A bait ball, or baitball, occurs when small fish swarm in a tightly packed spherical formation about a common centre. It is a last-ditch defensive measure adopted by small schooling fish when they are threatened by predators. Small schooling fish are eaten by many types of predators, and for this reason they are called bait fish or forage fish.

In the photograph above we see a bait ball of bigeye scad fish off the coast of Kona, Hawaii. Photographer Steve Dunleavy explains:

“Big Eyed Scad feed offshore on plankton at night, and tend to congregate in huge schools in shallow water for protection during the day. The baitball can be massive – as big as a house, hundreds of thousands of fish. If the water is clear and you can find the school, these are a blast to shoot. I’ve run out of air and camera battery more than once getting pictures like this.
 
This image was from last year [2010], shortly before some local fishermen moved in and constructed a huge pen to fish the school out. They gave up without catching the scad, but stressed them enough so they disappeared. A year later, I’ve heard reports that small schools are starting to reform at different locations.”