Humpback whales are skilled acrobats, emotive singers and the most ambitious migrators of all mammals. They are also incredibly creative foragers, capable of trying new approaches to catching a meal.
Now, a study has found that these titans of innovation have learned to feed on salmon released from man-made hatcheries in Southeast Alaska.
"This is a new source of prey, as far as we can tell," said Ellen Chenoweth, a doctoral candidate at the University of Alaska Fairbanks and lead author of the study, published Tuesday in the journal Royal Society Open Science.
Once nearly hunted to extinction, humpbacks are making a strong comeback in many regions of the world, in part because of their distinct ability to exploit different types of prey. The researchers studied the whales' dining habits in Alaska in hopes of starting to understand whether they might have an economic impact on the area's fisheries.
Alaska's modern salmon hatcheries have played an important role in supplementing wild fish stocks since the 1970s. Run by local fishers, the hatcheries rear juvenile salmon until they are ready to be released into the ocean, where they mingle with wild fish before returning to where they were raised.
In 2008, staff members at a hatchery filmed a humpback whale feeding on salmon they had just released. Steve Reifenstuhl, the general manager of an aquaculture association that runs several hatcheries in Southeast Alaska, brought the video to Chenoweth's lab.
Chenoweth, Reifenstuhl and colleagues devised a plan to study five salmon release sites over six years. An analysis found that the probability of humpback whale occurrence increased after releases of hatched salmon began and decreased after releases concluded every year.
"It's a small number of whales feeding at the hatcheries, but a couple individuals seem to be coming back and making this part of their annual foraging strategy," Chenoweth said.
Next, the researchers want to understand if humpbacks prefer salmon releases at certain depths, sizes, densities or times. That information could help the hatcheries figure out ways to deter whale predation.
Ron Heintz, a program manager with the National Oceanic and Atmospheric Administration's Alaska Fisheries Science Center, said the study succeeded at documenting a novel behavior for humpback whales. But Heintz, who was not involved in the research, said figuring out whether whale predation would have an economic impact on the hatcheries would be much harder, because salmon return rates vary widely and depend on many environmental factors year to year.
In terms of humpback whale biology, this study suggests the animals "are able to see, find and capitalize on a new opportunity," said Alyson Fleming, a cetacean researcher at the Smithsonian Institution who was not involved in the study.
Humpbacks are unique among baleen whales for feeding on a wide range of prey, including krill, fish, crab and squid. To do so, they use a variety of sophisticated techniques, such as bubble-net feeding, wherein a whale or group of whales creates a column of bubbles to corral fish toward the water's surface. Near hatcheries, the whales may use docks and fish pen enclosures to help them trap lots of salmon at once.
The whales' resourcefulness gives researchers some hope for their ability to adapt to climate change.
"They will develop new tactics and do stuff you've never seen before," Chenoweth said. "In a changing ocean, like the one we have now, that type of flexibility is useful."