The population of the world's smallest and rarest dolphins has dropped by half in the past seven years to an estimated 55 individuals, according to research released March 13 by the New Zealand Department of Conservation (DOC), the University of Auckland and Oregon State University. The critically endangered Maui's dolphins (Cephalorhynchus hectori maui), which can only be found off a small stretch of the west coast of New Zealand's North Island, were last surveyed in 2004, when the count was placed at 111 dolphins. Large, fixed gill nets set into place with anchors remain the main threat to the marine mammals, which are only protected in a portion of their range. A net was blamed for the death of one of the rare dolphins in February.

Maui's dolphins are a subspecies of the endangered Hector's dolphin (Cephalorhynchus hectori), which are also found around coastal New Zealand. Maui's dolphins are approximately 1.7 meters long and weigh about 50 kilograms. By comparison, the common bottlenose dolphin (Tursiops truncatus) grows up to four meters in length and weighs up to 650 kilograms. The Maui's live about 20 years, but only reach sexual maturity between seven and nine years of age; they can only calf once every three years, giving them a very slow reproduction rate.

The study, which surveyed the dolphins in February and March 2010 and during the same period in 2011, analyzed DNA samples collected via dart biopsies to estimate the population for Maui's dolphins over one year of age. Of the 39 Maui's dolphins sampled, 23 were female. The team also encountered two female Hector's dolphins during their survey—a never-before-observed occurrence, because their habitats do not overlap. There is no evidence at this time of Hector's and Maui's interbreeding.

The World Wildlife Fund, which was not involved in the study, has called for additional protections for Maui's, including a ban on the use of set nets throughout the range of both the Hector's and Maui's dolphins. Set nets are already banned in the main portion of the Maui's habitat. "The Maui's population has been declining since the 1970s, and protection measures introduced in 2008 have not succeeded in turning the situation around," said WWF New Zealand marine program manager Rebecca Bird in a prepared statement. "It is a national tragedy that our critically endangered dolphins are still dying needlessly in fishing nets. We need to act immediately to get nets out of the water, including harbors and estuaries, to protect these dolphins throughout their range."

In a prepared statement, the New Zealand government said it is considering additional protective measures that would extend the ban on set nets along a larger stretch of the North Island coast and extend the existing West Coast North Island Marine Mammal Sanctuary out 22 kilometers from the coast into the ocean. But Minister for Primary Industries David Carter also said the needs of the local fishing community must be considered.

Local fishermen were not pleased by the idea of expanding the net ban area, saying it would put them out of business. Egmont Seafood Managing Director Keith Mawson told New Zealand's 3 News, "There is a real possibility that the population will become extinct whether they extend the ban or not."

Photo: New Zealand Department of Conservation