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Ageing fish

Why do we need to know the age?

Age determination is a very important aspect of fisheries research. To assess the stocks, we need to know the age structure of the fish being caught. The age structure allows us to estimate the fishing rate, and hence the number of fish in the stock. The growth rate is used to convert numbers to biomass and the strength of each year class is used to forecast stock sizes in the future.

How are fish aged?

There are several parts of a fish's body that can be used for ageing. Scales, vertebrae and various bones can all be utilised but otoliths are widely accepted as the best structure for many marine species. Otoliths are small bony structures situated in the head and are composed mainly of calcium carbonate. They are part of the ear system and are often referred to as "ear stones". As the fish grows, new material is deposited on the surface of the otolith and changes in the environment and the fish's life cycle cause the structure of each layer to alter in the form of opaque and translucent material.

Photo: Otolith
North Sea cod otolith - section viewed by reflected light - Age 5

A section taken through the centre of the otolith reveals a pattern of rings similar to that seen on the cross section of a tree, the opaque zone representing the main growth period of the fish in the summer and the translucent zone the slower winter growth phase. Each pair of rings represents a year in the fish's life.

Photo: Plaice otolith
Plaice otolith - 4 years old

Otoliths of roundfish species such as cod, haddock and whiting have fairly large otoliths which have to be sectioned in order to observe the ring structure clearly whereas flatfish otoliths are thinner and the structure can often be seen clearly on the whole otolith particularly on young fish. Older flatfish have thicker otoliths and the rings are not so easily seen by this method. These, along with young fish that are not easily aged on the whole otolith need to be broken across the nucleus and lightly burnt to see the rings more clearly. Plaice, sole and other flatfish contain a protein band at the end of each translucent zone. When burnt this produces a thin brownish-black line, which assists in the identification of the ring structure. The protein band can also be stained and this method is used extensively for soles and shows the ring structure very clearly.

Photo: Stained sole otolith - approx 37 years old
Stained sole otolith - approximately 37 years old

Cefas have one of the foremost otolith processing and ageing sections in Europe. Each year we age approximately 60-70,000 otoliths from over 20 different species from around the British Isles. More information on ageing fish.

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Last Modified: 27 April 2014