One of the highlights of a trip I took to the Kermadec Islands in August was seeing five humpback whales, including a mother and calf, blowing, fluking and rolling in the water. We were lucky, says Rochelle Constantine, a University of Auckland marine biologist who still smiles every time she sees a whale. “We came very close to losing them during the whaling era.”
Humpback whales live in all the world’s seas, migrating between polar summer feeding grounds and winter tropical waters, where they breed. The estimated 4300 humpbacks in the Oceania population breed between New Caledonia and French Polynesia and feed in Antarctica. Although other humpback populations have been recovering well since commercial whaling was banned in 1966, this population is still listed as endangered. Constantine, who is involved in a long-running international project aimed at understanding the Oceania humpbacks, wants to know why they are bouncing back so slowly. Finding out exactly where they’re feeding is a good place to start, she says.
“The polar regions are places of intense seasonal productivity,” says Constantine. In the 24-hour summer sunlight, the phytoplankton blooms off the coast of Antarctica can be seen from space, colouring the otherwise blue ocean a soupy pea-green. Zooplankton feed on the phytoplankton and the exploding population of 2-3cm-long krill is what attracts baleen whales such as the humpbacks – they can swallow several thousand in one giant gulp. “With this massive reliable source of food, they’re able to lay down huge amounts of blubber to get them through the long migration north.” Whales, like us, are mammals, with a body temperature of 37°C.
Although the Southern Ocean is a great source of summer food, once the food becomes scarce, the whales swim to the warmer waters of the tropics where they, and their calves, don’t have to expend so much energy keeping warm. But no one knows where the Oceania humpbacks are going to feed. The key to finding out, says Constantine, is Raoul Island, the northernmost of the Kermadec Islands, about 1000km north of Auckland. “Raoul is the southernmost landmass they pass on their long journey to their Antarctic feeding grounds,” she says. Last October, Department of Conservation staff on Raoul Island counted 126 humpbacks in just four hours. Next October, Constantine plans to use this predictable proximity to collect genetic samples, take fluke identification photographs, and put satellite tags on 30 whales.
By comparing genetic samples and fluke photographs – the underside of each humpback’s tail has a unique pattern of black and white markings – with samples and photographs taken throughout Oceania, Constantine and colleagues will determine where the whales swimming past the Kermadecs have come from. The satellite tags, which are designed to stay on for a few months, will show where the whales are heading. Previous Antarctic surveys have confirmed they are not going to the Ross Sea region directly south of New Zealand. Constantine suspects they’re headed for the Bellingshausen Sea, about halfway between McMurdo Sound and the Antarctic Peninsula. Once she finds out where they are congregating in summer, Constantine will use information gathered by satellites to find out more. “We can gather information about things like chlorophyll a concentrations, sea surface temperature, water depth and proximity to sea ice.”
Comparing this data with that from locations where more successful populations of humpbacks are feeding might give some clues as to why this population is slow to recover. “We can’t have these whales still on the endangered list. Every potential threat – entanglement in fishing gear, ship strike, tourism, habitat disruption – becomes magnified when the numbers are low.”