Fig. 1. Plan of the experimental pool (40 mx6 mx2.5 m). Animals could only surface to breathe at the respirometry chamber (RC). The feeding patch (FP) could be placed at 40, 80 or 120 m away from the surface. This set-up allowed us to measure dive duration, `descent' speed (1), bottom time, `ascent' speed (2), oxygen consumption and quantity of prey eaten, for every dive.