The fish you control is surrounded by radar ping lines to represent the sense of smell.  Use this to find food or avoid predators. When there is a clump of food or predators around, the radar shows a circle around the fish.
The radar will show the direction to a clump of food. It also shows the distance visually by the number of ping lines.

A red colored radar indicates a predator is nearby and the direction. The fish will turn more red as the predator gets nearer.

The fish in the school also turn  red when they are fleeing a predator.