Age Determination

It is often very useful for scientists to know the age of a fish. For instance, it can be of great importance when devising management strategies or estimating the effects of fishing and fish stocks to know how many fish of each age are in a population (age structures), age at sex change, age at sexual maturation, or how long fish live.
Have you ever wondered how scientists tell how old a fish is? You can't tell by looking at them. To get an accurate age of a fish, scientists need to look a little deeper, and have a look at the fishes' ear bones.

Removing the otolith from a Spanish mackerel. Photo by CRC Reef.

All fish have ear bones, called otoliths, which are usually found under either side of the fish’s brain case. Otoliths function to help fish keep their ‘balance’, or stay upright in the water, but also play a key role in how we age fish.

While all fish have otoliths, (in fact they have six, three for each side of their head), they look quite different in different species. Some of the larger fish such as marlin and maori wrasse have very small otoliths (in the case of marlin these can be the size of a pin head), while some of the small snappers have much larger otoliths (about the size of a 5c or 10c piece).

Otoliths grow continuously throughout the lifetime of a fish by laying down bony calcified material at different rates throughout the year.

Growth is slower in the winter and faster in the summer months. The different rates of growth give a different appearance to the material laid down: in winter it looks milky white and opaque, in summer the material is much clearer, even translucent.

Fish ear bone Photo: CRC Reef

The result is a banding pattern on the otolith, alternating between the milky white (winter) and translucent bands (summer) as you can see in the picture. Each set of bands represents one year of growth. By counting the number of milky white bands on an otolith, the age of the fish can be determined, just like counting growth rings in a tree trunk.

There can be a large difference in how easy it is to see the banding pattern on otoliths from various species of fish. Coral trout are probably one of the most difficult species to age. With these otoliths, it is usually necessary to cut a very thin slice of the otolith with a diamond edged saw, polish the thin section and view the bands under a microscope.

Otoliths from other species of fish are much easier to read. For instance, the bands in the otoliths of red-throat emperor can usually be seen on the whole otoliths and do not require cutting. Interestingly, fish scales have similar annual bands in some species. Unfortunately, while being much easier to get, scales do not give a very reliable indication of the fishes' age because they can be lost and replaced over the life of the fish.

Scientists are now using otoliths for another purpose. The composition of the calcified material of the otoliths can also provide valuable information. Minute differences in the chemical composition of the otoliths can indicate patterns in the environment in which the fish have been growing, and possibly indicate geographic diversity in fish stocks or populations (often referred to as ‘stock structure’). For example, are red-throat emperor in the Swains a separate group from those off Townsville? Micro-chemical analysis of otoliths will help scientists provide the answer to this question. This can be taken one step further. Similar analysis of different sections of the otoliths can provide information on migration history and dispersal patterns of the fish as juveniles, and even during the larval stages. By relating these chemical signals incorporated in the otoliths as they grow to the water chemistry in different regions, scientists can build up a road map of where each fish has spent its time from hatching to death. Unfortunately these analyses are very expensive, so they are used only when we have a pretty high expectation that they will tell a sensible story.

CRC REEF FISH AGEING FACILITY AND EXPERTISE

The CRC Reef Research Centre possesses an advanced fish ageing facility (CRC Reef Fish Otolith Laboratory) as well as a team of experts who specialise in age determination of both tropical and temperate marine fish species. Established in 1994 with its particular focus of tropical coral reef fish species, the CRC Reef Fish Otolith Laboratory now has the most modern of fish ageing instruments, ranging from an automatic grinding and polishing machine to computerised otolith reading systems.

In addition, members of CRC Reef Fishing & Fisheries research team have substantial experience in ageing tropical fish. Led by Dr Dong C. Lou, the Fishing & Fisheries fish ageing team has aged approximately 50,000 fish otoliths from over 100 coral reef fish species (see below) in the last seven years.

List of major fish species aged by the Fishing & Fisheries Fish Ageing Team.