To age fish we use the fish's otolith. An otolith is a bone located just behind the ear which is used for hearing.

How: We count the number of lines in the otolith (one year= one line). just like counting the rings in a tree to see how old it is.

Why: Knowing the age of the fish helps us determine how long the fish live and how quickly they grow.

This otolith tells us this fish is years old.
(Hint: It is the same as the one drawn above, each little dot is marking one ring) Answer Here

Otolith under a microscope

We also use scales to determine the age of fish. Fish scales have lines similar to otoliths. With scales one year= one line also.