It is the first robust estimate of the pre-whaling abundance of the animals in the New Zealand region, rather than across the whole of the southern hemisphere.
The researchers used historical records from whaling logbooks, along with modern genetic data to come up with the estimates.
“We find that the population declined rapidly following early 19th century whaling, and came close to extinction in the early 20th century, with less than 20 mature females estimated as surviving the bottleneck,” the researchers said in a study published in Royal Society journal Open Science.
The low of around 110 whales was reached by 1925. Southern right whale numbers are slowly rebuilding and are now estimated to be at 12 per cent of pre-whaling levels.
Southern right whales had been protected from commercial whaling for more than 80 years, but illegal Soviet whaling of 372 whales from the southwest Pacific in the 1960s inevitably delayed recovery, the study said. Based on assumptions made for the study, it was thought the population would not recover to 95 per cent of its carrying capacity for at least 50 years, but that projection was uncertain.
According to the Department of Conservation, southern right whales can reach lengths of up to 18 metres. Whalers considered them the “right” whales to hunt, as they were easy to approach, lived close to shore and provided large amounts of meat, oil and whalebone.
The Maori word for the whales is tohora, and their head and lower jaw are covered with callosities – large, white, rough growths on the skin that are usually infested with parasitic worms, whale lice and barnacles.
Te Ara The Encyclopedia of New Zealand said there were so many right whales in Wellington Harbour during the winter in the 1840s that some settlers complained the whales’ blowing kept them awake at night.
The study, published on Wednesday, said southern right whales were the target of whaling around New Zealand from as early as 1791. Between 35,000 and 41,000 southern right whales were estimated to have been killed in New Zealand waters since 1827.
In the aftermath of hunting, no southern right whales were seen in New Zealand mainland waters between 1928 and 1963. Today, the species appeared to be slowly recolonizing former mainland wintering grounds from areas in the sub-Antarctic where it persisted during whaling.
Southern right whales remain at risk of being hit by boats and ships.
“They are vulnerable to vessels strike and too much attention when near the mainland coast. A few years ago we had a mother-calf pair and the calf was hit by a small boat that left propeller cuts across its back,” Auckland University marine mammal scientist Dr Rochelle Constantine said.
“These whales will often come close to shore and will approach boats so it’s easy to harass them or injure them.”
Increasingly they were being seen around the New Zealand mainland, most often in Southland but every few years some made it up to Northland.
Dr Will Rayment from the Department of Marine Science at Otago University warned that as the population of southern right whales increased they would increasingly be affected by human activities.
Elsewhere impacts on their wellbeing included fishing, shipping and pollution, and the lessons from problems in other parts of the world needed to be heeded if the animals were to recover to anything near their former abundance in New Zealand waters.
The new study was led by Professor Scott Baker from Oregon State University who is also adjunct professor of molecular ecology at Auckland University.
Citation : An integrated approach to historical population assessment of the great whales: case of the New Zealand southern right whale. Jennifer A. Jackson, Emma L. Carroll, Tim D. Smith, Alexandre N. Zerbini, Nathalie J. Patenaude, C. Scott Baker. Royal Society Open Science. Published 16 March 2016. DOI : 10.1098/rsos.150669