Each year in late summer, hundreds of blue whales migrate to rich feeding grounds in the cold California Current. There upwellings trigger an explosion in phytoplankton, the primary food source for krill, anchovies, and other small sea creatures. These animals cluster together in vast schools, providing enormous, but ephemeral, feeding opportunities for bigger predators.
Blue whales are generally found in the open ocean but frequently enter Monterey Bay on California’s central coast. Sightings of “supergroups” of up to 40 whales in a 1-kilometer radius support the idea that the animals know when upwelling is occurring and quickly hone in on it.
“I’ve been out when it just seems like you could walk across the ocean on whale backs,” said John Ryan, an oceanographer with the Monterey Bay Aquarium Research Institute. With drones, he continued, “we’re seeing whales streaming toward the place where [krill] were aggregating. So that’s another piece of the puzzle that made us say, they must know how to find these spots.”
Using acoustic technology, Ryan and his colleagues were able to listen to the whales, track their movements, and learn that they respond to changes in the strength and direction of wind, which influences upwelling events. They published their findings in Ecology Letters.
Listening to the Wind
Ryan and his colleagues used hydrophones to listen in and gathered evidence that the whales are cooperating. “They put out a certain type of call at a really high rate,” he said, “as if it was an acoustic signpost to say ‘here’s the place.’”
Although hydrophones can detect whale calls, they can’t tell which direction the calls come from. A sophisticated listening device called an acoustic vector sensor, however, can determine the direction of sound underwater. The device, originally developed for military use and integrated into hydrophone technology, allowed Ryan and his team to get a bearing on the origin of whale calls. “Because we could continuously track calling whales,” he said, “we could watch them move in response to the wind.”
A clear pattern soon emerged: When strong wind drove upwelling along the shore, whales quickly moved from deeper water to forage in the bay. Once the wind died down and the upwelling abated, the whales moved back out to sea.
Shipping Lane Safety
These rhythmic movements are putting the whales in harm’s way. “They’re crossing shipping lanes every time the winds transition,” said Ryan.
Elliott Hazen is an ecologist with NOAA and studies blue whale movements along the central California coast. “Ship strike is one of the largest sources of human-induced mortality for the whales,” he said, “but we also believe we’re only detecting 1 in every 10 ship strikes. A lot of ship-struck whales sink to the bottom before they’re detected.”
Hazen and his colleagues previously developed methods to monitor oceanic conditions to predict whale movements. “We use things like chlorophyll (from satellite data), temperature, and mix layer depth,” he explained, “which gives us a good idea of how strong the upwelling is at any given time.”
These data are fed into a modeling tool called WhaleWatch, which predicts the likelihood of whales being present in an area. Another tool, Whale Safe, uses this information, as well as aerial surveys and hydrophone data, to warn ships of the likelihood of encountering whales. It also grades ships on their response—in effect, “naming and shaming” those that fail to take precautions.
Tracking whales with directional hydrophones could reduce Hazen and other researchers’ reliance on modeling and prediction, allowing them to know for certain where whales are at any given moment.
“We need to be able to provide reliable, real-time information,” said Ryan. “Whether that informs speed reductions or a careful observation at the bow of the ships while they’re underway or even temporary changes in the location of shipping lanes.”
—Bill Morris, Science Writer