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 Lapillus can be used for aging as well because in some species the age is more easily determined from this otolith than the Sagittae. The lapillus is also widely used to determine daily age.

Image 1. Spotted Seatrout Sagittal otoliths.

Otoliths come in various shapes and sizes depending on the species (Image 2. Image 3.). There are different methods for sectioning and processing otoliths. Transverse sectioning is the preferred method meaning the otolith is sliced or broken through the nucleus. This is the method we use here at AES. If the otolith is large enough it can be sectioned using 2 saw blades. A common method for smaller sized otoliths (e.g. largemouth bass) is to embed the sectioned
otolith into epoxy resin and grind it down until it is readable under the microscope.

Image 2. Creek Chub otolith with Image 3. Black Drum otolith with
adult index finger for size reference. adult hand for size reference.

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 (Image 5.).

Image 4. Annuli on creek chub

 Image 5. Twelve-year-old largemouth bass.otolith (age 4).

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 owners determine 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.

Literature Cited

Berghahn, R. (2000). Response to extreme conditions in coastal areas: biological tags in flatfish
otoliths. Marine Ecology Progress Series, 192, 277-285.
Cappo, M., Eden, P., Newman, S. J., & Robertson, S. (2000). A new approach to validation of
periodicity and timing of opaque zone formation in the otoliths of eleven species of Lutjanus
from the central Great Barrier Reef. Fishery Bulletin, 98(3), 474-474.
Wright, P. J., Panfili, J., Morales-Nin, B., & Geffen, A. J. (2002). Types of calcified structures:
Otoliths. Manual of fish sclerochronology, 31-57.
Ricker, W. E. (1975). Computation and interpretation of biological statistics of fish
populations. Bull. Fish. Res. Bd. Can., 191, 1-382.

Authored by Ashley Fredricks, MS, Fisheries, Aquaculture and Aquatic Science