Diel vertical migration in the Lake Superior pelagic community. I. Changes in vertical migration of coregonids in response to varying predation risk

The distribution of fishes is influenced by a host of physico-chemical and biological variables, including temperature and oxygen, prey abundance, feeding or assimilation rates, and predation risk. We used hydroacoustics and midwater trawls to measure the vertical distribution of pelagic fishes during a series of research cruises on Lake Superior's western arm in 2001 and 2004. Our objective was to assess vertical structuring in the fish assemblage over varying light levels. We observed variability in vertical structuring of both ciscoes (Coregonus spp.) and their primary predator, the siscowet (Salvelinus namaycush siscowet). Our results indicate that deepwater predators and prey migrate extensively over a diel cycle. This migration pattern is most consistent with changes in the distribution of prey resources for siscowet and diel variability in predation risk controlled by changing light levels for ciscoes. The magnitude of vertical migration in ciscoes increased with higher abundance of siscowets, supporting predation risk as a driver of cisco distribution. This study describes the extent of vertical migration in each group of fish, provides a statistical description of the pattern, and discusses the implications for trophic interactions in the Lake Superior food web.