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.
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.
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.
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.