Marine Scotland Science collects biological information on the length and age of fish from sources such as: market sampling; observer trips and research vessel surveys. By determining the length and age of a large number of fish, the size and age structure of the whole population can be estimated. This is important in determining how a stock is affected by fishing.
The most common method used to age fish is by examining the ear bones, or otoliths, which are found in the head of the fish, lying on either side of the brain. These bony structures can be used to give an accurate picture of the life history of the individual fish.
Otoliths are easily removed and prepared by a variety of means to show their structure. They consist of a series of layers which are laid down as the fish grows, in a fashion similar to the rings on a tree. In summer the layers are close together, giving the appearance of a light band. In winter when growth is not so active, the layers are spaced further apart giving the appearance of a dark band. The age of the fish can be determined by counting the number of bands on the otolith (see picture below). The annual band pattern is used in large-scale population studies.
However, rings are laid down on a daily basis in the otolith and these are most visible in the early life of the fish.
This information can be used to determine when individual larvae actually hatched and provides possibilities for studying life history, stock structure and other aspects of fish biology at both the individual and population level.
The picture shows an otolith taken from a seven year old cod (magnification approximately x20)