I happened to see a picture of a scale of a fish recently, illustrating the age of the fish. Being able to age a fish could help fisheries managers with a variety of aspects of fisheries management, such as estimating growth rates, survival, and longevity. It could also help better understand aspects of habitat, food abundance and availability, and a myriad of other factors. But how could you age a fish?
Most everyone knows that the age of a tree can be estimated by counting growth rings. Growth rings in trees are a phenomenon of temperate climates and are the result of the seasonal activity of the cambium. Variations in how the cells are produced in an annual increment of wood (e.g. cell density and cell wall thickness) differ between what is called spring wood and summer wood. It is a reflection of the seasonality of the growing conditions of the year. As a result, these observable “growth rings” can be used to age the tree.
Just as the age of a tree can be estimated by counting the growth rings, a similar technique can be used to age fish. Because fish are cold blooded, in temperate climates they typically put on most of their growth during the warmer months, and very little growth during the colder months. Those differences in growth rate are recorded in some structures in the fish in much the same way as the growth rings form in trees.
Several techniques have been developed for aging fish, and these techniques use various parts of a fish. For some fish species the otolith or “earstone” can be used to estimate age. For other species the fin rays are used to estimate age. A thin section of vertebrae can also be used. However, these techniques usually require the fish to be dead or killed for age determination.
However, a method involving the collection and analysis of fish scales can be used to determine the age of live fish. The number of scales on a fish does not change. However, as a fish grows, the size of the scale changes and those changes occur in observable ways.
At the edge of a fish scale is a little ridge (circuli). When growth is slow, for example during winter, these ridges are close together. Conversely when growth is rapid, such as would be expected during the summer months, the ridges are more spaced apart. So with a little training and a microscope or dissecting scope, a person can learn to identify an annual increment in the scale, and thus age a fish.
~Chuck Lura
