In which fish is it impossible to determine the age of the scales? Why?

The simplest and most affordable way to determine the age of a fish is to study its scales. One scale is taken, examined under a microscope or a magnifying glass, and it is easy to see concentric rings on it, so their number will show age. The formation of such a ring is obtained due to a change in the growth rate, therefore this method is not very accurate, with prolonged fasting, such a ring may appear, but it is not as clear as the annual one.
Not all fish can be dated by their scales, for example, they are too small (pike perch, pike, perch, catfish), or none at all (flounder, eel, stingray, shark, moray).
In such cases, tubular bones are considered, and if they are not there, then along the rays of the fin or gill cover, which is easier to get.