How and why we age fish

By Fisheries Biologist Kevin Gallant

Having the ability to age a fish is a valuable tool for fishery managers.  Stocked fish often have clipped fins that tell us what age they are (by knowing the year the certain fin was clipped).  With wild fish (and unmarked hatchery fish), we have a few other options to give us the age of a given fish. 

One method is to collect a scale sample.  Scales are removed from the side of the fish near the dorsal fin with a dull knife.  When taken back to the lab and viewed under a microscope, biologists can examine the growth rings in a similar fashion to how a forester would age a tree.  From these growth rings, biologists can determine not only the age of the fish, but whether it is wild or stocked.  Even when the fish has spawned is sometimes visible in the pattern of the growth rings.  Scales work great for aging short-lived species (like brook trout) and can be obtained without harming the fish. 

Longer lived species often require using other fish parts to determine age. Calcified structures of the inner ear called otoliths are examined for ageing species like lake trout and lake whitefish.  The fisheries division utilized specialized equipment to cut a tiny cross section, capture a high-quality image, and then determine the age of otoliths.  The age rings show up because fish pack on more growth in the summer and slower growth in the winter.  This creates a pattern and allows each year to be counted.  Some of these fish have been aged to over 30 years old!  We can then use this information to learn about the age structure of a population.   

In addition to scales and otoliths, we can also use another structure on certain fish.  The cleithrum is a thin bone that can be used to age fish in the Esocidae family (pickerel, pike, muskellunge).  It can be removed and after boiling off the flesh you can read the age rings by holding it up to a bright light.

These methods allow biologists to gather critical age information that can be used to manage for healthy inland fish populations in the great State of Maine!