SEATTLE -- New research into the critically-endangered southern resident orcas shows that when they deep dive to catch salmon, they often come up empty.
Lack of food is one of the leading threats to their survival. The population is down to just 75 whales.
NOAA researchers released a study this week that examined how orca forage underwater. Between 2010 and 2014, teams attached suction-cup, multi-sensor tags, knows as DTAGs, to several orcas.
"It records depth and it records sound and it records three-dimensional movement under water," said Jennifer Tennessen, the lead author of the study.
Each tag contained flash memory to store all of the data and stayed on the orcas for an average of around eight hours, Tennessen said.
With the data, NOAA researchers got a glimpse into the underwater world of the endangered orcas.
"The killer whales were accelerating and decelerating, they were swimming in different directions, changing directions frequently and they also were turning onto their sides," Tennessen said.
Whenever the whales exhibited this pattern of movement, it coincided with the sound of buzzes -- echolocation clicks spaced very close together-- followed by the sound of handling prey, like crunching and tearing.
"Whenever that combination of three-dimensional movement was occurring, we were hearing these sounds, so we knew that that pattern of movement indicated that prey had been captured," she said.
These movements happened during the orca's "deep dives," between 100 and 600 or more feet underwater, where salmon swim. However, the data showed that less than half of those deep dives resulted in captured fish.
"It takes a lot of energy to dive really deeply and so presumably, they would need to only make such a dive if they thought a fish was down there," Tennessen said.
She said on average, females captured fish on just 22 percent of their dives. Males, while more successful, still caught fish just 48 percent of the time.
"What we don't know is whether that is typical or whether that could be due to disturbance," she said.
A known threat facing the orcas is vessel noise. The critically-endangered southern residents swim in the busy waters of the Puget Sound. Several studies suggest vessel noise can disrupt the whales' foraging.
Tennessen said the next step to their research is to compare the southern residents' hunting success rate with the northern resident orcas in Canada, where the water is quieter.
The study also revealed foraging differences between males and females. While they exhibited the same movements and sounds while foraging, "males captured more prey during a deployment than females, made more dives to Chinook habitat, and there was a trend that males spent more time engaged in deep diving," according to the study.
Females made prey-capture dives during just half of the tag deployments, while males made at least one deep dive on every tag deployment.
Scientists believe that is in part because males are bigger and need more calories. The disparity may also be because females need to spend more time at the surface taking care of calves.