We investigated whether pelagic food web dynamics, expressed through a trophic triangle, could influence the potential success of whole-lake fertilization to enhance juvenile sockeye salmon growth. Muriel Lake (145 ha), located on Vancouver Island, was fertilized during 1984 with no apparent effect on juvenile sockeye growth. Unlike most sockeye nursery lakes, Muriel Lake contains a substantial population of the invertebrate zooplanktivore Neomysis mercedis. We hypothesized that competition for zooplankton prey between Neomysis and planktivorous fish (juvenile sockeye, threespine stickleback) could counteract beneficial effects of either natural or fertilizer-induced increases in food for fish. To test this, we assessed (1985–1986) biomass, production, and consumption of planktivorous mysids and fish and then used bioenergetics models to quantify potential competitive interactions. Our analysis suggested that N. mercedis consumed 7–8% of the zooplankton standing stock per day, while planktivorous fish consumed <1.0%·d⁻¹. Although mysids were the main consumers of zooplankton, mysids were also consumed by fish. Late in 1986, an increase in mysid consumption by large, 1 juvenile sockeye precipitated substantial declines in Neomysis biomass. Although this event came too late to reduce mysid competition with sockeye fry in Muriel Lake in 1986, it did highlight the potential importance of trophic triangles in pelagic food webs. We suggest that mysids may be held in check by juvenile sockeye when exogenous recruitment events result in high sockeye biomass. By contrast, recruitment failures and low sockeye biomass promote increases in Neomysis populations, which then control zooplankton communities such that sockeye gain little benefit from either natural or fertilizer-induced increases of zooplankton. For food web ecologists, the implication is that trophic triangles might produce alternate stable states that are mediated by external factors. For fisheries managers, the implication is that lakes containing mysids should only be fertilized when sockeye densities are high or mysid densities are low.