Once upon a time, tens of thousands of right whales congregated along New Zealand’s coast—so many that people in seaside towns complained about how much noise they made.
Today the species is still recovering from a century of intense whaling and remains a rare visitor to the mainland.
The first comprehensive estimate of the population shows the devastation large-scale whaling left in the South Pacific: At the start of the 19th century, there were 28,000 to 47,100 New Zealand southern right whales.
At the start of this century, there were 3,300 to 5,700—just 12 percent of the pre-whaling population.
Southern right whales “seem to be re-colonizing the New Zealand coast from the sub-Antarctic islands, slowly,” said ecologist Jennifer Jackson of the British Antarctic Survey, who led the study, published Tuesday in the journal Royal Society Open Science.
“They used to be so numerous, it’s hard for us to imagine what the ocean would look like if they were back to their earlier numbers,” said Jackson.
The more thorough estimate of the population's ups and downs will, she hopes, help improve recovery and habitat conservation plans for the right whale.
Jackson and her colleagues estimate that after hunting was banned in 1935, the New Zealand right whale population rose slowly, from fewer than 200 individuals toward 1,000 by the late 1950s.
But then illegal Soviet whaling set off another population crash.
Soviet whalers killed more than 3,300 southern right whales in the South Pacific and Antarctic between 1946 and 1987, according to data revealed several years after the fall of the Soviet government.
That number included about 400 New Zealand right whales, taken between 1958 and 1963, Jackson and her colleagues determined.
To assess how many whales there were in 1800, the researchers used logs from American, British, and French whaling ships, along with other historic documents, to develop estimates of how many whales ships encountered and how many were killed.
The American logs in particular contained very detailed information on the number of whales the crews succeeded or failed to land, she said. “For every five whales caught in a sheltered bay, four were brought in, while in open ocean, for every three, one would be lost to the sea.”
Annual government import records listed how many barrels of oil were landed port by port, allowing Jackson and her colleagues to “convert oil back into whales,” she said. “We also converted whale bone imports back into whales.”
With these data, and taking into account the whale’s reproduction rate, they developed two population estimates. The lower number estimates whales in New Zealand waters only, while the larger one encompasses whales in waters off eastern Australia.
“Genetically, the Australian and New Zealand populations are very different,” Jackson said. “They don’t fully mix, but we know they could potentially move long distances.”
A dozen distinct genetic lines of New Zealand southern right whales have survived the whaling era, said Jackson, low but sufficient diversity to restore a healthy population.
But it’s an open question whether the species will ever again number in the tens of thousands amid 21st-century pressures such as warming temperatures, ocean acidification, and overfishing.
“We know the oceans are changing,” Jackson said. “Models that take into account environmental change will be the next thing we’d like to explore.”