Tennessee Wildlife Resources Agency

TWRA's Region 4 - East Tennessee

Reservoir Fisheries Management Program

Fish Age and Growth with Otoliths

How to remove otholiths video

The TWRA routinely collects age samples to obtain information regarding the age structure and mortality rates of fish populations. TWRA Biologists use otoliths or “ear stones” in the aging process for most species. Otoliths are hard, calcium carbonate structures located directly behind the brain of bony fish.

There are three pairs of otoliths in fish (sagitta, asteriscus, and lapillus) which aid in balance and hearing. The sagitta is the largest and is the one used to age fish.

When a fish grows slowly in the winter, a darker denser ring forms on the otolith. A clearer ring is formed in the warmer months when a fish grows faster. These growth rings are called annuli, and are similar to the rings found in trees. Other hard parts of a fish such as the scales can be used to age, but tend to get knocked off over time and have been shown to be far less reliable than otoliths. Otoliths can be very small in young crappie and bass or as large as half your thumb in freshwater drum.

Otoliths are broken in the middle prior to aging. The resulting rough edges are smoothed with an emory wheel of a rotory (Dremel) tool. The otolith is placed in a pool of gycerine with the broken edge up and a fiber optic light is used to illuminate the structures under a binocular scope.

Otolith growth rings

musky otoliths


Basic biological data is the foundation on which assessments of fisheries resources are made. This data includes parameters such as the size and age composition of populations, growth rates and maturation. Proper assessments require knowledge of the size and age structure of fish populations and relative abundance by age. Rates of growth, mortality and reproduction can only be calculated if changes in length-at-age can be monitored over time.

Following is the typical data obtained through our age analysis of fish:

Walleye age graph


Walleye mean length at age


Walleye mean length at age


Walleye mortality