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Hector's
Dolphins at Porpoise Bay
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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
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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.
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Declining
numbers of Hector's dolphins around NZ
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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
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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.
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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.
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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.
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