Temperature and depth preferences of adult sea trout Salmo trutta during the marine migration phase

We tagged 125 sea trout kelts (length: 460-925 mm) in Danish rivers with positively buoyant, depth-and temperature-sensing data storage tags. Eight tags were recovered from fish that had completed a full marine cycle (exit and return to natal river). Mean duration of the postestuary marine cycle was 96.1 d (range: 47-142 d). The trout resided at depths of 0-3 m for 63.8% of the time and exhibited a characteristic diurnal behavioural pattern with repetitive dives deeper than 5.0 m during daytime and residency at the surface during night-time. The number of dives increased with day length, but dive duration was unaffected. Mean dive duration increased with water temperatures from 9.79 min at 5-7 degrees C to 79.8 min at 17-19 degrees C, and mean residence depth increased with water temperatures from 1.95 m at 5-7 degrees C to 10.1 m at 17-19 degrees C. The fish showed a marked response to temperatures above 17 degrees C by residing at greater depths and by discontinuing the characteristic dive/surface residency pattern for prolonged periods of time during warm periods. Temperature data indicated that the fish were generally close to land in the beginning of the marine period and had migrated into open sea during summer. Our results suggest that Danish sea trout kelts aim to optimize their growth at sea by exhibiting a characteristic foraging pattern similar to that of Atlantic salmon and by seeking temperatures within the range reported as optimal for growth in the species.