A Project for Class
How to Determine the Age of Fish
Scales, bones, fin rays and otoliths have all been used to determine
the age of fish, since these and other bony parts of fish often form
yearly rings (annuli) like those of a tree. However, otoliths
generally provide the most accurate ages, particularly in old fish.
|The easiest way to
"read" an otolith is to
take a slice, or cross
section, out of the
otolith with a special
saw and then count
the rings under a
However, unless you
have access to a low-speed diamond-bladed
saw in a laboratory,
you won't be able to age the otolith this way.
If the otolith is thin enough, it may be possible to count the annuli
without having to prepare the otolith first. Try measuring the
thickness of the otolith. If it is 1 mm or less, or if the thickness is
less than 1/8 that of the total length, you may be in luck. If you can
see alternating light and dark zones, you're probably looking at
annuli. They probably won't be as clear as those in a cross section,
but they should look roughly similar.
If annuli aren't visible in the whole otolith, you'll have to crack the
visible. To do this, you'll need a dissecting microscope, a piece of clay
or plasticine, forceps or tweezers, an alcohol burner or candle, and
some vegetable oil. To start with, you'll need to break the otolith
along its centre (length-wise). The easiest way to do this is to place
the otolith flat on the pad of your index finger, sulcus side up. The
sulcus is the groove carved into the top of the otolith, and is usually
found on the convex side (outward-facing curve). Take your thumb
nail and place it over the otolith centre. Then press down firmly
until the otolith snaps in half. Large otoliths can take A LOT of
pressure before breaking. If you can't break it, try using pliers. But
keep in mind that it's harder to control where the otolith breaks with
pliers. And an otolith that's broken too far from the centre line
cannot be aged.
An experienced otolith reader can age the cracked surface of the
otolith with nothing more than a light coating of oil and the
microscope. But you'll find it easier to read if you lightly burn the
cracked surface first. The burning makes the annuli stand out as
To burn the otolith, light the alcohol burner or candle and attach it
firmly to a solid, non-flammable counter or bench. Grab one of the
otolith halves with the forceps, holding it so that the cracked surface
is oriented vertically, facing towards you. Then hold the otolith
about 1 cm over the top of the flame, fairly near the cracked surface.
In 5-15 seconds, the otolith should start to turn brown. Try to avoid
getting soot on the cracked surface (eg- use a still, clean-burning
flame). When the otolith is a medium brown colour, or if it starts to
turn grey, remove it from the flame and put it on the counter to cool.
BE CAREFUL - it will stay hot for at least a minute!
Once cool, take the otolith half and embed the non-cracked tip in the
clay so that the cracked surface faces up. Spread a drop of vegetable
or cedar oil over the whole cracked surface. Then put the clay
holding the otolith under the microscope and focus at a
magnification of about 10X. If a lot of soot is visible, try rubbing it
off with an old cloth, or gently rub the cracked surface on a
whetstone. Then add another drop of oil.
The annuli should be visible as thin but prominent brown or black
lines. Keep in mind though that not every line is a yearly ring. So
count only those rings or groups of rings which are most prominent.
If no dark lines are visible, try re-burning the otolith. As a general
rule of thumb, the annuli nearest the centre are furthest apart, and
contain the most non-yearly lines. Later annuli (those nearest the
edge), such as would be seen in an old fish, tend to be closer
together and more regular in spacing. As a result, otoliths from
older fish tend to be easier to age than those from younger fish!
otolith annuli tend to be much less clear than those of a tree. So
don't be discouraged if you don't end up with a clear-cut age,
especially in a fish less than 3 years old. To find out about how
many annuli to expect, try referring to a book which tells you how
fast your fish species grows. But after ageing 10 or 20 otoliths, the
patterns tend to become more clear, and you might be surprised how
much more confidence you develop in your ages!