Scales, otolilths, fin rays and opercula can all be used to age fish as they grow in tandem with the fish.
The first step is to sample the fish population. This can be by using a seine net, electric fishing equipment or even rod and line. The species captured are then identified, measured and two to four scales removed. Scales are usually used for ageing as minimal stress and damage is inflicted on the fish as they are removed.
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. However, should eels require ageing, otoliths are the preferred method.
Scales, otolilths, fin rays and opercula can all be used to age fish as they grow in tandem with the fish. As England and Wales has a temperate climate and fish are cold blooded (poikilothermic) they have a defined growth season according to temperature. Whilst optimum temperatures vary between species, generally the growth season is between April/ May and October.
The growth season is represented on the body structure as growth rings being widely spaced (growth rings are laid down as the structure grows). As the fish stops growing in winter, the growth rings have very narrow spaces between them.
With the onset of the new growth season the next year, an annulus is formed as the fish begins to grow again. The number of annuli is equal to the age of the fish. Knowing the age of each fish and the length at which they were caught gives the average length at age.