Otoliths help FWRI biologists to determine age and derive
age-related information as well as growth rates of various fishes.
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:
- 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
- Asteriscus: This type of otolith is involved
in the detection of sound and the process of hearing.
- Lapillus: This type of otolith is involved in
the detection of gravitational force and sound (Popper and Lu
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
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
Assessment section for more information.
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.