Low mitochondrial genetic diversity in the Indian Ocean humpback dolphin Sousa plumbea in South African waters

The Indian Ocean humpback dolphin Sousa plumbea has been described as South Africa’s most endangered marine mammal due to its low abundance, reliance on coastal habitats with increasing anthropogenic threats and high rates of mortality from bycatch in bather protection nets (BPNs). Although the species has been well studied in South Africa, only a single study has examined its molecular ecology to date, and its population structure remains poorly understood. However, understanding population structure is vital for the conservation and management of a species. To address these research gaps for S. plumbea in South African waters, we analysed the mitochondrial D-loop of 157 museum skin and tooth samples collected between 1963 and 2017 from across the species’ geographic range in South Africa. Our data show that the humpback dolphin has extremely low mitochondrial diversity (haplotype diversity, HD = 0.47; nucleotide diversity, π = 0.2%) with only 3 haplotypes identified, which is comparable to the Critically Endangered Māui dolphin Cephalorhynchus hectori maui and the Critically Endangered Mekong population of Irrawaddy dolphin Orcaella brevirostris. Mitochondrial genetic diversity has not changed significantly in the last 50 yr, despite the high levels of bycatch in BPNs over this time period. Furthermore, we found no evidence of differentiation between dolphins from the KwaZulu-Natal Coast and the Cape South Coast (Western Cape and Eastern Cape). The extremely low mitochondrial diversity we found adds to the growing body of evidence that the humpback dolphin is becoming increasingly vulnerable and that urgent conservation efforts are required for the survival of the species.

Habitat modeling of Irrawaddy dolphins (Orcaella brevirostris) in the Eastern Gulf of Thailand

AIM

The Irrawaddy dolphin (Orcaella brevirostris) is an endangered cetacean found throughout Southeast Asia. The main threat to this species is human encroachment, led by entanglement in fishing gear. Information on this data-poor species' ecology and habitat use is needed to effectively inform spatial management.

LOCATION

We investigated the habitat of a previously unstudied group of Irrawaddy dolphins in the eastern Gulf of Thailand, between the villages of Laem Klat and Khlong Yai, in Trat Province. This location is important as government groups plan to establish a marine protected area.

METHODS

We carried out boat-based visual line transect surveys with concurrent oceanographic measurements and used hurdle models to evaluate this species' patterns of habitat use in this area.

RESULTS

Depth most strongly predicted dolphin presence, while temperature was a strong predictor of group size. The highest probability of dolphin presence occurred at around 10.0 m with an optimal depth range of 7.50 to 13.05 m. The greatest number of dolphins was predicted at 24.93°C with an optimal range between 24.93 and 25.31°C. Dolphins are most likely to occur in two primary locations, one large region in the center of the study area (11o54'18''N to 11o59'23''N) and a smaller region in the south (11o47'28''N to 11o49'59''N). Protections for this population will likely have the greatest chance of success in these two areas.

MAIN CONCLUSIONS

The results of this work can inform management strategies within the immediate study area by highlighting areas of high habitat use that should be considered for marine spatial planning measures, such as the creation of marine protected areas. Species distribution models for this species in Thailand can also assist conservation planning in other parts of the species' range by expanding our understanding of habitat preferences.

Profiling and Spatial Variation Analysis of Persistent Organic Pollutants in South African Delphinids

The continuous disposal of persistent organic pollutants (POPs) in South Africa (SA) warrants concern about their detrimental effects on humans and wildlife. We surveyed six dolphin species (n = 90) incidentally captured in shark net installations or stranded off the SA east and south coast from 2005 to 2009 to study the POP exposure. Sousa plumbea, an inshore and estuarine species, was found to be the most contaminated by total POPs (21 100 ng g(-1) lw) of all the dolphins off SA, followed by Tursiops aduncus (19 800 ng g(-1) lw), Lagenodelphis hosei (13 600 ng g(-1) lw), and Delphinus capensis (5500 ng g(-1) lw), whereas POP levels in the offshore or pelagic delphinids were much lower. In all delphinids, dominant pollutants were dichlorodiphenyltrichloroethanes (DDTs), which represented more than 60% of the total concentration of total POPs, followed by polychlorinated biphenyls (PCBs, 30%). Concentrations of DDTs in S. plumbea and T. aduncus off SA were among the highest levels reported in delphinids globally. Approximately half of the adult T. aduncus had PCB concentrations above the effect threshold for impairment of immune functions. The concentrations of Mirex and Dieldrin in SA delphinids were higher than those found in species from other regions of the Southern Hemisphere.

On the adaptive benefits of mammal migration

Migration is well developed among mammals, but there has been little attempt to date to review common ecological constraints that may guide the evolution of migration among mammals, nor to consider its prevalence across different taxa. Here we review several alternate hypotheses for the evolution of migration in mammals based on improvements in energetic gain and mate-finding contrasted with reduction in energetic costs or the risk of predation and parasitism. While there are well-documented examples of each across the order Mammalia, the available evidence to date most strongly supports the energy gain and predation risk hypotheses in the terrestrial realm, whereas a combined strategy of reducing energetic costs in one season but improving energetic gain in another season seems to characterize aquatic mammal species, as well as bats. We further discuss behavioral and physiological specialization and provide a taxonomic cross section of mammalian migration.