An experimental test of the genetic component of the ontogenetic habitat shift in Arctic charr (Salvelinus alpinus)

Fry of the Arctic charr, Salvelinus alpinus, were experimentally stocked in to a small fish-free lake to test the hypothesis that the size-dependent ha bitat shift from the epibenthic to the pelagic habitat is genetically deter mined. The charr originated from a nearby lake inhabiting predatory brown t rout Salmo trutta. The cohort of stocked charr was investigated for three y ears. The Arctic charr started to exploit the pelagic habitat in their firs t summer at a size of 7-9 cm in contrast to about 15 cm in the donor lake. In the next two summers, the pelagic fraction of the cohort increased. The main fraction lived in epibenthic areas, utilizing the same prey as pelagic charr. Water temperature moderated the habitat use of juveniles such that they avoided warm (>16 degrees C) waters and resided in cool, deep areas. T he result was consistent with the hypothesis of a tradeoff between feeding benefit and the predation risk producing spatial segregation of Arctic char r and demonstrated that the fish can facultatively respond to predation ris k and adjust the size at which they migrate to the pelagic zone to feed on zooplankton.