Being the largest creature on Earth hasn’t been much protection for blue whales. They were nearly wiped out by the mid-20th century due to whaling. And the population in the eastern North Pacific, which numbers only about 2,500 cetaceans, hasn’t really recovered since whaling officially ended in 1966.
Now, researchers say they have figured out why the whale numbers haven’t increased as much as expected: Shipping lanes off the coast of California intersect areas of high whale use during certain times of the year. Ladd M. Irvin of Oregon State University in Newport and colleagues report their findings July 23 in PLOS ONE.
The scientists tagged 117 blue whales off the coast of California between 1993 and 2008. They tracked the whales as they moved up and down the west coast of North America, paying particular attention to the times when the whales were in the U.S. Exclusive Economic Zone, the area of U.S. jurisdiction extending 200 nautical miles from the U.S. coastline.
In the heat of summer, blue whales can be found off central to southern California. In those months, upwelling brings cold water full of nutrients to the surface from the ocean depths. Those nutrients feed the local food web, producing lots of tiny crustaceans known as krill. Blue whales feast on dense patches of that krill.
In late summer to early winter, that bloom of productivity moves northward, and the blue whales follow. A few were tracked swimming as far north as Alaska. (The rest of the year, the whales are outside the EEZ, traveling as far south as the equator.)
Given the size of the ocean and that there aren’t that many whales, it would seem encountering one would be rare. But from July to October, many whales are found at sites where international shipping lanes cut through — near the Channel Islands off the coast near Santa Barbara and the Gulf of the Farallones off the coast of San Francisco. And as the whales move north, they run into the many shipping lanes that run into the San Francisco harbor.
The crossing of the whales and heavily trafficked paths of large ships raises “the likelihood of anthropogenic impacts on the whales, either through increased underwater sound, or through vessel collisions,” the researchers note.
Getting rid of the ships entirely is not an option. A decade ago, a shipping lane in the Bay of Fundy was rerouted so that it no longer cut through an area with a dense concentration of right whales. The whale population there subsequently began to grow. But the situation off California is more complicated. The blue whales move throughout the year, and there are other whale species to worry about as well. Moving most of the shipping lanes would just put them in other spots where the ships would encounter whales.
The researchers did make two recommendations for changes to shipping routes that could substantially help: In the western part of the Santa Barbara Channel, “moving the shipping route southwards would reduce the risk of ship strikes for blue whales, particularly during July to October,” they note. And the northern lane ferrying ships into and out of San Francisco could be closed during August to November or altered in such a way that it limits crossing areas of high whale density.
Last year, the National Oceanic and Atmospheric Administration established new rules for the shipping lanes in San Francisco Bay in an effort to limit ship strikes there. So further modifying the shipping routes off California is feasible. But established shipping lanes may soon be on their way out anyway— researchers are working on apps and other methods that would allow routes to be altered based on the current locations of whales, further reducing the interactions between whales and ships.