What Lake or Reservoir Characteristics Allow Kokanee to Persist with Introduced Opossum Shrimp and Shared Predators?

When introduced species cause variable impacts on biological communities, identifying the mechanisms driving these impacts can aid prediction and management efforts.  Introductions of the opossum shrimp Mysis diluviana and apex fish predators such as lake trout Salvelinus namaycush have altered hundreds of lake and reservoir ecosystems throughout western North America.  Following these introductions, many kokanee Oncorhynchus nerka and other planktivorous fish populations have declined or become extirpated, but these outcomes have varied widely in severity and timing.  Two indirect trophic mechanisms have been proposed to explain these impacts: Mysis may impact kokanee by reducing densities of shared zooplankton prey (exploitative competition) or enhancing densities of shared predators (apparent competition).  A prevailing hypothesis asserts that exploitative competition explains most Mysis impacts on kokanee.  We propose a new hypothesis linking both trophic mechanisms to lake bathymetry.  Deep-water habitat in lakes and reservoirs may provide Mysis with a refuge from daytime predation by lake trout.  Lake trout populations should therefore be less subsidized by Mysis prey in lakes with more deep refuge space for Mysis, which should in turn weaken predation impacts on kokanee.  In contrast, Mysis should compete with kokanee for zooplankton prey in all lakes regardless of depth.  We synthesize new empirical evidence and literature data from North American lakes and reservoirs to test these two hypotheses.  Comparisons of lake trout diet within and among lakes reveal that lake trout consume less Mysis prey in deeper habitats.  We quantify deep refuge habitat for Mysis using a model of diel vertical migration.  We test whether lake bathymetry is an important mediator of food web dynamics by comparing kokanee population trends among systems.  Finally, we discuss the implications of our findings for the expected effectiveness of alternative kokanee enhancement strategies such as stocking, lake fertilization, and predator removal.