Competitive Interactions Between Invertebrates and Fish Mediated by the Vertical Migration of Shared Prey

By inducing vertical migration in prey, predators may indirectly affect competitors through non-consumptive pathways. To understand the importance of these pathways relative to exploitative competition (direct consumption of prey), we evaluated the indirect effects of the invasive invertebrate planktivore, Bythotrephes longimanus, on age-0 alewife (Alosa pseudoharengus) in Lake Michigan’s offshore pelagic community. Using field data, we developed relationships between Bythotrephes density and the fraction of the prey, Daphnia mendatoe, that inhabited the hypolimnion. These relationships, as well as direct predation rates, were incorporated into a model of Bythotrephes effects on the population dynamics and vertical distribution of Daphnia over the course of a season (June 1 - October 31), which was linked to a second model estimating the consumption and growth of age-0 alewife over the same time period. We found that Bythotrephes can strongly impact alewife through non-consumptive pathways by not only diminishing alewife-Daphnia spatial overlap but also by severely reducing Daphnia birth rate due to exposure to cooler water temperature. Further, these effects were greater than effects due to Daphnia density reductions caused by Bythotrephes consumption. Our findings indicate that competitive interactions can be strongly influenced, and even dominated, by the induced vertical migration of shared prey.