Article content continued

On average, the whales keep their mouths open 18 seconds but as long as 90 seconds before chomping down.

Researchers believe that seabirds feeding on herring may actually be helping the whales. The herring are thought to use the whales’ bulk as protection against the birds, unaware they are setting themselves up to be swallowed.

“The whale is providing a shadow or shelter,” McMillan said. “It’s like a Venus flytrap, a sit-and-wait idea. It’s not working so well for the herring, but it’s working for the whales.”

Seabirds such as rhinoceros auklets and common murres dive into the ocean and swim beneath the herring so the prey forms tight schools known as bait balls. Gulls feed on the fish at the surface.

The frenzy of birds feeding also attract the whales.

When herring are startled, their instinct is to group up tight — get close to your neighbour, and hopefully he’ll get eaten instead.

McMillan was puzzled when she first observed a humpback trap feeding. “It was lying there at the surface with its mouth open. I thought, ‘This is really strange.’ Then we started seeing it happen over and over again.”

She said humpbacks tend to use a more energetic fishing tactic known as lunge feeding to target larger schools of herring, and trap feeding when there are fewer fish more widely distributed.

It’s kind of like snacking between meals.

“Trap feeding is an energetically efficient way to be feeding on less dense schools of fish because there is no high-speed acceleration,” McMillan said. “The whales are just opening their mouths and remaining relatively stationary.”

It appears that the longer a humpback remains off northeastern Vancouver Island, the greater the chance of learning to trap feed from those whales that have already adopted the method.

More than 2,000 humpback whales are thought to feed in the rich waters of B.C. before winter migrations to Hawaii or Mexico, where they give birth.

lpynn@postmedia.com