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Habitat utilization by sympatric arctic charr {Salvelinus atpinus (L.)) and brown trout (Salmo trutta L.) in Lake Atnsj0, south-east Norway SUMMARY. 1, Habitat utilization, as well as inter-and intraspeeific relations of different size groups of aretie charr (Salvelituis alpimts (L.)) and brown trout {Stilmo tnitta L.) in Lake Atnsjo. south-east Norway, were investigated by analysing food and spatial niches from monthly benthic and pelagic gillnet catches during June-October 1985, 2. Small individuals (150-230 mm) of both arctic charr and brown trout occurred in shallow benthic habitats. However, they were spatially segregated as arctic charr dominated at depths of 5-15 m and brown trout at depths of 0-5 m.3. Larger (>23() mm) arctic charr and brown trout coexisted in the pelagic zone. Both species occurred mainly in the uppermost 2-3 m of the pelagic, except in August, when arctic charr occurred at high densities throughout the 0-12 m depth interval. On this occasion, arctic charr were segregated in depth according to size, with significantly larger fish in the top 6 m. This was probably due to increased intraspeeific competition for food.4. The two species differed in food choice in both habitats, Arctic charr fed almost exclusively on zooplankton. whereas brown trout hud a more variable diet, consisting of surface insects, zooplankton, aquatic insects and fish.5. The data suggest that the uppermost pelagic was the more favourable habitat for both species. Large individuals having high soeial position occupied this habitat, whereas small individuals lived in benthic habitat where they were less vulnerable to agonistic behaviour from larger individuals and less exposed to predators. The more aggressive and dominant brown trout occupied the more rewardingpart of the benthic habitat.

Growth, mortality, moulting rate and cheliped loss were examined in aquaculture experiments with noble crayfish, Astacus astacus L., juveniles from four different Norwegian populations in the period September 1987‐June 1988. The juveniles were hatched indoors in May 1987 from parents wild‐caught in 1986. The experiments were performed at 18‐19°C in communal rearing units. Mean total length at experiment start was about 20mm in all groups. There were no significant differences in mean size (final mean total length: 32‐2‐34‐8mm). mortality (83a‐900%) or cheliped loss (429‐700%) among juveniles from the different populations when exposed to similar conditions (food: dry pellets; photoperiod: L:D = 8:16). Among crayfish fed fish and potatoes in addition to dry pellets, and with continuous light conditions, mortality was reduced significantly to about 70%. Continuous light conditions significantly increased mean size (final mean length: 36‐1‐389mm). Moulting occurred from November throughout the experiment period. Moulting rate was positively correlated to mortality and growth rate (i.e. mean length increase). A positive correlation between mortality and cheliped loss, along with direct observations of cannibalism, indicated that agonistic behaviour and cannibalism were the dominating mortality factors. It is suggested that continuous light conditions reduce agonistic behaviour and thereby decrease mortality.

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