Humans no longer hunt blue whales, but we’ve found a new way to put the endangered cetaceans at risk: plowing into them with our ships. After using satellites to track 171 blue whales that spend time off the west coast of the Americas over a 15-year period, scientists have fingered whale-ship collisions as a possible factor in why blue whale population numbers have remained low despite international protections.
The 171 tracked whales are members of what’s known as the eastern North Pacific population, which comprises about 2500 individuals. (There are about 10,000 blue whales worldwide.) “It’s an amazing and unprecedented sample size; there’s nothing like it for any other species of whale,” says Phillip Clapham, a cetacean biologist at the National Marine Mammal Laboratory in Seattle, Washington, who was not involved in the study.
The study began in 1993 after Bruce Mate, a cetacean biologist at Oregon State University’s Marine Mammal Institute (MMI) in Newport, developed methods for attaching satellite tags to the backs of large baleen whales. The researchers approach a whale in a rigid inflatable boat and then deploy a tag designed to implant in cetacean skin from modified crossbows or compressed air guns, aiming for an area near the animal’s dorsal fin. Of these baleen whales, the blues (Balaenoptera musculus) are the largest; indeed, they’re the largest animal to ever live (even beating out the heftiest dinosaurs), with some of the creatures measuring 30 meters in length and weighing 170 metric tons. Yet despite their great size, little was known about their range or movements.
“Blue whales began showing up off California in the early 1990s,” says Ladd Irvine, a cetacean researcher also at MMI and the lead author of the new study. “But we had no idea of where they came from or where they went to breed, or even their numbers at that time.”
Mate, Irvine, and their colleagues began to fill in those blanks by tagging blue whales found along the California coast and tracking their movements. They were also curious about why there weren’t more blue whales, because the populations of other species of baleen whales have increased dramatically since the International Whaling Commission banned industrial hunting of some whale species in 1966.
“When compared to other large whales that inhabit the same area, the blue whale population doesn’t appear to have increased at a similar rate,” says Monica DeAngelis, a marine mammal biologist at the National Oceanic and Atmospheric Association’s (NOAA’s) National Marine Fisheries Service in Long Beach, California, who collaborated with the paper’s authors on this project. Humpback whales in the North Pacific, for instance, now number about 20,000, up from 1400 since the ban.
To find out more about the blue whales’ movements, the researchers programmed some of the tags to transmit location data either daily or every other day; other whales were fitted with tags that transmitted every day for the first 90 days, then switched to every other day until the tags’ batteries ran out. Tags implanted by crossbow lasted 58 days on average; those deployed by the air gun method kept ticking an average of 85 days. One whale’s tag continued beaming data for a remarkable 504-day period.
The team’s analysis of the tagged cetaceans’ movements, reported today in PLOS ONE, showed that they faithfully return each summer to the rich, upwelling zones off Santa Barbara and San Francisco that produce masses of krill, their main prey. But these same areas are also crossed by major shipping lanes, with vessels traveling to and from the busy ports of Los Angeles and San Francisco.