Diel vertical migration in Mesocyclops edax: implications for predation rate estimates

Abstract

The diel migration patterns of Mesocyclops edax and its prey in a small lake were followed in two studies separated by approximately one year. Gut contents of the predators were examined and selectivity indices calculated at each depth at 0100 h during 1980. The three principal zooplankton prey found in the guts of M. edax were Keratella, Kellicottia , and Bosmina . The predator and all three major prey species exhibited unique and different diel vertical distribution and migration patterns. The complex nature of the spatio-temporal variation in prey density to which M. edax is exposed, demonstrates the dangers of using selectivity indices without knowledge of the distribution patterns of both predator and prey. An increase in vertebrate predation pressure from one year to the next is thought to be responsible for an increase in the abundance of small zooplankton species, the disappearance of two out of three of the large zooplankton species, and the onset of a pronounced nocturnal migration pattern in the third large species.