Hector’s dolphins are the smallest and rarest marine dolphins in the world. They are found only in New Zealand.
There are at least three genetically separate sub-species of Hector’s dolphins – North Island, West Coast South Island and East Coast South Island. There may also be a fourth group, in Southland. These subspecies do not interbreed. In total there are only a few thousand of these dolphins left, most of which are around Banks Peninsula or on the West Coast of the South Island.
Hector’s dolphins stay within a range of about 30 km of coast all their lives, within ten kilometres of the shoreline. They feed on fish that dwell near the seafloor in shallow waters, making frequent short dives to find food.
Hector’s dolphins are similar to the endangered parrot the kakapo, in that they do not breed very often, which causes problems for the species‘ survival. Hector’s dolphins live for up to 20 years, but female dolphins only produce one calf every 2-4 years and do not start breeding until they are seven to nine years old. This slow rate of reproduction makes Hector’s dolphin populations particularly vulnerable to deaths caused by human activities such as fishing.
The world’s smallest marine dolphin
While common dolphins reach about 2.6 metres in length, Hector’s dolphins grow to a diminutive 1.4-1.6 metres, and weigh about 50 kilograms. Apart from their size, Hector’s dolphins are identifiable by their rounded dorsal fin, which is unique among dolphins. They also have distinctive black markings on their fins, tails, flippers and faces. They swim in pairs, or in groups of up to 12.
Declining numbers of Hector’s dolphins around NZ
Maui’s dolphins are now found only between Baylys Beach (near Dargaville) and Mokau (north Taranaki), on the west coast. The other species of Hector’s dolphin are found in the coastal waters on the West Coast, in Te Waewae Bay, and around Banks Peninsula and Southland. The largest population of Hector’s dolphin is on the West Coast, estimated at 5400, making a total of 7300 in the South Island.
In the nineteenth century North Island Hector’s dolphins were spread around the North Island coastline, from Kapiti Island to the Bay of Islands. In the 1970s the majority of strandings (the main indicator of where the dolphins are) were around the lower North Taranaki Bight. But by the 1980s only a small proportion came from this area, and now the dolphins have not been seen there for at least ten years. The population range of the species is clearly getting smaller and smaller.
Hector’s dolphin killers
Gill nets used in commercial and recreational set net fishing are the most significant threat to the survival of Hector’s dolphins. These nets are used to catch fish in coastal waters where Hector’s dolphins live – the dolphins get entangled in these nets and drown. Spotted dogfish (also known as ‚rig‘) are the fishery that is considered likely to have the biggest impact on Hector’s dolphins. In a 1999/2000 survey of twelve commercial fishers who used set nets, a quarter admitted they had caught Hector’s dolphins, while all but one admitted catching other dolphins and seals.
Trawlers are used in the West Coast snapper fishery – these vessels are known to have caught dolphins around Banks Peninsula and on the West Coast of the South Island. At least six dolphins have been reported caught by trawlers.
Pollution could affect the long-term survival of Hector’s dolphins. Pollutants such as PCBs, dioxins and dibenzofurans have been found in the dolphins‘ blubber in higher concentrations than in other dolphins and whales in New Zealand’s waters. If the concentrations of these chemicals in the dolphins increases, their immune systems and fertility could be damaged.