Diving of Otters (Lutra lutra) in a Marine Habitat: Use of Depths by a Single-Prey Loader

Abstract

1. We test the hypotheses that otters Lutra lutra, diving in a marine habitat, prefer shallow water because (a) breathing is more efficient, (b) searching is more efficient, or (c) body heat is lost less rapidly. 2. Otters were observed along the coast in Shetland over a 3-year period, and dives were plotted on a depth-contour map. The duration of the dives and, when possible, the hunting bouts were timed. Success rate was scored. Swimming and diving speed were derived from film-sequences of a captive otter. 3. Otters strongly preferred diving in shallow water 0-3 m deep. The recovery time was linearly related to underwater time. Dive efficiency decreased from 0 to 7 m deep, but tended to increase again at larger depths. Although (in unsuccessful dives) the time spent at the bottom initially increased with depth, searching efficiency decreased with depth because of a rapid increase of the travel time. The maximum duration of hunting bouts was shorter for those in which more deep dives were included. 4. Our data support hypothesis (c), but are not in full agreement with the hypotheses (a) and (b) which were developed for multiple-prey loaders unlike the otter. The otters' preference for shallow water is possibly a result of the lower thermoregulatory costs, and the shorter travel time.