What determines the width of annual rings on fish scales?

The width of the growth rings of the fish depends on the growth of the scales, so it is easy to determine the age of the fish from them. In the process of growth, large young scales appear, they grow over the old ones, forming annual rings. In summer, the fish grows quickly and the rings on the scales are wide, and in winter it slowly and the ring turns out to be narrow. The scales become stronger every year.