SPATIAL SEGREGATION BY AGE AND SIZE IN ARCTIC CHARR - A TRADE-OFF BETWEEN FEEDING POSSIBILITY AND RISK OF PREDATION

1. Arctic chaff Salvelinus alpinus underwent phenological and ontogenetic habitat shifts between the epibenthic and pelagic zones in five Norwegian study lakes. Juveniles exploited epibenthic areas until they reached a body length of 13-18 cm when many moved into pelagic waters during summer. 2. Pelagic juvenile Arctic charr had larger age-specific lengths than their epibenthic counterparts, and few juvenile Arctic chaff moved from epibenthic to pelagic areas in the presence of large brown trout, the main potential fish predator present. 3. The mean size of the smallest Arctic chaff in the uppermost 2 Secchi disc units of the pelagic area was positively correlated with the body size of the largest brown trout that were present, and with the water transparency of the lakes. The size of pelagic charr declined significantly with depth in all but one lake. 4. Our results indicate that juvenile Arctic charr demonstrate a trade-off between food demand and presence of predators, and thereby distribute themselves according to food availability and risk of predation.