|Can an otolith tell a fish's age?
|Question Date: 2003-04-27|
How to tell how old a fish is by Alison
Otoliths are ear stones, made of Calcium
Carbonate, located near the brain cavity of a
fish. Each fish has three types of otoliths,
and two of each type. The largest is called a
sagita, the medium a lapillus, and
the smallest asteriscus. For the fish,
these ear stones function for balance and
hearing, much like our ear
bones, but scientists use these to determine the
age of a fish.
The otoliths grow with the fish as
it ages. Usually a fish will have periods of fast
growth and periods of slow growth during a year,
these growth changes will provide changes in
patterns in the otolith causing annual rings to
form much like the trunk of a tree has a ring for
each year of its life. If the fish experiences
strong seasonal changes these annual rings, called
annuli, are easier to see. The otolith, usually a
sagitta, can be cut and examined under a
microscope to count the rings. Some fish are
easier to age than others.
Many parts of a fish can be used to tell its
including the scales, bones, fin rays, vertebrae,
and otoliths. These structures all provide
indications of age, since they tend to produce
annual growth rings, called annuli, like a tree.
Of these, the otolith (which is kind of like
an ear bone) tends to be the easiest to interpret.
As the fish grows, it undergoes annual changes in
its growth rate due to seasonal variations in
temperature, food abundance, etc. As the growth
rate changes the structure in the deposited ear
bone changes, and thus rings in the ear bone
correspond to these annual environmental changes.
However, it can be very difficult to count these
rings accurately, especially since sometimes
growth rates change due to factors other than
seasonal change in the environment (for example,
the fish might have been injured).
create false rings. Also the patterns tend to be
complex. Therefore, it takes a lot of experience
to read these otoliths accurately, and most
scientists like to use other forms of dating (such
as radioisotope analysis) to bolster their
Yes, by analyzing the shape and layered growth
pattern of an otolith, the age of a fish can be
determined. These layers bear a slight
resemblance to the growth rings of trees. Using
always newer and more powerful analysis tools,
scientist have been able to determine not only the
age but also many details of the life of different
fish species, previously unknown.
research trip to Japan in 2000 I heard a
presentation that explained how by measuring the
ratio of calcium to strontium by electron
microprobe analysis in the otoliths of eels the
researchers can tell the time spent by the eels in
river water and the time spent out in ocean
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