Abstract

The study of secretive marine mammals, like narwhals (Monodon monoceros), has been greatly facilitated by the development of autonomous satellite-tracked transmitters. In order to attach these transmitters, the animal to study has to be captured and restrained, which is not an easy task when dealing with large mammals like narwhals. We describe as follows the technique used to capture individuals of this species for the installation of such transmitters. These whales were caught in Tremblay Sound, a narrow inlet in Baffin Island, Northwest Territories. This 45 km-long fjord with maximum depths of up 275 m is known to be explored by large numbers of narwhals at the end of the summer. Three 5×50 m stationary nets, made of 4 mm-diameter green nylon twines forming meshes of 20×20 cm, were mounted in a row close to the surface. The net was set perpendicularly to the shore and was constantly monitored. Between 10 and 23 August 1997, four males, and one female, measuring from 2.4–4.4 m, were caught. When the narwhals were captured, they vigorously struggled and became rapidly entangled into the net. Afterward they were restrained at the surface of the water using three industrial straps secured on two inflatable boats located on each side of the whale. These straps were installed cranially to the pectoral fins, under the abdomen, and around the caudal peduncle. Radio-transmitters weighing from 950–1450 g in air (150–225 g in water) were installed on the tusk of the adult males using steel collars, and on the dorsal ridge of a female and a juvenile male using transcutaneous plastic rods. Skin biopsies were also taken for genetic study. The total procedure lasted 45–60 minutes. The initial response to the capture was strong, but of short duration. All five animals remained relatively calm when removed from the net and supported by the straps. Their breathing remained stable throughout the procedure, and all whales dived for a long period of time when released. Lesions caused by the net were limited to superficial cutaneous lacerations of most likely little significance for animals of this size. Based on these observations we believe that these animals experienced a significant, but acceptable, level of stress during a limited length of time. Fatal captures of non-target species were limited to one ringed seal, and two arctic chars. Several Greenland sharks were also caught in the net but could be released. The method of capture used in this study was therefore proven to be safe for narwhals and did not cause any significant harm to the local fauna.