At the start of the experiment, S. alpinus from the holding stock were anaesthetized with tricaine methanesulphonate (MS-222, 0·15 g l−1), weighed and measured, and the left side of each S. alpinus was photographed using a Canon EOS 500D digital camera (www.canon.com) under ambient light kept constant throughout the session. Salvelinus alpinus were classified into one of three different size categories, small (591·9 ± 149·9 g, 322·0 ± 28·1 mm, n = 19), medium (922·1 ± 76·1 g, 376·3 ± 12·3 mm, n = 18) or large (1308·3 ± 256·8 g, 408·9 ± 32·1 mm, n = 23), for the following experiments. Individual S. alpinus were moved to isolation overnight in an aquarium prepared for an open field test. The aquaria were of two different sizes (73 cm × 40 cm × 49 cm, 143 l; and 71 cm × 49·5 cm × 49 cm, 172 l) and supplied with water at ambient temperature from a nearby lake (also used for the holding stock) and the same photoperiod as in holding tanks (10L:14D). Each aquarium was separated into two compartments of equal size; one compartment was covered in black plastic, thus being dark, whereas the other compartment was exposed to light. The two compartments were connected by an opaque screen with a round opening (10 cm diameter) in the middle, covered by a sliding trapdoor. Initially, the S. alpinus were placed in the dark compartment. The second day, the trapdoor was removed and latency to enter the lit area and number of movements between the dark and the lit compartments (activity) was observed for 90 min. The observers were sitting motionless c. 2 m from the aquaria, and took care not to disturb the fish during the behavioural test. On the third day, S. alpinus were exposed to an acute confinement stress test. Salvelinus alpinus were placed in one of the three different confinement aquaria based on size of individuals (27·5 cm × 18 cm × 17 cm, filled with 3 l water for small size, n = 19; 38 cm × 26 cm × 23·5 cm, filled with 4 l water for medium size, n = 18; 42 cm × 31 cm × 25 cm, filled with 5 l water for large size, n = 23). After 30 min, S. alpinus were sacrificed with a blow on the head, and blood was sampled. The blood was subsequently centrifuged, plasma collected and stored at −70° C until further analysis.