Passengers and crew aboard the National Geographic cruise ship Endurance as it sailed parts of Antarctica got more than expected last year when the ship happened to pass through a huge group of whales foraging together.

A team of oceanographers from multiple institutions in the U.S. studied images and videos taken by the passengers to learn more about the whales and have just reported their findings in a paper published in the journal Ecology.

Passengers and crew reported that as the ship sailed just north of Coronation Island, the sea around them came alive with whale spouts. Many also reported that as the ship sailed into the area, they could hear the breath of whales as they blew. Some noted that there were so many whales that the air became moist due to so much blowing.

The research group found that most of the whales were fin whales—the second largest living whale, behind only blue whales—they weigh on average 80 tons. Also sighted were a few humpbacks and blue whales. The researchers estimate that the whole group of whales was made up of between 830 and 1,153 members, making it the largest group of whales seen since the late 20th century.

They note that formations of such large groups of whales have become extremely rare as whale population numbers have fallen—fin whale numbers have been estimated at just 2% of what they were centuries ago. They are listed by the International Union for Conservation of Nature as vulnerable to extinction. The research team notes that the numbers of fin whales have been increasing slowly in recent years and suggest that finding so many in one place is encouraging.

All of the whales in the group were baleens, which explains why so many congregated in one place—it was rife with krill, the main food source for many types of whales, including fin. One downside to the sighting of the large group of whales was the observation of numerous trawling ships competing with the whales to capture krill in the same area.

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