An area around Banks Peninsula in Canterbury has been declared New Zealand’s first marine mammal sanctuary.
The former Minister of Conservation, Helen Clark, accorded special status to these waters in December 1988 to provide protection for the local population of Hector’s dolphins.
The diminutive Hector’s dolphin is unique to New Zealand and is thought to be the world’s rarest oceanic dolphin. Surveys have put its numbers at fewer than 4000 animals
Averaging just 1.5m in length, Hector’s dolphins are recognisable by their distinctive grey and black markings and rounded dorsal fins. These likeable animals are attracted to any water craft, around which they cavort in small family groups, body surfing and bowriding in the boat’s wake.
Graceful and intelligent though they are, the dolphins are in considerable danger from commercial and recreational setnetting activities. In the past four years at least 223 animals have drowned in setnets in the Canterbury region. This loss represents between a quarter and a third of what is thought to be the dolphin’s largest breeding population.
Given the dolphin’s relatively short lifespan (18 years) and low reproductive rate (females give birth in two or three year intervals from the age of seven to nine upwards), such a drop in numbers poses a serious threat to the dolphin population.
The highest number of deaths occurs during the summer holiday season, when recreational setnetting is at its peak and dolphins move inshore for calving. From November to February the mothers gather in “nursery groups” less than 800m from the shore and there they suckle their young all year.
One six-month-old calf was found dead in a net at Akaroa only metres from the shore. An autopsy revealed tooth marks on its snout, indicating that the frantic mother had struggled to free her calf from the net.
The sanctuary concept was put forward to drastically reduce the chances of entanglement during the critical calving period. Now, with a ban on the use of setnets between November and February in the 1130 sq km protected area, the population may have a chance to recover.
Further restrictions on net size and catch per boat are designed to help build up fish stocks in the area. These had been severely depleted by heavy setnetting over many years.
The plight of the dolphins was only realised recently, by two zoologists studying for their PhD at Canterbury University. Elizabeth Slooten and Steve Dawson began a survey of Hector’s dolphin four years ago with an epic 8400km circumnavigation of New Zealand. They were surprised at the small population size, between 3000 – 4000, but were horrified when they learned of the numbers being lost by drowning.
The problem, Dawson explains, lies in the dolphin’s behaviour when close to shore and in familiar territory. It has been known for some time that dolphins and other cetaceans navigate using a remarkably sensitive inbuilt “sonar”. Initially it was thought that thin nylon fishing nets were invisible to the sonar signals sent out by the dolphins. Now it appears likely that they temporarily “turn off” their sonar in familiar waters; they fail to detect the nets in time and simply blunder into them.
“It’s similar to driving at excessive speed and failing to take a bend,” says Steve Dawson.
Once snagged, the animals are unable to reverse out of the net and are trapped underwater, drowning within minutes. Like all marine mammals, Hector’s dolphins must surface to breathe — even a two-minute dive is exceptionally long.
New Zealand is following the example of several other nations in putting restrictions on setnetting to safeguard marine mammals. A similar ban has been implemented in California, where restrictions have reduced the number of harbour porpoise drownings. In Australia the use of gill nets has been prohibited in all states except Tasmania.
With growing concern about the effects of offshore “wall of death” nets, it is likely that further netting restrictions will surface in the future. Meanwhile, the country’s first marine mammal sanctuary offers new opportunities for tourism in Canterbury. Whale-watching is a multimillion dollar industry in Hawaii and Boston, and the South Island already has one dolphin- and whale-watching company operating from Kaikoura.
With the right promotion, Hector’s dolphins could be a big hit in the South.