Trophic triangles and competition among vertebrate (Oncorhynchus nerka, Gasterosteus aculeatus) and macroinvertebrate (Neomysis mercedis) planktivores in Muriel Lake, British Columbia, Canada

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