Otoliths, removal and ageing

Otolith definition, how to remove otoliths and determine age.

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Otolith definition

Otoliths (ear stones) are found in the head of all fish other than sharks, rays and lampreys.

These pearly white stones are about the size of a pea, and can be found in the fish's skull just below the rear of the brain. They aren't attached to the skull, but rather float beneath the brain inside the soft, transparent inner ear canals.

There are 3 pairs of otoliths in each fish, including 1 large pair (the sagittae) and 2 small pairs (the lapilli and the asteriscii).

The largest pair is usually used for determining age. The smaller pairs are about the size of the tip of a pin. However, despite their size, the smallest pair (the lapilli) is most often used for daily ring ageing.

Properties

Otoliths are rocks, not bones. This property makes them more durable than bone.

The growth of the otolith is a one-way process. New otolith material can be (and is) added to the outside surface through time, but existing material can't be removed. This one-way growth process explains why otoliths can form and retain such delicate structures as daily rings, whereas bone cannot.

Shape

Otoliths have a very distinct shape, which is characteristic of the species of fish. That is, different fish species have differently shaped otoliths.

The otolith shape is so distinctive that biologists can use otoliths recovered from seal and bird stomachs and droppings to determine the type of fish they ate. Even the size of the otolith can be used to indicate the size of the fish that was eaten.

Otoliths examples, including the sagittae, lapilli and asteriscii.

Otoliths examples, including the sagittae, lapilli and asteriscii.

Otolith examples, including those from a cod, a redfish and a hake.

Otolith examples, including those from a cod, a redfish and a hake.

Removing otoliths

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.

To remove a pair of otoliths:

  1. use a knife with at least a 15 to 20 cm blade
    • the blade 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
    • 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 30 degree angle
  4. slice back and down about 1 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
  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
  7. use forceps to pull out both otoliths
    • they won't be attached to anything other than soft tissue
    • clean off the otoliths with water or your fingers
    • store dry in a paper envelope until you're ready to age them

This approach works well for most fish species. However, other approaches work better for some fish. For a more detailed list of alternatives, see Dr. Dave Secor's online manual, Otolith Removal and Preparation for Microstructural Examination.

A diagram showing the lines along which to cut in order to expose otoliths.

A diagram showing the lines along which to cut in order to expose otoliths.

Head and skull of a fish cut open to expose the brain (central white portion of image) and sagittal otoliths on either side.

Head and skull of a fish cut open to expose the brain (central white portion of image) and sagittal otoliths on either side.

Determining age

We've used many parts of fish to determine their age, including:

  • bones
  • scales
  • otoliths
  • fin rays

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.

Reading thin otoliths

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 microscope. 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's 1 mm or less, or if the thickness is less than 1/8 that of the total length, you may be able to read it
  • 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.

Cracking otoliths

If annuli aren't visible in the whole otolith, you'll have to crack the otolith in half, then lightly burn it, to make the annuli visible. To do this, you'll need:

  • some vegetable oil
  • forceps or tweezers
  • a dissecting microscope
  • a piece of clay or plasticine
  • an alcohol burner or candle

To crack the otolith, 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, usually found on the convex side (outward-facing curve). Place your thumbnail over the otolith centre and 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. 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 can't be aged.

Burning otoliths

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 dark rings.

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
  • hold the otolith about 1 cm over the top of the flame, near the cracked surface
    • in 5 to 15 seconds, the otolith should start to turn brown
    • try to avoid getting soot on the cracked surface (use a still, clean-burning flame)
  • remove the otolith from the flame when it's a medium brown colour, or if it starts to turn grey, remove it from the flame
  • put it on the counter to cool

Reading otoliths

You can read the otolith once it's cool. To do this:

  • 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
  • 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 with an old cloth or on a whetstone, and then add another drop of oil

The annuli should be visible as thin but prominent brown or black lines. Keep in mind 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. In general, 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.

Sectioned cod otolith showing annular growth increments (annuli).

Sectioned cod otolith showing annular growth increments (annuli).

Intact gaspereau otolith showing annuli.

Intact gaspereau otolith showing annuli.

Flounder otolith which has been cracked and burned to show annuli.

Flounder otolith which has been cracked and burned to show annuli.

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