Results of the Grays Harbor Coho Survival Investigations 1987-1990

Wild and hatchery coho smolts emigrating through the Chehalis Basin and its estuary have consistently survived at lower rates than coho originating from other coastal watersheds. This survival difference is important since 80 to 100 thousand coho plus additional chinook and steelhead could be added to annual coastal catches if it were corrected. In addition, the low abundance of wild coho from the Chehalis River has often put constraints on ocean fisheries.

From 1987 through 1990, scientists from the Washington Department of Fisheries, National Marine Fisheries Service, Oregon Cooperative Fishery Research Unit-Oregon State University, University of Washington, U.S. Fish and Wildlife Service, Washington Department of Ecology, and U.S. Environmental Protection Agency investigated where, geographically, fish from the Chehalis watershed were being impacted. A secondary objective was to identify the factors that were stressing or directly killing the fish. The approach used was to compare coho juveniles sampled throughout the Chehalis River and its estuary (the inner harbor) with fish that had been collected from the Humptulips River and North Bay where survival is considered normal.

Four possible explanations for the survival problem were investigated. These were: 1) that freshwater rearing areas in the Chehalis and Humptulips watersheds possess environmental characteristics that produce coho with dissimilar abilities to survive in seawater, i.e. to complete smoltification; 2) that pathogens or parasites present in the Chehalis Basin or its estuary directly induce mortalities or interfere with smoltification; 3) that the eventual death of any smolting coho that emigrate through the Chehalis River estuary is caused by chronic physiological stress and reduced immunocompetence initiated by poor water quality in the inner harbor; and 40 that predators in the Chehalis Basin and estuary cause some or all of the observed losses of coho salmon.