Fish predators, food availability and diel vertical migration in Daphnia

Diel vertical migration of zooplankton is a highly variableand complex behaviour which apparently cannot be explained byany single factor. We determined the relative importance offish predation, food availability and water depth in shapingthe migratory behaviour of Daphnia. A modified 2x2x2 factorialexperiment provided two levels of fish density (present/absent),food availability (ambient/high) and depth (4–10 m); shallowtreatments with fish were excluded. Triplicate 1.2 m diameterenclosures for each of the six treatments were held in an 18unit array in Peter Lake, Gogebic Co., MI, USA. Repeated measuresANOVA identified significant trends in daphnid density, migrationand fitness (determined by lipid-ovary-egg index, LOE) as wellas in chlorophyll a content of the water column for part ofa 4-week experiment in July 1988. In deep enclosures with fish,Daphnia performed significantly more intense migrations thanin fishless enclosures, save those in fishless ambient-foodenclosures. Daphnia in deep fishless enclosures without abundantfood at depth performed significantly reduced migrations. DaphnidLOE index was significantly influenced only by food content.Our results were consistent with the predator-avoidance hypothesisas well as with observations of greatest migrations where largevertical differences in food abundance exist. They support ahierarchical view of vertical migration, with presence of fishthe primary factor, and food availability the secondary factor.