A common environment experiment was conducted to assess the magnitude of the difference in growth, consumption,

activity rate, and spatial and temporal patterns of habitat use between morphologically different populations of

Arctic char (Salvelinus alpinus) originating from two Norwegian lakes. These two lakes contrasted sharply in terms of surface

area, depth, elevation, length of the winter period, and fish community structure (presence–absence of brown trout,

Salmo trutta). The experimental framework encompassed four littoral enclosures (average volume, 146 m3) stocked with

char from either of the two populations with duplicated treatments. Char morphology was quantified with numerical image

analysis, food consumption was estimated using caesium analysis (133Cs), and activity cost and patterns were determined

using video cameras. Char populations were morphologically distinct and reacted differently in growth (1.9-fold difference

between populations), food consumption (3-fold difference), and spatial activity patterns (20-fold difference) to the conditions prevailing in the enclosures. The results highlight that functional differences between morphologically distinct char

may drive important differences in their bioenergetic and behavioural responses when exposed to similar environmental

conditions. Such functional differences should be incorporated when developing habitat or trophic cascade models.