Dusky dolphin (Lagenorhynchus obscurus) subgroup distribution: are shallow waters a refuge for nursery groups?

Mammalian young are especially vulnerable to harm because of their small size, poorly developed survival skills, and strong dependence on maternal care. Tactics for rearing young, including avoiding harm, vary largely by environment type. Off Kaikoura, New Zealand, mother-calf pairs of dusky dolphins (Lagenorhynchus obscurus (Gray, 1828)) are often found in small groups with other mother-calf pairs, with calves of roughly the same age. These groups, termed nursery groups, may distance vulnerable calves from hazards associated with males seeking mating opportunities, predation by sharks and killer whales (Orcinus orca (L., 1758)), and boat traffic. We conducted systematic boat-based surveys to determine the relative distribution of dusky dolphin group types, predators, and boats in the area. Nursery groups were encountered in shallow waters (<= 20 m) significantly more often than in deeper waters (p < 0.001). Boats were found more in shallow water, while predators were almost exclusively encountered in deeper water. Other dusky dolphin group types did not show a preference for deep or shallow waters. Potential harassment from males and the presence of boats were both significant predictors of the depth category in which a group was found (p < 0.03). We recommend that managers focus efforts on protecting this vulnerable subgroup of the dusky dolphin population by protecting their preferred nearshore shallow habitat and by reducing recreational boating and fishing in these areas.