Book 30, Question 55
Why do fish sometimes gulp at the surface of a pond?
Fish breathe oxygen dissolved in the water.
In hot, still weather, the oxygen in a pond can run low, especially at night, when lots of algae or decaying plants use it up. When oxygen is low, some fish rise to the surface and gulp at the thin top layer where air mixes in. If they do this a lot, it can be a sign the pond is stressed.