How can you tell the age of a fish?

Like the rings on the cross section of a tree, fish scales also have rings. Fish scales have concentric ridges called circuli which reflect the growth patterns of the fist and can be used to estimate the age. From “The Handy Science Answer Book,” the portion of the scale that is embedded in the skin contains clusters of these circuli, called annuli. Each of these clusters mark one year of growth.

So what about fish that don’t have scales? Like catfish, dolphins or sharks? I’ve heard that a seasoned biologist (or ichthyologist) could possibly make a fair estimate by looking at their teeth. But I would take this answer with some skepticism and say that this is probably difficult to do.