Dear Dinah Bollasina,
Biologists find many years ago that the fish can have growth marks on several hard parts of their body, like bones, scales and otoliths.
Normally we use the otoliths (that are calcified structures of the fishes hearing) to determine fish ages, but scales and bones can also be used.
Like with the trees these structures forms growth rings annually, that we can count and determine the age. These rings are only formed in climates that had pronounced seasons of higher and lower food abundance. This alternation of high and low abundance of food makes the otoliths (or scale rings) to grow more or less, and this forms different growth rings more or less transparent (Winter or Summer), when we observe them at a binocular or microscope. We called this rings annuli, because they normally corresponds to one year, in temperate regions.
This is not very easy you must gain experience to read otoliths ages. Some species are easy than others. In the tropics, because there isn’t a pronounced year seasons, these rings are not so visible in the hard parts of the fishes, and age is almost impossible to determine in this case.