A recent study has shown that groups of dolphins in the western Indian Ocean do not mix freely with one another and that dolphin populations are kept separate by currents and other unseen factors. Researchers have found that genetically distinct populations of the Indo-Pacific humpback dolphin may be formed in part by currents, surface temperature differences, and other environmental barriers.

The study was made possible by using both genetic data from dolphins and environmental information from remote-sensing satellites and is a breakthrough in high-tech research on marine wildlife and a foundation for ensuring sound future management decisions on the Indo-Pacific humpback dolphin, a distant relative of the more familiar bottlenose dolphin. It is one of the first examinations of how environmental factors in marine environments can influence population structure in marine species, and can potentially enhance an understanding about the environmental factors that may drive the evolution of new species.

The team started its examination of Indo-Pacific humpback dolphin populations using genetics, analyzing mitochondrial DNA data from more than 90 individual dolphins from the coastal areas of Oman, Mozambique, Madagascar, Tanzania, and South Africa. The scientists used this "genetic marker" to statistically measure gene flow between dolphin groups at different locations. The researchers then compared their molecular findings with 13 years of data from NASA's satellites on environmental factors such as currents, temperature, turbidity, levels of chlorophyll, and dissolved organic matter.

Dr. Mendez and his co-authors have succeeded in finding support for the hypothesis that environmental differences between regions could influence the population structure of marine species. Specifically, they found correlations between regional environmental differences and measurable genetic breaks between populations of dolphins from Mozambique and Tanzania in Africa, and Oman on the Saudi Arabian peninsula. On the largest regional scale, data on currents seem to correlate with genetic distinctiveness between certain populations. In particular, the South Equatorial Current which runs west across the Indian Ocean before diverging north and south as it meets the African continent seems to represent a barrier between genetically distinct populations of Mozambique and Tanzania; the current may play a role in creating them. Seasonal monsoons also potentially contribute to what researchers found was a lack of southbound migration (or detectable gene flow) along the African coast.

The researchers also found agreement on smaller spatial scales. Differences in temperature, chlorophyll, turbidity, and dissolved organic matter between regions also coincided with genetic differences between dolphin populations in Mozambique, Tanzania, and Oman. The two coastal regions without detectable genetic distinctiveness between dolphin populations?Mozambique and South Africa?also lacked significant environmental differentiation between them, a finding in agreement with the correlation of both genetic and environmental differences detected in other areas.

Named for a distinctive hump under the dorsal fin of some individuals, the Indo-Pacific humpback dolphin grows up to 10 feet in length and ranges from dark gray to pink and/or white in color. The species generally inhabits coastal waters, deltas, estuaries, and occurs throughout the Indian Ocean basin to the coasts of Australia. The Indo-Pacific humpback dolphin is listed as "Near Threatened" by the International Union for Conservation of Nature (IUCN) and is threatened by habitat loss, disruption, and fishing activity.