A 14-year study of habitat use and diet of brown trout (Salmo trutta) and Arctic charr (Salvelinus alpinus) in Lake Atnsjøen, a subalpine Norwegian lake

Abstract

We studied the spatial and temporal variation in relative abundance (CPUE), habitat use, and diet of Arctic charr (Salvelinus alpinus (L.)) and brown trout (Salmo trutta L.) in Lake Atnsjøen in southern Norway over a period of 14 years (1985–1998). Fish were sampled with gill nets in epibenthic and pelagic habitats in August each year. Stratified sampling in the epibenthic habitat was carried out with gill net series at three depth zones (0–10, 10–30 and 30–70 m) at one station (1985–1993), and at seven depth zones between 0–3 m and 50–70 m at 10 different stations with multi-mesh gill nets (1994–1998). Lake Atnsjøen is a relatively deep (maximum 80 m), unregulated, subalpine, oligotrophic lake which is not affected to any extent by local or long-range pollution. In shallow epibenthic areas, brown trout is the most abundant species, while Arctic charr dominate deeper epibenthic areas. Epibenthic catches of brown trout at 0–10 m depth were inversely correlated with the catches of Arctic charr. The catches of epibenthic brown trout and Arctic charr did not correlate with mean water temperature in the sampling period. In the pelagic zone, brown trout were mainly caught close to the surface (0–6 m), while Arctic charr were caught between the surface and 12 m depth. Pelagic fish of both species were significantly larger than individuals caught in the epibenthic habitat. The mean size of epibenthic brown trout increased with depth. The opposite trend was evident for Arctic charr, although some larger fish also occupy deeper areas. The size of brown trout in epibenthic areas correlated inversely with CPUE, while no such relationship existed for Arctic charr. The most important food items for Arctic charr in both habitats in August were various species of zooplankton; Daphnia longispina, Bosmina longispina, Bythotrephes longimanus and Polyphemus pediculus. Brown trout mainly foraged near the surface on insects in both habitats, but B. longimanus and P. pediculus were preyed upon to some extent by pelagic specimens. The number of piscivorous individuals was low in both species (1.9–4.5%).