A small boat was lifted out of the water when it struck a grey whale off the coast of Washington State on the weekend, in an incident caught on camera by shocked whale watchers.

The boat struck the marine mammal on Sunday morning near Whidbey Island, about 50 kilometres north of Seattle.

The incident was captured by a camera on the San Juan Clipper, a whale watching vessel operated by Clipper Vacations.

A woman onboard the vessel at the time told NBC affiliate King 5 that a group of whale watchers had stopped to watch a pod of grey whales, when a smaller boat zoomed through the area. One of the whales surfaced directly in the boat's path, and video shows the boat lift completely out of the water upon striking the whale.

Witnesses said the boat's operator stopped briefly, then left the area.

A naturalist on the San Juan Clipper took photos of the boat and its operator, and said she sent the pictures and video to the Washington Department of Fish and Wildlife.

The WDFW told King 5 that staff was able to track down the driver, and said that they are cooperating with its investigation.

The Vancouver Aquarium's head veterinarian, Martin Haulena, said wildlife officials have tracked the whale since the collision and assessed its injuries.

"There's a large area of swelling behind the head, so that's obviously a sign of traumatic injury," he told CTV News on Monday.

Many believe the strike may have been preventable – in most cases, boats in the U.S. are told to stay about 100 metres from whales – but incidents happen more often than most people realize, Haulena said.

There are a lot of boats and people in areas close to the U.S. and Canadian coastlines, and while it's unclear exactly how the latest strike happened, mariners are calling it a reminder to slow down.

With files from King 5 and CTV Vancouver's Sheila Scott