Low early marine survival rates of Puget Sound steelhead (Oncorhynchus mykiss), the state fish of Washington, have contributed to its dramatic population decline and current listing as a “Threatened Species” under the Endangered Species Act. Determining the cause(s) of this elevated mortality remains a primary objective of the Salish Sea Marine Survival Project, a large US / Canada effort investigating factors affecting salmon survival.  This project contributes to the existing international effort by evaluating the contributions of disease-related mortality in the southern Salish Sea.

The digenean trematode Nanophyetus salmincola has a complex life cycle involving freshwater snails Juga spp. as the first intermediate host; freshwater and anadromous fishes as the second intermediate hosts; and birds and mammals as the definitive hosts. Its geographical range in coastal watersheds from northern California through central Washington is determined primarily by the distribution of the first intermediate snail host. However, the encysted parasite commonly occurs in the tissues of marine-phase salmonids beyond this narrow geographic range as infected juveniles outmigrate into the Pacific Ocean.

Direct and indirect impacts of Nanophyetus to salmonids in the Puget Sound region are increasingly observable.  For example, recent surveys documented 100% infection prevalence in outmigrating wild steelhead from the Nisqually River, with mean parasite loads occurring well-above lethal intensities. Early marine survival and smolt-to-adult return rates are lower among steelhead from Nanophyetus-positive watersheds in southern Puget Sound than among those from Nanophyetus-free watersheds in northern Puget Sound and the northern Washington coast. Unmanageable mortality from Nanophyetus resulted in the closure of at least two Washington Department of Fish and Wildlife (WFDW) salmon enhancement facilities in the southern Puget Sound region.  Further, adaptive disease management strategies are currently implemented other locations to mitigate juvenile steelhead trout and coho salmon mortalities resulting from heavy Nanophyetus infections.

All available data indicate that Nanophyetus infections likely compromise the health and survival of outmigrating steelhead smolts from south Puget Sound; however, the magnitude and mechanisms of these disease impacts remain unresolved.  For example, it is unclear whether the proximate cause of death is from direct disease-related mortality or indirect mortality stemming from parasite-induced reductions in fitness and swimming performance that predispose infected cohorts to predation.  Here, we propose to examine these Nanophyetus effects using an approach that combines field and laboratory investigations. Results from these studies will provide a basal understanding of Nanophyetus epizootiological characteristics that will be used to develop disease mitigation strategies.


In 2013, the Washington Department of Fish and Wildlife (WDFW) and Puget Sound Partnership (PSP) initiated an effort to determine why steelhead are dying in Puget Sound.  A multi-disciplinary, ecosystem-based research approach - the Salish Sea Marine Survival Project (SSMSP) - was formed as a consortium of experts from state and federal agencies, Puget Sound Treaty Tribes, and academic representatives. This project is an integral component of the SSMP.

Long Live the Kings, Sea Doc Society, WDFW, NOAA - Manchester Research Station


Juga sp. snails

Juga sp. snails, the molluscan intermediate host for Nanophyetus salmincola, populate the bottom of a stream bed in a south Puget Sound watershed. Credit: Bonnie Besijn, USGS - Western Fisheries Research Center. (Public domain)

Kidney of a steelhead with heavy N. salmincola infection

Kidney of a steelhead with heavy N. salmincola infection. White spots, occurring primarily in the posterior region of the kidney, represent parasite metacercariae. Credit: Paul Hershberger, USGS - Western Fisheries Research Center. (Public domain)