Paul, how do you know the age of a fish? | Wyoming Game & Fish Department

Fish are aged the same way as trees – by counting growth rings called annuli on bony structures. The more rings, the older the fish. Scales, spines and ear bones - known as otoliths - are the most common body structures Wyoming biologists use to age fish.

Scales can be placed directly under a microscope or in a projector to count annuli, whereas cross-sections must be cut out of spines and otoliths with a saw. Otoliths are the most accurate ageing structure because the annuli are clearer and easier to count than those on scales and spines. However, a disadvantage of obtaining otoliths is that fish must be killed, whereas obtaining spines and scales is nonlethal.

Here are some examples. The yellow dots mark the rings.

Large mouth bass scale