Empirical evaluation of predator-driven diel vertical migration in Lake Superior

Recent studies on Lake Superior suggest that diel vertical migration (DVM) of prey (generalized Coregonus spp.) may be influenced by the density of predatory siscowet (Salvelinus namaycush). We empirically evaluated this hy- pothesis using data from acoustic, midwater trawl, and bottom trawl sampling at eight Lake Superior sites during three seasons in 2005 and a subset of sites in 2006. We expected the larger-bodied cisco (Coregonus artedi) to exhibit a shal- lower DVM compared with the smaller-bodied kiyi (Coregonus kiyi). Although DVM of kiyi and cisco were consistent with expectations of DVM as a size-dependent, predator-mediated process, we found no relationship between siscowet density and the magnitude of DVM of either coregonid. Cisco appear to have a size refuge from siscowet predation. Kiyi and siscowet co-occur in demersal habitat > 150 m during the day, where visual predation is unlikely, suggesting predator avoidance is not a factor in the daytime distribution of kiyi. Seasonal patterns of kiyi DVM were consistent with reported DVM of their primary prey Mysis relicta. Our results suggest that consideration of nonvisual foraging, rather than light- based foraging theory (i.e., the antipredation window), is necessary to understand the processes driving DVM in deepwater systems. Resume´ : Des etudes recentes sur le lac Superieur laissent croire que la migration verticale journaliere (DVM) des proies (les Coregonus spp. en general) est influencee par la densitedu predateur, le touladi siscowet (Salvelinus namaycush). Nous evaluons cette hypothese de maniere empirique a l'aide de donnees provenant d'echantillonnages acoustiques et de prelevements aux chaluts pelagique et benthique dans huit sites au lac Superieur durant trois saisons en 2005 et dans un sous-ensemble de sites en 2006. Nous nous attendions ace que le cisco de lac (Coregonus artedi) de plus grande taille ait