why a fish appears to be at less depth than its actual depth with respect to a bird flying in the air
This is because the rays coming from the fish get refracted at the interface of the air and the water.
As the rays gets refracted at the interface the tend to bend away from the normal as they are travelling from denser to rarer medium
due to this for the bird flying in thge air the refracted rays appear to meet at a distance above the fish virtually!