The capture of four Hector's dolphins by commercial fishers has prompted fresh concerns about the lack of camera monitoring on boats.
On Monday, Fisheries New Zealand said it had received two reports from commercial fishers about rare Hector's dolphins caught off the South Island's East Coast this summer.
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One report related to a single dolphin being caught in trawling nets, while three were captured on another occasion.
Manager of Inshore Fisheries, Steve Halley, said while he was happy the commercial fishers reported the captures quickly, it was still "extremely disappointing" to hear of the deaths.
"Hector's dolphins are nationally endangered taonga and we're interested to find out as much as we can about these captures so we can work with the industry to avoid them occurring," said Mr Halley.
But Karen Bair, from Forest & Bird, said the events proved electronic monitoring of boats through cameras was necessary.
"In many fishing industries, people understand the benefit of having cameras, increased transparency, and monitoring of by catch, among other things," she told Newshub.
"If you only have low observer coverage, you can't rely on that to give you a good indication of the number of animals that have been killed."
Ms Blair said the Government's implementation of the cameras was being blocked "by a few rotten apples in the fishing industry".
"These four dolphins were correctly reported by the skippers, but the fact is MPI have no idea what is being pulled up in the nets of hundreds of boats all around our coastline," she said.
"Without observers or cameras on most of these boats, we have no idea how many dolphins are killed every year, and no ability to protect them."
The World Wildlife Fund said the conventional trawling nets were a significant threat to Hector's dolphins, which has an estimated population of 15,000.
"Until we find new, safe ways of trawling that don't put our precious dolphins at risk, we need to remove all dangerous fishing nets, as well as other human threats, from the homes of the most vulnerable Hector's dolphin populations around the South Island and the home of their critically endangered Maui dolphin cousins in the North Island," said WWF's New Zealand senior campaigner David Tong.
In February last year, five Hector's dolphins were killed by commercial set nets, resulting in the Government reviewing the use of set nets.
A review of the Hector's and Maui dolphin Threat Management Plan is currently underway and will be reported back on later this year.