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.

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Patterns of die1 food selection in pelagic Arctic charr, Saluelims alpinus (L.) and brown trout, Salmo trutta L. were investigated in Lake Atnsjs, SE Norway, by gillnet sampling during JulySeptember 1985. Arctic charr feed almost exclusively on zooplankton both day and night, while brown trout had a diurnal shift in diet. For this species zooplankton made up a considerable part of the diet in the daytime, while at night the diet consisted mainly of surface insect and chironomid pupae. Both species had a selective feeding mode on zooplankton during the day and night. Arctic charr had a higher gill raker number and a denser gill raker spacing compared with brown trout. Still, the differences in prey size between the two species were small. We argue that the observed differences in food selection between Arctic charr and brown trout can be explained by differing abilities to detect food items under low light conditions.

Individual variation in reproductive investment is important for recruitment and population dynamics in fish populations. Thus, it is crucial to understand how these decisions are influenced by environmental conditions. Here, the influence of growth variation during gonadogenesis or early life on reproductive investment in vendace, Coregonus albula (L.) was investigated. A set of cohorts of mature females were sampled over two different years with contrasting climate conditions and food levels, using total length at age 1 year as a proxy for juvenile growth. Relative reproductive investment and individual egg mass were lower in the favourable year (elevated temperatures and zooplankton densities) when the females were in better condition compared with a less favourable year (lower temperatures and zooplankton densities). Female vendace demonstrated plasticity in their allocation to relative reproductive investment and egg mass, probably induced by diverging environmental conditions. This phenotypic response to growth may have strong effects on recruitment and population growth rate.

Native and native-stocked brown trout (Salmo trutta) in Lake Tesse, a regulated hydroelectric reservoir (southern Norway), were spatially segregated according to size: small individuals occurred mainly in the epibenthic habitat and larger individuals mainly in the pelagic habitat. In contrast, all size groups of non-native stocked brown trout were mostly restricted to the epibenthic habitat. Age-specific lengths were generally larger for non-native than for native stocked trout, which were larger than native fish. However, growth rate between age 3 and 4 was significantly lower for non-native stocked fish than for native and native stocked fish. Differences in body length were mainly due to strain but also to some extent to habitat. Native fish had significantly fuller stomachs in the pelagic than in the epibenthic habitat in summer. Epibenthic non-native fish had significantly fuller stomachs than native and native stocked fish in August but not in July. Native and native stocked fish fed mainly on surface insects and planktonic crustaceans in both habitats. We hypothesize that the non-native brown trout stocked in Lake Tesse do not use the pelagic habitat in the home lake and are therefore less adapted to utilize such habitat than populations originating from lakes where pelagic habitat is available.