A whale flipped a fishing boat with people on board. Experts think they know why.

Boaters off the New Hampshire coast have a whale of a tail to tell after a humpback whale slammed into a fishing boat and a pair of teen brothers caught the whole thing on video , then rescued the two fishermen who were tossed into the sea.

In a highly unusual event, the humpback lunged out of the water Tuesday morning, then landed on the back of a boat, flipping the vessel onto its side. Video shows the fishers on-board, Greg Paquette and Ryland Kenney, fell into the water. They were soon rescued by Wyatt and Colin Yager of Eliot, Maine .

The incident with the 21-foot vessel happened near the mouth of the Piscataqua River, located on the border between Maine and New Hampshire, according to the National Oceanic and Atmospheric Administration. The boaters were uninjured, but it isn’t known whether the whale suffered injuries, NOAA said.

Seeing the whale close to shore shouldn't have been a huge shock to boaters. The same whale or a different humpback has been spotted a few times times in or near the river in highly publicized sightings since July 2 , said Jen Kennedy, executive director of the Blue Ocean Society for Marine Conservation, a nonprofit based based in Portsmouth, New Hampshire.

In three decades Kennedy had never heard of a humpback in the Piscataqua River, until just three weeks ago. And she’d never heard of a humpback lunging on to the top of a boat in the region, she said. “I’ve never heard of that ever and I’ve been whale watching here since the mid-1990s."

Why did the whale lunge out of the water?

In the Tuesday morning incident in the ocean off Rye, the whale appeared to be lunging in a classic humpback fishing tactic, said Linnea Mayfield, a natural manager at Boston City Cruises, affiliated with the New England Aquarium, after viewing the video.

The whales blow large frothy bubbles in the water to help corral fish, then they lunge up through the bubbles to scoop up the fish, Mayfield said. The incident was almost certainly accidental, she said. Humpbacks have a blind spot, and it’s “very possible the vessel was in the animal’s blind spot as it came up to lunge and feed.”

Both Kennedy and Mayfield said they occasionally hear reports of whales bumping into vessels in the region. While rare, such incidents happen from time to time, NOAA said. Almost exactly two years ago, a humpback leaped out of the water in the middle of a cluster of boats and landed on the back of a fishing vessel off Plymouth, Massachusetts .

How unusual is a nearshore humpback sighting?

A humpback thought to be a juvenile has been seen several times over the past three weeks, including between July 2 and 4 in or near Pepperell Cove along the Piscataqua River in Kittery, Maine. At the time, the Society and the Kittery harbormaster had warned the whale was feeding so close it could be a threat to boaters, the Portsmouth Herald, a USA TODAY Network property, reported. The whale had attracted large crowds of curious boaters.

On Sunday, the same juvenile humpback was seen further upriver near the Portsmouth Naval Shipyard, Kennedy said. The juvenile whale seen in the river was thought to be chasing menhaden, a prey fish, NOAA said.

It isn’t unusual for a humpback to linger in an area if it presents easy access to food, Mayfield said. “If their prey items are sticking around in one spot, they’re not going off in search of food somewhere else. They’re going to stay where that food is readily available.”

Officials hope to identify the whale

NOAA Fisheries is looking into whether the whale that hit the boat was the same whale previously seen in the river, said Andrea Gomez, a NOAA spokeswoman.

The Blue Ocean Society is pleading with the public to share any photos taken of the whale off Rye, New Hampshire on Tuesday, Kennedy said. With better photos, biologists might be able to identify the whale, comparing it to a photo catalog of known humpback whales in the region.

To make the identification, biologists need clear photos of the whale’s dorsal fin and the underside of its tail fluke, said Mayfield.

Whale advocates and NOAA offer the following tips for boaters:

• Watch for bubbles — which turn patches of water a frothy, seafoam green — and avoid them.
• Maintain a healthy distance from whales, at least 100 to 600 feet from humpbacks and 500 yards from North Atlantic right whales, NOAA advises.
• In an area where whales are present, move at only 10 knots so whales and vessels have the ability to safely navigate out of the area.
• If a fisher has lines in the water and a whale is seen, the lines should be retracted as soon as possible.
• Immediately report all stranded or entangled marine mammals to NOAA’s marine mammal and sea turtle stranding hotline at (866) 755-6622.

Are whales fishing close to shore this summer?

It's possible more than one whale was on the hunt for menhaden, said Regina Asmutis-Silvia, executive director of Whale and Dolphin Conservation North America.

"NOAA has been getting multiple reports of humpback whales close to shore between Maine and Massachusetts as schools of bait fish are abundant close to shore," Asmutis-Silvia said. "We just reported three whales in Plymouth's outer harbor to NOAA earlier this week."

The whales are primarily young and focused on relatively fast-moving bait and the typical lunges happen pretty quickly, she said. "These whales have the single-minded focus of that driver who eyes that one last open parking space after circling the parking lot for a while, they are not paying attention to anything but the food so it really is on the boaters to stay on alert."

Contributing: Glenn Sabalewski and Ian Lenahan, Portsmouth Herald

Dinah Voyles Pulver covers climate change, whales and other wildlife for USA TODAY. Reach her at dpulver@gannett.com or @dinahvp.

This article originally appeared on USA TODAY: A whale flipped a fishing boat with people on board. Experts think they know why.