During 2005-13, at Osoyoos Lake, British Columbia, we investigated trophic relationships among fry of Oncorhynchus nerka Walbaum (Sockeye and kokanee), a suite of limnetic planktivores including Mysis diluviana, and their zooplankton prey.

Our goal was to quantify the impacts that a recently introduced population of Mysis would have on density, growth and survival of resident age-0 Sockeye Salmon.

Evidence of Mysis impact was based on (a) simple correlation analysis between various biophysical performance measures and (b) production and bioenergetics models used to identify the strength of bottom-up (i.e. production-driven) and top-down (i.e. consumption-driven) processes.

This nine-year study indicated that the Osoyoos Lake food web was strongly influenced by external events These included: large annual variations in river discharge, an earthen dam failure and effluent input from an Okanagan River tributary, and highly variable recruitment of O. nerka fry given out-of-basin factors (harvest, marine survival) controlling adult salmon returns. Surprisingly, large annual variations in O. nerka recruitment (0.63 - 7.0 million fry), did not induce significant “top-down” associations in growth, survival or subsequent production among the macro-planktivores (pelagic fish and Mysis) and their zooplankton prey. A single significant correlation (p ≤ 0.05) between O. nerka fry abundance and their von Bertalanffy W_∞ parameter emerged from a set of 14 potential top-down associations tested. By contrast, we identified several strongly positive “bottom-up” effects in which survival of O. nerka fry was significantly associated (p ≤ 0.01) with annual variations in total zooplankton biomass, Daphnia biomass and Epischura biomass. Our results indicate that Mysis played a dual role in the Osoyoos Lake pelagic food web. As predators, they accounted for an average (June-October) of 64% of the total prey biomass consumed by fish and Mysis. As prey, Mysis contributed an average of 35% of the prey biomass consumed by fish. Consumption by fish and Mysis together accounted for daily losses of only 4.5% of non-mysid zooplankton biomass and 34% of daily zooplankton production.

We conclude that in all years, combined prey consumption by Mysis and fish was never high enough, acting alone, to reduce the availability of their potential zooplankton forage base. However, we also estimate that in the absence of Mysis, O. nerka fry could experience a 43% increase in their daily food intake and that fish and Mysis might control their principal prey taxa when exogenous factors (e.g. annual discharge) induced major reductions in zooplankton biomass. Finally, although Mysis has clearly altered the energy flow pathways from plankton to fish in the Osoyoos Lake food-web, mysids have not precluded rebuilding O. nerka abundance to levels at or exceeding historic maxima.