We have a couple of ways to get age data for crappie. One way is to take scales from the fish. Fish put down daily growth rings on their scales similar to the rings on a tree. When growth slows down, usually in the winter, these daily growth rings start to bunch up forming a thinker, darker ring. When you press the scales and look at them under a microscope you can count these annual rings starting in the center of the scale moving out. Each thick ring equals 1 yr. of life. You can also tell how much growth occurred between yrs. based on the distance between these annual rings. A bigger distance between annuli indicates better growth, tighter lines stunted growth. The good thing about using scales is that it is a non-lethal method. The problem with using scales is that often times they can be difficult to read and may cause us to get an inaccurate age. These rings really stack up on older fish and become hard to read towards the outer margins of the scale and fish can also regenerate damaged scales, leading to inaccurate readings.
Another way to get more accurate age data is to take an inner ear bone out of the fish called an otolith. These structures also put down daily and annual growth rings. The good thing about otoliths is that you don't have to worry about regeneration and I think they are much easier to read. We cross section the small bone, sand it down flat, and put it under a microscope. When you shine a fiber optic light on the otolith the annuli kind of glow making them easier to count. The bad part about taking otoliths is that it is a lethal method.
In both cases, scales and otoliths, we usually take around 5 fish per half inch group to get an accurate data set. Obviously some fish grow faster than others so taking multiple fish is a must in order to get an accurate length/age data.