Biologists use otiliths to determine the age of fish
from The Fishing Wire

Ever wonder how biologists figure out how old a fish is and how fast it’s growing? Here’s how they do it, from the Florida Fish & Wildlife Commission.

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

Age is one of the most important pieces of data researchers collect about both freshwater and saltwater fish. Biologists use bones in the inner ear of the fish called otoliths, or ear stones, to determine how old an individual fish is. These bones have rings very much like a tree trunk, and every year environmental triggers cause a new ring to form. Biologists remove the otoliths 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.

Largemouth bass 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 FWRI biologists have seen 10 pounders that range 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.