A 15-year study of blue whale movement off the coast of California reveals how busy shipping lanes cut right through their favorite feeding grounds. Moving or halting the shipping traffic during seasons when whales are the most abundant could significantly decrease their risk of ship strikes, according to the findings published in PLoS One this week.

Blue whales (Balaenoptera musculus) living in the eastern North Pacific can travel from the Gulf of Alaska all the way to the Costa Rica Dome near the equator. The population has been depleted by commercial whaling, and even though the International Whaling Commission began protecting them in 1966, there’s been no evidence of substantial population growth. Something’s still impeding their recovery. 

About 2,500 of the world’s 10,000 blue whales spend time in waters off the west coast of the U.S. To determine which areas are the most important and when, a team led by Ladd Irvine from Oregon State University attached transmitters to 171 blue whales off California and tracked their movements via satellite from 1993 to 2008. Scientists in inflatable boats attached the tags near the dorsal fin using a modified crossbow or a gun shooting compressed air.

During the summer and fall, Santa Barbara and San Francisco were the most heavily used areas, leaving the whales in the path of vessels traveling to and from the Los Angeles and San Francisco ports. Once, five blue whales were hit within a seven-week period.

“The main areas that attract blue whales are highly productive, strong upwelling zones that produce large amounts of krill -- which is pretty much all that they eat,” Irvine says in a news release. They’re the biggest animals that have ever lived, yet their diet is primarily these 5-centimeter (2-inch) shrimp-like critters. The whales maximize their food intake during the summer and fall before migrating south for the winter.

“It’s an unhappy coincidence," Irvine tells the Los Angeles Times. "The blue whales need to find the densest food supply. There’s a limited number of those dense places, and it seems as though two of the main regular spots are crossed by the shipping lanes.” 

Researchers don’t know how many are being hit by ships, Science reports, and they can’t say if these strikes are what’s definitely keeping the population numbers low. “At a minimum, it’s not helping,” Irvine adds. 

Blue whales may not be as acoustically aware as species who use echolocation to find prey, and the rear placement of engines creates an acoustic shadow that makes it hard for whales to hear oncoming ships. 

Speed restrictions could help, and moving the shipping lanes off LA and SF to slightly different areas -- at least during summer and fall when blue whales are the most abundant -- could lower their threat of injury and mortality, the researchers say. A similar shipping lane relocation in the Bay of Fundy off eastern Canada lowered the likelihood of vessels striking right whales by 80 percent. Here's a blue whale spouting! 

Images: Craig Hayslip, Oregon State University Marine Mammal Institute (top), Oregon State University (middle), Flip Nickin, Minden Pictures (bottom)