system
Given the following post, respond as if you're commenting on Reddit:
human
How does forming a bait ball act as protection against predators in schooling fish?

After watching quite a few documentaries that show large shoals of fish forming bait balls to protect against predation, I am genuinely confused on how this is a protective behavior.  In EVERY documentary (that I’ve seen at least) the entire school of fish gets consumed.   The fish seem to all form a tight ball which allows for large predators like whales, eat large swathes of the school in one pass.  After that, they flock to the surface to avoid predators from below, allowing birds to pick them off one by one.  It just never seems to work out well for the fish. Can someone please explain why fish do this, and what advantage it provides?  Thank you.
gpt
It's not a great defensive measure, but rather a natural consequence of every individual fish trying to escape the predators. It's a last ditch effort when they find themselves unable to escape from their normal fish schooling behavior. Any fish by itself will quickly get eaten but if you're hiding yourself among thousands of other fish, it's more likely someone else will get eaten. 

One way it _is_ actually defensive is that a sphere has the smallest possible surface area vs its volume, thereby the fewest possible fish are exposed to danger at a given time.