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The common dolphin is in danger of falling victim to its name and could already be an endangered species, a marine biologist believes.

The Government has released pictures of 22 dolphins killed accidentally in December by fishing trawlers, and urged fishing companies to try and minimise the number of mammals they catch.

However, Massey University marine biologist Karen Stockin said as many as 300 common dolphins could be falling victim to human activities every year.

"With a name like common we assume them to be common ... but this is one of the few native marine mammal species we have that there is no population estimate for," Ms Stockin said.

"At the moment they are classified as being not threatened, but when you go through the literature and you go back to the people that designate that status, they have admitted that on no grounds do they have any data to give them that status."

Common dolphins suffered losses from pollution, inland and offshore fishing and tourism activities, Ms Stockin said.

They should be reclassified as "data-deficient", and a population survey should be carried out, she said.

"Quite frankly, we don't have any evidence to suggest that they are not threatened in these waters.

"In fact, all the impacts we are currently having on them makes me think the other way - that we are having quite a big impact on this population."

Fishing industry groups agreed that the number of common dolphins was unknown, but said they were still plentiful. Seafood Industry Council chief executive Owen Symmans said common dolphins could form schools several thousand strong, and the average number caught was low.

The numbers of dolphins killed by fishing trawlers each year varies, with some years seeing almost no deaths and some seeing as many as 20.

The 22 dolphins killed in December fell victim to four trawlers fishing for jack mackerel off the west coast of the North Island.

The Department of Conservation and Ministry of Fisheries were investigating how the catch happened and how to prevent other fishing-related deaths.

Deepwater Group chief executive George Clement said trawlers catching dolphins was a rare and regrettable occurrence.

"We are in the same space, and it's just very, very unfortunate. We do absolutely everything we can to minimise it, and when you do get a capture like this is regrettable, but it is part and parcel of the business."

Mr Clement said the common dolphin was the most prolific species in New Zealand waters and the most likely to fall foul of human activity.

"Regrettably it has occurred, and regrettably the interactions will continue because we share the same planet."

Green MP Metiria Turei said the death of so many dolphins in one incident was not a one-off, and that 20 of the mammals were killed by a single fishing vessel in 2002.

"The Ministers of Conservation and Fisheries must recognise the extent of the problem and act immediately to protect these animals, instead of offering paltry threats of action at some undefined period in the distant future."