Age Of Fish

Introduction

For many species, analyzing hard body structures can make a direct measure of age. In temperate-zone waters, both fish and shellfish species exhibit seasonal growth patterns indicative of age. Generally, growth is rapid during warm "summer" months, and slow during cold "winter" months.

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.

Depending on the species, scale samples are taken from different locations on the body. Scales must be taken from an area the fish known to exhibit complete and clear growth patterns.

Any scale removed is re-generated by the fish. Otoliths, fin rays and gill opercula can all be used to age fish, but usually their use means the fish must be killed, so scales are preferred.

As fish grow, they deposit minerals in their skeletal tissues, producing characteristic growth patterns. One year of growth consists of one summer zone plus one winter zone. In bones, these patterns are called annuli and in scales they are called circuli. The annulus is usually defined as the winter zone. Summer and winter growth zones differ in appearance, thus providing the basis for age determinations.

Length proportional to the growth of the age structure is used as a basis for empirical relationships. Different periods of growth can be determined bycounting the light and dark bands typical of annuli or by observing the differences in spacing of the circuli. By assessing these patterns the age of the fish can be determined.

How old is a fish scale ?

On the right is dried scale of a barramundi showing the growth rings, or annuli. There are four main types of scales which cover the fish. They are

• Cycloid Scales - smooth scales that sprout on true bony fish such as carp.
• Ctenoid Scales - these scales have tiny teeth around their edges, characteristic of Bass
• Ganoid Scales - shiny, hard and diamond-shaped, characterize primitive bony fish, such as the Gar
• Placoid Scales - thorny scales characteristic of many cartilaginous fish, such as the shark

As cycloid and ctenoid scales increase in size, growth rings called circuli become visible. These rings look a little like the growth rings in the trunk of a tree.

During the cooler months of the year the scale grows more slowly and the circuli are closer together leaving a dark band called an annulus . By counting the annuli it is possible estimate the age of the fish. This technique is extensively used by fisheries biologists.

Otoliths

Otoliths are found in the ear of all teleost (bony) fish (ie. all fish except sharks, fins and rays). They are located in the saccular endolymph of the brain cavity, and aid in hearing and balance in the fish.

Otoliths are made up of crystalline aragonite and a protein matrix. They grow concentrically from inside to outside, much like the rings of a tree. Light and dark bands on the otolith represent periods of high and low growth respectively, either on yearly, monthly or daily cycles.

Otoliths are composed of around 90% calcium carbonate (CaCO3), mostly in the form of aragonite. The other 10% of the otolith is minor and trace elements within the aragonitic matrix that are derived from the water surrounding the fish. These impurities reflect the water chemistry, as well as the fish's metabolism.

Otoliths are of particular use to scientists because they are metabolically inert, meaning that there is no re-adsorption and once any material is added to the otolith, it remains there un-altered. The concentric nature of the otolith also provides a chronological life history of the fish, from the fish's embryonic stages at the center to its death in the outside ring.

Removal of Otilith

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.

   

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.

Reading an Otilith

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 !