Diel dive depth in penguins in relation to diel vertical migration

Diel vertical migration by zooplankton is thought to allow manne animals to exploit the more abundant food resources in the upper water strata at night while minimizing predation from optically orientating predators. Contrary to this, consistantly shallower dives at night made by marine reptiles, mammals, and birds have been attributed to enhanced feeding conditions as predators exploit prey that have become accessible near the surface. We found foraging success in penguins reduced at night and dive depth limited exclusively by ambient light levels. Consideration of foraging strategies adopted by air-breathing predators may explain typical vertical migration patterns of prey, as well a s departures from them. Diel vertical migrations of zooplankton and necton, in which organisms reside in deeper water strata during the day but rise to the surface by night, have been attributed to efficient utilisation of food resources near the surface (McLaren 1974) while minimizing predation from optically orientating predators (Gliwicz 1986). However, some species, despite being cited as conforming to this pattern, show partial diel vertical migration in that they frequent the upper water strata by night but distribute themselves over a wide variety of depths during the day (Kalinowski & Witek 1980). Mammals, birds and reptiles are major consumers in many marine ecosystems (e.g. Duffy et al. 1987, Huntley et al. 1991) and also show consistent periodicity in depth utilisation, diving deeper during the day than at night (Croxall et al. 1985, Eckert et al. 1988, Kooyman et al. 1992). This pattern is thought to occur because predators track prey throughout their diel vertical migrations (Croxall et al. 1985, 1988, Eckert et al. 1988, Kooyman et al. 1992). Nocturnal feeding in particular is considered to be facilitated as predators do not have to engage in energetically costly deep dives (e.g. Kooyman et al. 1992). This premise is, however, inconsistent with the hypothesis that many pelagic species minimize predation through diel vertical migrations (McLaren 1974,