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

Abstract

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.

Two social communities in the Pearl River Estuary population of Indo-Pacific humpback dolphins (Sousa chinensis)

“…South African female S. chinensis are suspected to have increased site fidelity while nursing, likely in more favourable habitat, with males searching for mates among female groups (Karczmarski 1999). A preference for more nearshore habitats by mothers with calves has been noted in other dolphin species (e.g., dusky dolphins (Lagenorhynchus obscurus (Gray, 1828): Weir et al 2008), so between-community associations could be attributed, at least in part, to common habitat preferences by breeding females from both communities. Investigating the role breeding plays in between-community association will depend on the development of reliable sexing methods and determining mother-calf relationships with certainty in future studies.…”

“…Mother-infant bonding is thought to be the evolutionary basis for social attachment in most mammals (Broad et al 2006) because the social environment experienced while young influences social tendencies in adulthood (e.g., nursery groups of bottlenose dolphins: Mann and Smuts 1998; allomaternal care among female sperm whales: Whitehead 1996). Certain habitats may also be safer for calves or may be better suited to supporting the higher energetic demands of nursing mothers, so social patterns may correspond to habitat partitioning by sex or age classes (e.g., Weir et al 2008). As such, we expected that groups with calves might use certain areas of habitat more than others and might associate significantly more often with fishing vessels than groups without calves (Fertl and Leatherwood 1997).…”

Impacts of fisheries on the Critically Endangered humpback dolphin Sousa chinensis population in the eastern Taiwan Strait

“…Females with calves tend to prefer areas of high prey density (e.g. Weir et al 2008), which can result in greater overlap of habitat use by fisheries and mother−calf groups than other individuals. Over the long term, behavioural habituation to feeding in association with fishing activities can result in permanent social structure changes because calves learn foraging behaviours from their mothers (e.g.…”

Parental care behavior in the Guiana dolphin, Sotalia guianensis (Cetacea: Delphinidae), in Ilha Grande Bay, southeastern Brazil

“…In the open and deep waters of the outer bay, offspring may encounter predators and, therefore, risk death. WEIR et al (2008), working with the Dusky dolphin, Lagenorhynchus obscurus (Gray, 1828) in New Zealand, reported that groups with calves preferred shallow waters, a possible strategy for avoiding predators. Moreover, feeding behavior involves the other adults in the group in prey capture.…”