Tourists on an Alaskan fishing boat caught a rare glimpse of how a killer whale stalks and kills its prey after seeing an orca tossing what seemed to be a sea lion into the air. 

The marine mammal was hiding near a fishing boat operated by Anglers Adventures of Ketchikan in Alaska when the orca began circling nearby.

In a YouTube video posted by Anglers Adventures, the animal is seen nestled against the boat. When the boat decided to leave the area, the whale attacked, tossing the marine mammal six metres up into the air.

Cetacean expert Paul Spong with OrcaLab said he believes the whale's prey may have been a seal rather than a sea lion, but agrees the video is remarkable, because humans don't often see how whales hunt.

"They are ferocious predators and have many strategies in dealing with their prey," says Spong.

"In this case, the poor little guy was hanging out under the boat for some time, very aware there was an orca very close by interested in him."

Spong said once the boat was leaving, the sea lion became an easy target for the killer whale and the rare interaction was captured on video. 

Hunters and prey know each other

Killer whales are particularly adept and fierce hunters, according to Spong.

He has seen videos of a group of orcas targeting an elephant seal on a floe of ice. The killer whales washed water on to the surface, rocked the ice and eventually washed the seal into the water and consumed their prey.

Killer whale tosses sea lion into air

The crew of an Alaskan fishing boat posted this video after witnessing an orca toss what they thought was a sea lion into the air. (Anglers Adventures/YouTube)

In another hunt caught on tape, Spong said you could see an orca jumping up and landing down vertically on a sea lion in order to kill it.

Sea lions have also developed techniques to evade being captured by killer whales.

When a group of sea lions on a surface notices a predator coming, they will often jump into the water and begin thrashing around to scare off the killer whale.

In this case, the sea lion — or seal — flipped by the orca in view of the tourists likely didn't escape, according to Spong.

"There's no question about the end of this. Sea lions are totally aware of orcas and the dangers they represent."