How to Remove an Otolith
Otoliths of adult fish can generally be removed with nothing more than a
sharp fish knife and a pair of forceps or tweezers. With a little practice, the
large pair of otoliths (the sagittae) can be removed in 15 seconds. Marine fish
such as cod and haddock have otoliths which are relatively large and therefore
easy to find (about 1 cm long in a 30 cm long fish). Smaller fish, such as
minnows, may require the use of a microscope.
There are many ways to remove a pair of otoliths. Here is one way:
1) Use a knife with at least a 15-20 cm blade. It should be as sharp
as possible. You'll also need a pair of forceps or tweezers about 10 cm long.
2) Grip the head of the fish by putting your thumb and forefinger in its eye
sockets (it IS dead remember!). Lay the body of the fish on a counter with the
tail pointing away from you.
3) Put the knife blade on the top of the fish's head about 1 eye diameter
behind the eyes. Slant the blade AWAY from you, at about a 300
angle. 4) Slice back and down about one head length. You should feel the knife cut
through the top of the skull. For flatfish and some other species, a vertical
cut through the top of the skull directly over the preopercle (the curved line
3/4 of the way back on the gill flap) also works well.
| 5) Check to see if you've cut the top off the skull. If you haven't, make
another slightly deeper cut. An ideal cut removes the top of the skull,
revealing the full length of the soft white brain underneath. Note that the
brain joins the much narrower (but still white) spinal cord at the rear. Once
the brain is visible, expose the brain even more by pressing the nose and body
down and towards each other. This should "snap" a portion of the
skull, and push the brain and otoliths up. Very often, this exposes the otoliths
and allows them to be removed immediately. |
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6) Push the rear of the brain to one side, or cut it out all together. The
large pair of otoliths should be visible underneath the rear of the brain, still
inside the skull. They may or may not be resting inside hollows in the base of
the skull.
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| 7) Use
forceps to pull out both otoliths. They will not be attached to anything other
than soft tissue. Clean off the otoliths with water or your fingers and store
dry in a paper envelope until you're ready to age them. |
8) Try another one! You might be surprised how much faster this one goes!
The above approach works well for most fish species. However, other
approaches work better for some fishes. For a more detailed list of
alternatives, see the on-line manual "Otolith Removal and Preparation for
Microstructural Examination".
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