Info for students

What is an Otolith?

What is an Otolith?

Some examples of otoliths. Shown here are the sagittae, lapilli, and asteriscii

Some examples of otoliths. Shown here are the sagittae, lapilli, and asteriscii

More examples of otoliths. Shown here are the otoliths from a cod, a redfish, and a hake.

More examples of otoliths. Shown here are the otoliths from a cod, a redfish, and a hake.

Otoliths, or "earstones", are found in the head of all fishes 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 are not 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; 1 large pair (the sagittae) and 2 small pairs (the lapilli and the asteriscii). It is the largest pair which is usually used for determining age. The smaller pairs are about the size of the tip of a pin. Despite their size however, the smallest pair (the lapilli) is most often used for daily ring ageing.

Otoliths are rocks, not bones. This property makes them more durable than bone. Indeed, 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 cannot be removed. This one-way growth process explains why otoliths can form and retain such delicate structures as daily rings, whereas bone cannot.

Otoliths have a very distinct shape which is characteristic of the species of fish. That is, different fish species have differently shaped otoliths. Indeed, the shape is so distinctive that biologists can use the 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.

Removing Otoliths

Removing Otoliths

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 (Centeral white portion of image) and sagittal otoliths on either side

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

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 30┬░ 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.

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 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".

Determining Age

Determining Age

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

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 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 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 otolith in half, then lightly burn it, to make the annuli 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 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. 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!

Accurate age determinations of fish using otoliths requires A LOT of experience. And 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!

A Class Project

A Class Project

Fisheries science can be interesting as well as educational. The class project described below combines hands-on biology, data collection, data interpretation and computer skills in a modular package well suited to a class of high school students or to individuals preparing an exhibit for a Science Fair. Though intended to expose the student to a range of scientific activities, this is not just a "make work" project: this type of activity represents a genuine portion of the job of a fisheries biologist.

The project is focused on the analysis of a group of fish, and the calculation of their rate of growth. Materials required include the following:

  • a sample of about 30 whole or gutted fish of various sizes; where the costs of purchasing these fish are too great, the heads alone can be used. Given appropriate lead time, fish processors and seafood markets will often save heads for teachers at no cost
  • measuring boards or rulers; at least one balance for weights
  • surgical type gloves (optional)
  • fish knives and forceps for otolith removal
  • dissecting microscope(s) for age determination, as well as clay and vegetable oil
  • computer program capable of simple graphs or charts; alternatively, graph paper can be used

Biological Sampling (1 hour): Working in pairs, the group weighs and measures each fish or head (a minimum of 2 fish per pair). Lengths should be recorded from the tip of the nose to the fork in the tail (or the tip of the tail if there is no fork). If heads are being used, the length is the tip of the nose to the back of the gill flap. Each head is then cut open and the otoliths removed, as described in How to remove an otolith. Otoliths can be stored dry until the next class, or if there is sufficient time, can be aged immediately.

Age Determination (1 hour): Work in pairs. Prepare and age each pair of otoliths (described in How to determine the age of a fish). Once completed, each person should circulate to the other stations to examine the other prepared otoliths. This is intended to familiarize the students with a broader range of otoliths and preparations. Upon return to their own otoliths, the pair should then reexamine their own otoliths, and if warranted, revise their ages. Once satisfied, an average age for each pair of otoliths (each fish) should be calculated, since the 2 otoliths may or may not result in the same ages. The average age for each fish should then be put up on the board, along with the corresponding length and weight. This will result in 3 columns of numbers - one each for age, length and weight. The data for a given fish must be all in the same row. All students should copy down the lengths, weights, and ages for all of the fish put down on the board (eg- there should be about 30 rows of numbers on the board). These numbers make up the data for the following class.

Data Analysis (1 hour): The data for any one fish may or may not be representative of other fish. It may not even be correct. But when a sufficient number of fish are examined, the overall trend is much more likely to be correct.

Working individually, the students should enter the data into a computerized spreadsheet or charting program. The data should be entered as they were recorded on the board; that is, one column for length, a second for weight, and a third for age, with one row per fish. Using either the spreadsheet program (or graph paper if computers are not available), the following graphs should be prepared:

Graph of fish length versus fish weight

Graph of fish length versus fish weight

Graph of fish length versus fish weight after log transformation

Graph of fish length versus fish weight after log transformation

Graph of fish weight versus fish age

Graph of fish weight versus fish age

Graph of fish length versus fish age

Graph of fish length versus fish age

Regressions (linear or exponential) can be fit to each of the above graphs as part of any spreadsheet. Alternatively, lines can be drawn through the data by eye. Students can then be asked to interpret the results.

The weight vs length graph indicates that the weight of the fish increases much more rapidly than does the length. Indeed, the second graph shows that the relationship is exponential (just like that of bank interest). The slope of the length vs age graph indicates the growth rate of the sample of fish. The spreadsheet will display the value for the slope when the regression is fitted. But if it doesn't, the slope can be calculated as the "rise over run" of the fitted line, or (greatest length on line minus smallest length on line) divided by the (greatest age on line minus the youngest age on line). This growth rate is the average expected growth of your fish each year. The fact that the growth rate in weight is curved indicates that its growth is much faster, and increases exponentially compared to that of length.

To calculate the expected length of the fish at, say, an age of 10 years, multiply the age (10) by the slope and add it to the intercept value from the regression. This calculation can be assigned to the students. They can also be asked how large the fish would be at an age of 100 years? And why is it dangerous to predict beyond the range of your data? If they answer that the relationship between fish length and age could change at non-observed ages, they would be right!

Graduate Student

Job and Graduate Student Opportunities

The Otolith Research Laboratory is part of a federal government (DFO) laboratory, and thus must follow government hiring practices. Very few permanent positions have been offered in recent years, and we do not expect that situation to change in the near future. If a position becomes available, notification will be posted here so that candidates may apply through the Public Service Commission. Preference is usually given to Canadian citizens. Regretfully, no permanent positions are currently available.

Some students have expressed an interest in volunteering their time on an unpaid basis in order to gain some hands-on experience with otoliths. We neither encourage nor discourage this practice, but will generally accept volunteers only if there is a specific duty requiring attention.

Although there are normally excellent opportunities for graduate (MSc or PhD) research on otoliths, no openings are currently available. Nor do we consider short-term (less than 1 year) visits by those seeking work experience as part of their university program. Areas of research are flexible, but recent projects have been more age-based. Interested students must have an excellent academic record, including full scholarship support. Quantitative skills are a plus.

Scholarship

Scholarship

For those who have reached tentative agreement to do graduate research under Dr. Campana, application should be made to the Biology Dept. of Dalhousie University. Since Dr. Campana is an Adjunct Professor there, his name should be listed as the prospective supervisor on the application.

The Biology Dept. requires that all graduate students receive a yearly stipend of $15,000 ($18,000 for foreign students), either from scholarship or the supervisor. This covers their tuition fees ($7000/year; $13,000 per year for foreign students), plus it gives them a modest living allowance. Self-funding is not permitted by Dalhousie, and we receive too many applications to consider taking on students without full scholarship support. NSERC is the primary source of scholarships for Canadian students with an A average. However, there are sometimes smaller scholarships available to specific groups of students (such as through employers). Some of these are listed on the Dalhousie Faculty of Graduate Studies web site. Others can be found through your undergraduate university. Foreign students generally find their best scholarship opportunities at the Canadian embassy, through their own country's government, or through the European Union (for European students). In general, Dalhousie does not offer scholarships to students other than those who already have NSERC support.

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