In sonar target strength refers to the ability of a target to return an echo.
The target strength or acoustic size is a measure of the area of a sonar target. This is usually quantified as a number of decibels. For fish such as salmom, the target size varies with the length of the fish and a 5 cm fish could have a target strength of about -50 dB.
The fish have a range of decibels in which the signals that targets them is effective and can give an echo back. Each target such as a submarine or fish has a specific target strength and there are losses due to bubbles which means that the reproducing back echo has a specific identity that the sonar needs to catch. A fish that has acoustic target strength of 25 decibels means that at this sound its back signal is the most effective and detectable back. The fish sends obviously also pulses back at 26 or 24 decibels as examples but because there is transmission loss the optimum acoustic target strength should be found for the maximum effectiveness in operation.
As the amount of reflected sound intensity measured one meter away from the target, relative to the amount of energy incident upon the target this parameter is called the target strength (TS) and expressed in dB.
When there is a difference in pressure then the acoustic signal behaves similarly. Fish have morphological differences such as the swimbladder the gonads the shape of their body etc that result in the echoing back to have differences in loudness according to these features. The fish can be spheroid or cylindrical, their bone fats etc make the scatter back of sound complex.
For typical fish lengths (1-100 cm), swimbladders scatter sound over a range of three orders of frequency magnitude (hundreds of Hz to hundreds of kHz)
Backscatter intensities of fish without swimbladder are much lower than any swimbladdered species.
Acoustic scattering by a swimbladder is four or more times greater than the scattering by fish bodies at any given frequency . An air-filled swimbladder can contribute up to 90% of backscattered sound [8-9].
One of the conclusions of these studies is that the kind of species of fish can be detected because if the fish have a swimbladder that would produce a bigger sound compared to a fish without a swimbladder. When the fish has the swimbladder full of air so as to come up in the water column then again the sound would be louder according to the depth. Therefore a stronger sound would come probably from the shallow water and that would give a clue whether the fish was recently deep or high in the water column.
All these information are useful for the fisherman so as to arrange the fishing tool accordingly in depth according to the data the fish finder gives about the position of the fish. For example a fish that has emptied its swimbladder so as to go down in the water it would be more difficult to detect because it makes a less strong echo back and in that case the intensity of the search should be bigger by having a fish finder with a larger frequency so as to discern the weaker signal.