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Introduction to Aging Fish: What Are Otoliths?

Otoliths, commonly known as “earstones,” are hard, calcium carbonate structures located directly behind the brain of bony fishes. Otoliths are important to scientific age and growth studies.

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Otoliths, commonly known as “earstones,” are hard, calcium carbonate structures located directly behind the brain of bony fishes. X-ray images of a spotted seatrout show the location and orientation of the otoliths.

There are three types of otoliths, all of which aid fish in balance and hearing:

1. Sagitta—the largest of the 3 pairs of otoliths; involved in the detection of sound and process of hearing, or converting sound waves into electrical signals
2. Asteriscus—involved in the detection of sound and the process of hearing
3. Lapillus—involved in the detection of gravitational force and sound (Popper and Lu 2000)

There are many different shapes and sizes of otoliths among different species. Cartilaginous fishes, which include sharks, skates, and rays, do not have otoliths. Otoliths are important to scientific age and growth studies. This figure shows the growth rings of a sagittal otolith section viewed under reflective light. The darker area or “translucent zone” represents a period of fast growth. The whiter area or “opaque zone” represents a period of slower growth. The age of the fish is estimated by counting the annuli, or opaque bands, of the thin sections, as one would count rings on a tree to determine its age.

Before age data can be used, the method of estimating age by counting annuli must be validated for each species to which it is applied. There are several ways to validate age, or prove that “one annulus is equal to one year.” Most obvious might be to simply rear fish from spawn, sacrifice the fish after a few years, and compare the number of rings to the known age of those fish. This process can be time consuming and expensive. It also creates the possibility of abnormal growth patterns caused by laboratory settings (Campana, 2001). Although this method may not be practical for validating annular ring formation, a similar method is effective in validating daily ring formation (Campana and Neilson, 1985).

To avoid the effects of long-term laboratory exposure, tag and release of wild fish can be useful in validating annulus deposition. This figure shows the fluorescent tag of a common snook otolith. A captured common snook was injected with oxytetracycline (OTC), a chemical that is incorporated into calcium-rich structures including otoliths. The fish was then tagged and released. Seven years later, the fish was recaptured, sacrificed, and processed for aging. The OTC, which binds to the calcium in the otolith, appears as a glowing band when the otolith is viewed using fluorescence microscopy. The number of annuli between capture and recapture is also seven. Information like this is key to linking a single annulus to one year of growth, but such information relies heavily on time and chance.

The most common method of age validation is marginal increment analysis. The marginal increment is the measurement from the last annulus to the margin. Each graph above represents the average monthly marginal increment values for one year for ages one through four. The marginal increment cycles down only once during the year, meaning one annulus was deposited once per year.

The age data gathered from otolith examinations allow scientists to model growth rates, maximum age, age at maturity, and the trend of future generations. Visit the Stock Assessment section for more information.

Literature Cited
Campana, S.E. 2001. Accuracy, precision and quality control in age determination, including a review of the use and abuse of age validation methods. J. Fish Biol. 59:197–242.

Campana, S.E. and J.D. Neilson. 1985. Microstructure of fish otoliths. Can. J. Fish. Aquat. Sci. 42:1014–1032. Popper, A.N. and Z. Lu. 2000. Structure-function relationships in fish otolith organs. Fisheries Research 46(2000):15–25.

Taylor, R.G., J.A. Whittington, H.J. Grier, R.E. Crabtree. 2000. Age, growth, maturation, and protandric sex reversal in common snook, Centropomis undecimalis, from the east and west coasts of South Florida. Fish. Bul. 98:617.