Otolith Aging--how we can age your fish.
Otoliths, commonly referred to as “ear stones” are hard structures composed of calcium
carbonate. All members of the superclass Osteichthyes (bony fish) have otoliths. They are
located in the head, underneath the brain, and are responsible for the fish’s ability to sense
vibrations, hear and maintain balance. There are three pairs of otoliths in a fish: Sagittae,
Asteriscus and Lapillus.
The process of removing, sectioning, and analyzing otoliths to determine fish age has been
used since 1899. There are several structures from fish that have been and are currently used
for aging such as scales, opercula, fin spines and vertebrae. Vertebrae are often used to age
fish that lack otoliths (e.g. sharks and rays) however, otoliths have been proven to be the most
reliable and accurate aging method among the scientific community. Sagittae (Image 1.) is the
largest of the pairs, so it is the most utilized otolith for yearly aging and the Lapillus is the
smallest pair.
The two zones present in otoliths are determined by opacity. These areas are visible under
transmitted light. The dark zones are opaque, and the bright zones are translucent (Image 4.).
By counting the number of opaque zones on the otolith, an age can be determined. The ageing
process of otoliths is often compared to the aging process of a tree. You count the number of
rings, which are referred to as annuli, and assign a yearly age to the fish
The larger a fish is and the more it weighs does not necessarily mean it’s older. When determining the age of a fish it is protocol to have two different readers separately analyze the otolith without knowledge of the fish’s length or weight as to avoid biased aging.
This protocol also helps to avoid confusing false annuli as yearly annuli. Research has shown that environmental stressors such as salinity, hostile weather, temperature, oxidative stress and food consumption are associated with false annuli. The presence of false annuli and distance
measurements taken between annuli formations can provide life history information about the fish.
At AES, we age around 1,000 bass per year. Age determination of these bass help pond ownersdetermine how well or poorly their bass are growing. For new AES clients, it is not uncommon for us to see a 12” bass that is 6 years old. In a healthy and properly maintained pond a bass that age should fall into the size range of 18”-22”. Fish aging is an essential element in the field of fisheries science. Age data can help biologists understand the effects that fishing, anthropogenic factors and other environmental changes have on fish populations. It also aids in determining maturity, growth rates, life history and lifespans of different species.