Fish of a Certain Age

By Justin Hill

Pictured is an otolith from a largemouth bass. The bottom is a cross-section revealing the rings of this 8 year old.

Size isn’t everything. I know we all want to catch a bass big enough to brag about, but from a scientific point of view, maturity can be much more important. If you focus too much on how big it is and not enough on how long it took to get there, you can miss out on the big picture. One of the most important pieces of data researchers collect about both freshwater and saltwater fish is age. Now, you may ask yourself, “How do they know how old a fish is?” It’s not like they can knock on the door and ask politely. No, we can’t. Things would be a lot easier if we could just ask each fish, “Hey, how old are you?” What biologists do use are bones in the inner ear of the fish called otoliths, or ear stones. Otoliths have rings very much like a tree trunk. Every year, environmental triggers cause a new ring to form. Biologists remove these bones from the fish and count the rings. There are several things researchers can gather from this information.

Size at age: Size at age graphs are created by comparing a fish’s age to its length. This tells researchers how fast the fish are growing and at what age they become big enough to catch. The information from size at age can be used by management officials as part of the decision making process on length limits, and to evaluate the quality of the food sources and habitat in a water body.

Year Classes: Researchers can also use age data to follow groups of fish born each year called “year classes.” For example, biologists observed large year classes of bass following drawdowns on lakes Toho and Kissimmee. These fish went on to produce many trophy bass and biologists were able to document long-term improvements resulting from management practices.

Mortality: Biologists can estimate the rate that fish die from the number of individuals collected from each year class. This is used to predict how many fish will be available to anglers in future years.

The oldest largemouth I have personally aged was 13 but they can reach 16 years old in Florida. After about 8 pounds some say you can guess the age at about a year per pound. This is nothing more than a good guess, though, as biologists here have seen 10 pounders that ranged from just 4 to 14 years old. Black crappie can make it to 10, but rarely make it past 6 years old. The same goes for most of the bream like bluegill and shellcracker. The next time you catch a fish, think to yourself, “How old are you?”…. Then ask out loud and let us know if it tells you. It would make our jobs much easier.

For a closer look at FWC’s research like our Facebook page at facebook.com/FWCResearch and check out this bass fact at the Trophy Catch Florida Facebook page at http://on.fb.me/YzJTsV

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