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01/28/2013 07:37 pm ET

Gray Whale Migration Video Shows One Of The Largest Pods Spotted Off California Coast

Whale watchers in Southern California are exhilarated over a sighting that's been deemed a "once in a lifetime encounter."

A pod of 20-25 gray whales, spotted off the California coast near San Diego, is believed to be the largest seen there in at least three decades.

"Just when you thought that it was over, MORE whales came up," Alisa Schulman-Janiger, an American Cetacean Society board member, told NBC San Diego in an email. "Over 30 years of watching and studying whales, and I was awestruck! I will NEVER forget this day!"

Gray whales regularly migrate past San Diego en route from the Arctic to Baja, where they breed and calve, the San Francisco Chronicle notes. About 15,000 of the creatures make the journey every year.

At 10,000 miles roundtrip, the migration is considered the longest for any mammal on Earth, according to the National Park Service.

Gray whale populations have increased since the 1970s, but sightings off the California coast have decreased in that time, the organization notes.

Dr. Jim Sumich, a whale biologist with Grossmont College in San Diego County, told the National Park Service the decrease in their appearances could be attributed to a number of factors, including some combination of water quality changes, military and commercial boat activity, natural shifts in migration routes, and an uptick in overeager whale-watching vessels that get too close to the creatures.

Gray whales were removed from the United States endangered species list in 1994, according to National Geographic. The mammals can grow up to measure about 50 feet long and weigh up to 40 tons.

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