Lakewide Estimates of Alewife Biomass and Chinook Salmon Abundance and Consumption in Lake Ontario, 1989-2005: Implications for Prey Fish Sustainability

Stocking levels of Chinook salmon Oncorhynchus tshawytscha for Lake Ontario have been highly controversial since the early 1990s, largely because of uncertainties about lakewide abundance and rates of prey consumption. Previous estimates have focused on years before 1995; since then, however, the Lake Ontario ecosystem has undergone substantial changes, and there is new evidence of extensive natural recruitment. Presented here are new abundance estimates of Chinook salmon and alewives Alosa pseudoharengus in Lake Ontario and a reevaluation of the potential risk of alewife population collapse. We found that Lake Ontario has been supporting, on average (1989-2005), 1.83 x 10(6) (range, 1.08 x 10(6) to 3.24 x 10(6)) Chinook salmon of ages 1-4, amounting to a mean annual biomass of 11.33 x 10(3) metric tons (range, 5.83 x 10(3) to 23.04 x 10(3) metric tons). During the same period (1989-2005), the lake supported an alewife biomass of 173.66 x 10(3) metric tons (range, 62.37 x 10(3) to 345.49 x 10(3) metric tons); Chinook salmon of ages 1-4 consumed, on average, 22% (range, 11-44%) of the alewife biomass annually. Because our estimates probably underestimate total consumption and because Chinook salmon are only one of several salmonine species that depend on alewives, predation pressure on the Lake Ontario alewife population may be high enough to raise concerns about long-term stability of this predator-prey system.