RESOURCE PARTITIONING AND NICHE SHIFT IN ARCTIC CHARR SALVELINUS-ALPINUS AND BROWN TROUT SALMO-TRUTTA

(1) Habitat use and resource utilization of sympatric Arctic charr and brown trout in four lakes and allopatric Arctic charr in three lakes were investigated in central Norway. Fish were sampled both in epibenthic and pelagic habitats simultaneously with sampling of zooplankton resources. (2) Depth distribution of both Arctic charr and brown trout was correlated with Secchi disc transparency. Sympatric brown trout lived mainly in littoral areas down to 1 Secchi disc unit and were even more confined to near-surface waters in the pelagic zone. Sympatric Arctic charr were most abundant in epibenthic areas between 2 and 5 Secchi disc units and between 1 and 2 units in pelagic habitats. However, allopatric charr used chiefly littoral areas down to 2 Secchi disc units, but they also utilized depths down to 8 Secchi disc units. (3) Generally, the food of allopatric and sympatric populations of Arctic charr differed greatly from that of brown trout. Typically the food overlap index calculated was lower than 70%. (4) A comparison between habitat use of allopatric and sympatric charr indicated that Arctic charr were excluded by brown trout from littoral areas during summer. However, this habitat segregation was broken down in November through winter. (5) Habitat and resource utilization between Arctic charr and brown trout was explained by selective differences and asymmetric competition with brown trout as the dominant species in shallow areas from spring through autumn. In late autumn through winter (November-May), Arctic charr may be superior to brown trout because of higher activity at low temperatures.