**The title, authors, and abstract for this completion report are provided below. For a copy of the completion report, please contact the GLFC via e-mail or via telephone at 734-662-3209**
Effect of Exotic Cercopagids on Fish: Food Web Disruption through Density- and Behaviorally-Mediated Effects
Scott Peacor2, Kevin Pangle2, and Katrina Button2
2 Department of Fisheries and Wildlife
Michigan State University
East Lansing MI. 48824
The invasive predatory cercopagid cladocerans, Cercopagis pengoi and Bythotrephes longimanus, are recognized as potential serious threats to Great Lakes fisheries. Cercopagids may disrupt food webs through their effects on their zooplankton prey, which can divert energy away from the early life history stages of fishes, and thus limit the recruitment potential of fishes. Typically ecologists examine the effect predators have on prey through direct reduction of zooplankton abundance (called “density” or “consumptive” effects). But ecologists are becoming increasingly aware of the potential large influence of “nonconsuptive effects” (also “behaviorally mediated effects”) that occur through induced modification of the prey behavior or morphology.
Our field surveys of Lake Michigan and Lake Erie indicated that Daphnia and Bosmina, which are principal zooplankton prey of YOY fish, are much lower in the water column in the presence of Bythotrephes. Laboratory experiments confirmed that these zooplankton prey can sense Bythotrephes kairomones (scent) and respond by swimming to lower depths and increasing diel vertical migration. A series of such laboratory experiments with a suite of different zooplankton prey and different predators indicated that prey respond in relationship to their vulnerability to predators, independent of whether the predator is invasive or not. Therefore the short evolutionary history between the invasive predator and endemic prey does not exclude the potential for large phenotypic responses. Vertical migration to lower depths reduces predation risk because Bythotrephes densities are much lower, likely due to light requirements of Bythotrephes. However, migration by zooplankton prey to the deeper “refuge” comes at a cost in growth and reproduction due to the lower temperatures. Indeed, in laboratory experiments that simulated the temperature gradient in Lake Michigan, Daphnia mendotae grew 36% slower in water columns with Bythotrephes kairomones. We parameterized the effect of direct predation (the “consumptive effect”) of Bythotrephes on Daphnia population growth rate in the field. We used field survey data of the vertical position of the predator and prey, and light-dependent predation rates derived from mesocosm experiments. We further parameterized the cost to population growth rate of the induced vertical migration (the “nonconsumptive effect”). Our laboratory experiments of Daphnia growth rate in water taken from the field showed that changes in water temperature dominated the net effect on Daphnia growth rate relative to changes in encountered resources. Comparison of the two consumptive and nonconsumptive effects showed that the nonconsumptive effect greatly contributes to, and can even dominate, the net effect of the Bythotrephes on Daphnia growth rate. We further examined the indirect effect of Bythotrephes on age-0 alewife growth rate and survival with a fish growth rate model that incorporates the natural prey assemblage, light levels, and temperatures, in Lake Michigan. When only the consumptive effect of Bythotrephes was included in the model, alewife mean length entering winter was 110 mm and over-winter survival was 87 %. However, when the nonconsumptives effect were also included, due to the cost of migration on Daphnia population growth rate and the reduction in overlap between fish and Daphnia resulting from migration, the mean length of alewife entering winter was 72 mm and predicted over-winter survival was 19 %.
Our results therefore indicate that Bythotrephes can have dramatic effects on zooplankton populations through non-consumptive effects, and initial models suggest that the induced effect on zooplankton may contribute strongly to the net indirect effect of Bythotrephes fish recruitment. Therefore, this factor which is typically ignored, may significantly contribute to the net effect of the invasive predator on the system. Our study indicates that to fully understand the effect of this invasive predator, field studies must consider the induced changes in the vertical structure of zooplankton. Importantly, the underlying mechanism examined here is likely pervasive in the Great Lakes; many prey (including fish) respond to predator by modifying their traits. Managers are well aware of such trait changes, but their implications to the food web are not well understood, and implicitly assumed negligible in food web models when not included. This study suggests that non-consumptive effects can contribute significantly to net effects of predators in the Great Lakes food webs and deserve further attention.