Here are some comments on the chasing behaviour shown in the photograph:

1. It is unlikely that the small fishes are chasing and trying to catch and eat the larger fish. They would be no match for the larger one.

2. Could the small fishes simply be curious about the big one? This is certainly possible. Such "predator-inspection visits" are a known behaviour of schools. They may allow members of the school to assess the identity and motivational state of the predator and, additionally, to let the predator know that they, the potential prey fishes, are alert.

3. If you have an idea that the school members might be angry at the big fish, of some piscine equivalent of this, then it's a good one. It may be that the small fishes are trying to chase the large fish away. Such behaviour in birds is known as "mobbing". Not only can a predator be physically disturbed by such an action and move away, but the behaviour may serve as an alarm to other members of a large school."cartoon explanation of a mixed school of reef fishes apparently chasing a predator

4. Is the school trying to be large and frightening? This is possible and is reflected in many cartoons, as the adaptation shown here, depicting a school mimicking the shape of a larger fish, which is intended to frighten the predator. The behaviour is not thought to be intentional; rather, the menacing shape of the school results from a few of its members swimming faster than the others, thus drawing the school into the shape of an apparently larger creature. The predator does not like the large shape coming up from behind and naturally swims away from it.