Ever wonder how biologists age fish? It’s not by size. Surprisingly, it’s by counting growth rings on a fish bone like foresters do patterns on a tree stump. Researchers also use scales or specimens from dorsal fins, but the most accurate procedure is dissecting a small bone in the fish’s head called an “otolith”.
According to Dr. Skip with Gulf Shores Alabama, the otolith’s biological function is aiding equilibrium. When removed, the bone chip is sectioned into thin slices and viewed under a microscope. Rings appear dark and light. Darker rings are narrow and depict dense growth in winter months when fish experience slower metabolism. During warm, fast growing months, rings appear clear.
Otoliths may contain heavy metals like lead and can be used as indexes for pollution.
Scale analysis can reveal fish environmental conditions and growth periods as little as two weeks. As fish age, scales become larger. Scales removed for sampling are regenerated. Experienced biologists can estimate fish length by examining the angle of growth rings along the scale’s center.