Introduction to Ageing Fish: What Are Otoliths?

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, sagitta is involved in the detection of sound and the process of hearing, or converting sound waves into electrical signals
2. Asteriscus: This type of otolith is involved in the detection of sound and the process of hearing.
3. Lapillus: This type of otolith is involved in the detection of gravitational force and sound (Popper and Lu 2000)

Different species have otoliths of different shapes and sizes; and cartilaginous fishes, such as sharks, skates, and rays, have none. This figure shows the growth rings of a sagittal otolith section viewed under reflected light. The dark translucent zone represents a period of fast growth. The white opaque zone represents a period of slower growth. Biologists estimate fish age by counting these opaque zones, called annuli, just as one would count rings on a tree to determine its age.

For each species to which it is applied, the method of estimating age by counting annuli must be validated to prove that one whole annulus is equal to one year of growth. There are several ways to validate age. The most obvious might be to raise 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, and the artificial environment may cause abnormal growth patterns (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).

Tagged and released wild fish can yield more accurate information on annulus deposition. This figure shows the fluorescent tag of a common snook otolith. The snook was captured and injected with OTC, which binds to the calcium in the otoliths. The fish was then dart-tagged and released. Seven years later, the fish was recaptured, sacrificed, and processed for aging. When the otolith was viewed using fluorescence microscopy, a glowing band appeared that corresponded to the injection of OTC. The number of annuli between the OTC band and the edge of the otolith was also seven, thus linking a single annulus to one year of growth (Taylor et al., 2000). However, 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 (or edge of the otolith). 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 each year, which means that one annulus was deposited once per year.

The age data gathered from otolith examinations allow scientists to estimate growth rates, maximum age, age at maturity, and trends of future generations. These data are used in age-based stock assessment models, to estimate mortality and population structure, to follow cohorts, to know species' longevity, and more. 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. Fish. Res. 46:15-25.

Taylor, R.G., J.A. Whittington, H.J. Grier, and 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. Bull. 98:617.