Whale watchers were thrilled to see a giant pack of 24 orca killer whales right off the coast of the Farallon Islands, but you might be less thrilled to learn that those orcas are probably there to feast on sea lions, elephant seals, and baby gray whales.

It’s whale migration season, when SFist readers are routinely delighted and dazzled by whale frolicking antics in the Pacific Ocean and San Francisco Bay. But would it delight and dazzle you to know that the latest highly unusual whale sighting is because a gang of two dozen whales is there to devour seals, sea lions, and baby gray whales? Because that is indeed the case here, as the Chronicle reports an unusually gigantic pack of 24 orcas was spotted off the Farallon Islands. Orcas usually only show up near the islands in groups of four to six.

The video above is from May 7, but the observers from the Oceanic Society, a nonprofit that runs Farallons whale watching tours, have just posted their thorough documentation of the encounter. “We encounter orcas out there maybe two to three times a year,” Oceanic Society naturalist Michael Pierson told the Chronicle.

The Oceanic Society blog describes the encounter in detail. “In the distance we saw several dark dorsal fins rising above the ocean’s surface,” they explain. “We knew right away we were seeing orca. They were also moving north and doing so at speed, so we initially struggled to catch up to them. Once we did, we realized what a special sighting we were witnessing as the size of the group became apparent. This is when the killer whales slowed and seemed to be watching us as much as we were watching them. They made several very close passes by the boat, allowing good views for identification.”

This particular breed of orcas is known as Bigg’s killer whales, or Transient killer whales. And they are indeed ruthless killers. Their diet is exclusively marine mammals, preferring sea lions, elephant seals and baby whales. Biologists suspect they were there to attack baby gray whales migrating with their mothers, or because pregnant sea lions like to give birth on the Farallons. The mothers and females generally lead the hunt, separating the youngster prey from their mothers, and then devouring the little ones to shreds.

And yes, orcas are having themselves a media moment of a so-called “uprising”, in light of reports that orcas are attacking boats lately in Europe. Researchers aren’t sure why they’re doing this, but they have not discounted a “revenge” theory. One of the whales nicknamed “White Gladis” is believed to have had a negative interaction with a boat, and may be training her young to attack boats.

Related: Video: Killer Whales Attack Gray Whale Calf Off Sonoma Coast As Mother Tries To Defend It [SFist]

Image: Michael Pierson, Oceanic Society