Decline in dusky dolphin population

SONIA BEAL
Last updated 11:43 08/01/2014
Dusky dolphin
Dusky dolphin

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Marlborough Sounds tourism operator Danny Boulton is calling for the removal of mussel farms in Admiralty Bay following a study pointing to the area's declining dusky dolphin population.

Researchers from the United States have found that the dusky dolphin encounter rate, average group size and individual re-sighting rate in Admiralty Bay, near French Pass, declined during the winters of 2011 and 2012 compared with 2005 and 2006.

Texas A&M University PhD candidate Sarah Piwetz, a member of the research team, said the reason behind the change in habitat use was unknown.

"It could be due to natural or other changes in habitat," she said.

Their data had not yet been fully analysed statistically or published in a peer-review scientific journal.

Mr Boulton has lived at French Pass for the past 25 years and has worked with the researchers since the start of their project in 2001. His business was operating before the introduction of mussel farms.

"As aquaculture has increased, the dusky dolphin population has decreased, and that should be cause for concern," Mr Boulton said.

He called on the Marlborough District Council and the aquaculture industry to halt any further applications to establish mussel farms in the area.

The government was placing economic advancement ahead of environmental balance, Mr Boulton said. "The Government can do what they want with regard to their economic target, but at what cost to the environment?"

Admiralty Bay was a historically important foraging habitat for dusky dolphins, according to the research.

The dolphins had a seasonal preference for Admiralty Bay in winter and early spring where they formed co-ordinated groups to trap schooling fish into bait balls.

Some of the dolphins travelled between ecologically diverse habitats: Kaikoura's deep water and Admiralty Bay's partly enclosed shallow bay.

Findings from the research suggested a higher number of dolphins were travelling through Admiralty Bay but the encounter rate, which is the number of groups observed per hour of surveying, was in decline.

Between 2001 and 2006 the encounter rate declined from 7.5 groups an hour to 1.6 groups an hour. This rate further declined to 0.5 groups and hour in the 2011-2012 season.

The average group size of dolphins travelling together had dropped from seven to three.

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Individual re-sighting rates decreased and co-ordinated prey foraging typical of dusky dolphins in Admiralty Bay was rarely observed in 2011-2012.

Potential contributing factors of the decline in dusky dolphins included the climate, human-generated disturbance such as intensive shellfish aquaculture and inter-species competition, the report said.


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Admiralty Bay has long been known for its dolphins, with one dolphin in particular bringing the bay worldwide attention – Pelorus Jack.

The rare risso's dolphin gained fame in the late 1800s for meeting and guiding ships from Pelorus Sound to treacherous French Pass. It continued to do this for 24 years until 1912.

Pelorus Jack attracted many tourists to the area including American writer Mark Twain, The Encyclopedia of New Zealand records, and is possibly the first individual sea creature protected by law in any country.

- The Marlborough Express

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