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.

“It’s an unhappy coincidence,” Irvine says. Over a 2-week period in 2007, for example, at least three blue whales were killed by ships striking them near California’s Channel Islands. (Two other blue whale carcasses were spotted during the same 2 weeks, but the scientists weren’t able to study them.) “That made everyone say, ‘Whoa,’ ” says Irvine, who recalls seeing the injury to one of the dead whales. “It had a huge hematoma from what was clearly a massive, blunt trauma.”

The scientists don’t know how many of the eastern North Pacific blue whales are being hit by ships. (“Some vessels are so big they may not even know that they hit a whale,” Irvine says.) And they still can’t say if such strikes are the most important factor keeping the cetaceans’ population numbers low. “At a minimum, it’s not helping,” Irvine says. And because the eastern North Pacific population is the best studied stock of blue whales, scientists suspect that the whales are suffering elsewhere from ship collisions. “The reality is … ship strikes are a global threat to this species,” DeAngelis writes in an e-mail.

Happily, there is a model for improvement. In the Bay of Fundy on the Atlantic coast of North America, lethal ship strikes posed a similar problem for right whales in the 1990s. Then, 11 years ago, the maritime industry there moved one key shipping lane and agreed to have ships enter and exit ports at slower speeds—which was especially effective, Clapham says. “Speed restrictions give whales time to get out of the way.” The changes reduced the likelihood of vessels striking the endangered whales by about 80%.

In this case, the researchers say shipping lanes near the Channel Islands and off San Francisco would need to be moved to end the collisions. They recommend altering the routes through the Santa Barbara Channel during the height of the whales’ feeding season, from July to October, and closing the northern shipping lane to and from San Francisco Bay ports between August and November. And although such changes are not easily made, NOAA is taking a first step. It will be conducting a review—informed by this study—of the especially busy shipping lanes off southern California and consulting with the various stakeholders about a “plan that would be beneficial to everyone,” Irvine says. After all, he points out, a ship hitting a whale that weighs as much as 25 elephants isn’t good for the vessel, either.