Fraser river sockeye salmon marine survival decline and harmful blooms of Heterosigma akashiwo

Abstract

A two-decade decline of Fraser River sockeye salmon, historically the most valuable west coast Canadian and United States salmon fishery, was linked with blooms of the harmful raphidophyte flagellate Heterosigma akashiwo in the Strait of Georgia (“Strait”), British Columbia. This region has the most intense and prolonged Heterosigma blooms of all B.C. regions analyzed. Marine survival of Chilko stock averaged 2.7% in years when juvenile sockeye salmon seawater migration in the Strait coincided with major Heterosigma blooms versus 10.9% in no or minor bloom index years. Since the mid 1990s, September young-of-the-year (“YOY”) herring abundance was strongly correlated with marine survival rates of Chilko stock Fraser River sockeye salmon and Heterosigma bloom timing in the Strait. Juvenile sockeye salmon and YOY herring co-occur for only six weeks in mid-May through June during initial salmon migration in the Strait. Sockeye salmon marine survival rates were therefore determined in that early period. Fraser River discharge was a bloom-controlling factor with earlier and larger spring and early summer flows linked to major blooms in that period. Heterosigma is a most versatile and allelopathic harmful algal bloom species and may adversely affect sockeye salmon through acute and chronic toxicity or food web impoverishment.