Abstract

The dorsal fin of odontocetes has, among other roles, hydrodynamic and thermoregulatory functions. In captivity, the dorsal fin sometimes bends laterally. Bending of the dorsal fin was described in 13 captive harbor porpoises and one captive striped dolphin. All had stranded and been given veterinary care in a small indoor treatment pool. After recovery, most of the animals were moved to a large outdoor floating pen for rehabilitation before being released at sea. The degree of bending of the dorsal fins was related to one, or several, of the following situations: (1) sickness, (2) recovery, (3) addition of pool-mates, (4) change in the direction of current in the pool, and (5) the move to the floating pen. In general, fins began to bend soon after stranding when the animals were ill, floating at the water surface and hardly moving in the small pool. Later in the recovery phase, when the animals could dive and had stereotypical swimming patterns, the fin bent even more. The fin often straightened when the social situation changed due to the introduction of pool-mates; the increase in social interactions usually led to irregular swimming patterns. When the direction of the current in the pool was changed, animals often changed their swimming direction. This often coincided with a change in the angles of their dorsal fins. When recovered, healthy animals were moved to the large floating pen to allow them to become fit enough for release. In the pen, their dorsal fins generally straightened. In order to avoid dorsal fin bending in captive cetaceans, we recommend that the direction of the current in pools should be changed often and that environments should be made as stimulating as possible. Bent dorsal fins of captive harbor porpoises can straighten to some degree. Rehabilitated stranded animals should spend some time in a large pool or pen, preferably with conspecific pool-mates, prior to being released.