Just read a piece in the NY Times about electric Volta's eels that form packs in shallow water and swim in ever tighter circles to force small fish into a tight mass. Then they turn on the juice and the shocked fish rise to the surface, where the eels gobble them up. Evidently that's more efficient than individual eels chasing down tiny fish. Group hunting has been observed in fish only on rare occasions. Whales and other marine mammals seem to hunt in groups more often.
In a small boat on Sitka Sound, Alaska, I watched humpback whales swim in circles, blowing bubbles. The bubbles create a visual barrier that confines prey fish, such as herring. As they crowd closer, the humpbacks charge to the surface, jaws wide, and gulp them down.
You do not want to be inside a bubble ring when the whales surface: like being hit by several buses. I was in a small inflatable kayak, and paddled madly out of several bubble rings before the whales breached, close enough to splash me.
