The study, led by Oregon State University (OSU)’s Marine Mammal Institute, is significant because there are several oil and gas rigs in the South Taranaki Bight (STB) between the islands, and the New Zealand government recently issued its first permit for ironsand-mining of the seabed.

The industry has until now claimed that the blue whales were migratory rather than resident.

“We had five hydrophones deployed for two years in the STB and we never heard any Australian blue whale calls – just the local New Zealand population,” said Leigh Torres of the OSU, a co-author on the study.

The whales were said to show a high level of residency, with their calls recorded on 99.7% of days in 2016. “When we conducted biopsies of individual whales, we also discovered that they are genetically distinct from other blue whale populations.”

In 2013, Torres wrote a paper suggesting that blue whales might favour the region because of its steady supply of krill, but said that the industry and even other scientists had resisted this idea because the whale was listed as a “migrant” under the New Zealand Threat Classification System.

The following year she led a research expedition that identified some 50 blue whales foraging in the area, but could not prove at the time that they were resident rather than migrating from Australia or elsewhere.

Longer surveys in 2016 and 2017 employed biopsy darts to determine the whales’ genetics and photo ID comparisons with whales from other regions as well as deployment of the hydrophones.

A total of 151 individual New Zealand blue whales were identified, leading the researchers to believe that the population numbers more than 700.

Blue whales found off New Zealand, Australia and Chile are smaller than those of the Antarctic, which can grow as long as 30m and are believed to be the largest animals ever to have existed. Blue whales such as those in New Zealand grow to a maximum of 22m.

Torres said that the OSU researchers were now “working closely with resource managers in New Zealand... so they can apply best management practices to minimise impacts from industry.”

They are due to meet political leaders and industry representatives in July, and will also present their findings to the International Whaling Commission.

Their study is published in the journal Endangered Species Research.

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