The target strength of individual fish depends on swimbladder inflation. Measured target strength from salmon in a net pen before, during and after the surface was closed with a netting roof. Lack of access to the surface deprived the fish from access to air for filling their swimbladders. Measurements in three different net pens showed the same trend (one colour for each net pen).
The measured calibration gain for a transducer should be identical for different calibration spheres, but this example shows gaps in the gain, in particular located close to the deep minima of the spheres.
