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Every year between January and April, humpback whale mothers and their calves can often be seen in Hawai‘i’s warm waters. The adult whales flock to Hawai‘i from Alaska and British Columbia to breed and rear their young. To keep their calves safe, humpback whale mothers usually prefer to stick closer to shore. This lets them avoid sharks, the potentially lethal advances of male humpbacks, and other threats. But as a new study shows, humpback whales’ habitat is being pinched between increasing inshore vessel traffic and the dangers of deeper water.
During the winters of 2005 and 2006, Adam Pack, a whale researcher at the University of Hawai‘i at Hilo, and his colleagues observed the humpback whales off western Maui from a vantage point atop a nearby hill as part of a separate research project. They noted the positions of mother-calf pods and pods without calves (which mostly included lone whales or courting pairs), as well as the locations of whale watching vessels and other craft.
Years later, after more was known about humpback habitat preferences, Pack was interested in revisiting and analyzing this data set. He had expected to see similar behavior to that documented in previous research—that the mother-calf pairs would remain closer to shore than whales without calves. “What we found was the direct opposite, which was confusing, and also kind of interesting from a scientific point of view,” says Pack. For the 161 mother-calf pods Pack and his colleagues observed, the researchers noticed that the whales started the day near the shore and, as the day crept on, moved into significantly deeper waters.
Pack says the whales’ daily commute is likely the consequence of them avoiding non–whale watching vessels such as fishing boats or recreational watercraft. The researchers draw a distinction between whale watching tour boats and other boats because, based on their analysis, the whales’ shift to deeper water was related to the density of non–whale watching boats, which increased during the course of the day. Whale watching boats, they say, were much fewer in number and didn’t have the same effect. The finding deviates from previous research in which vessels were absent.
Pack says that boats can be very noisy, which interferes with whales’ communications and disturbs the mother-calf pods. The study suggests mother-calf pods are being edged into deeper water during the day by boats, and at night, after the boat pressure has let up, are swimming back inshore.
“One of the remarkable things about [adult] humpback whales is that they don’t feed while they’re in their tropical breeding grounds,” explains Alison Craig, a marine mammal researcher at Edinburgh Napier University in Scotland, and one of the study’s coauthors. It is vital for nursing mothers to conserve their energy during this fasting period, she says. “If exposure to too much inshore vessel traffic causes females with calves to head into deeper waters, they’ll be more likely to encounter harassment from males, and this in turn will cause them to use more energy.”
Joe Mobley, a whale researcher at the University of Hawai‘i at Mānoa who was not involved with the study, says it was good that Pack and his team were able to highlight this problem.
“I think the largest problem these animals face is climate change,” says Mobley. “But in the meantime, we control the things that we can control.” It would be relatively feasible, Mobley says, to enact vessel-traffic policies to reduce stress for the humpbacks.
Before considering any policy changes, however, Pack says it would be important to conduct this research in other areas around Hawai‘i to have a better understanding of how pervasive the problem is. He’d also want to conduct the survey again since the data he collected was from 12 years ago and vessel traffic has only increased since then.
Humpback whales were nearly wiped out by commercial whaling that continued into the mid-20th century, and the population that visits Maui “is still very fragile,” Pack says. “It is extremely important to continue to monitor their preferred breeding grounds.”