Use of space and food by resident and migrant brown trout,Salmo trutta

Abstract

Parr and resident forms of brown trout,Salmo trutta, from Vangsvatnet Lake, Norway live in freshwater, while migrant forms live in coastal waters during summer and in freshwater during winter. About 80% of parr and residents live at depths <5 m, smolts and migrants are more confined to near-surface water. Brown trout partly segregate by size, age and sex from spring through autumn. More than 90% of parr and residents in the tributaries are 0–2 years old, 2–14 cm in length, in the littoral zone 0–3 years old, 7–24 cm in length, and in the pelagic zone 2–6 years old, 18–32 cm in length. The mean body length of equal-aged fish increases from tributaries through littoral to pelagic zones in the lake. Smolts (2–7 years, 14–29 cm) leave the lake from April through August and return during September–October. Migrants (2–11 years, 23–67 cm) leave the take in April–May and return during August–September; sexually mature fish return earlier than immatures. Female brown trout are less stream-dwelling, but more migrant and pelagic than males. Most individuals in the lake population spend the winter in the littoral zone. In the tributaries, diet differs significantly between age-groups of parr; young fish feed on smaller food items than do older fish. In the lake, parr and residents living in the same habitats feed on the same food items. Littoral brown trout feed mainly on insect larvae and chironomid pupae, pelagic brown trout feed on zooplankton and surface insects. Migrants feed little while staying in freshwater, except for matures which feed on young salmonids and surface arthropods during the 2 first months after they had returned from coastal waters. The results are discussed in relation to growth possibilities and mortality risks of the different habitats.