Seabass is a long-lived species and can reach up to 30 years of age. To determine how old a fish exactly is, scientists usually count the number of annual rings on the scales with a microscope. Each of these growth rings equals one year.
Bass scales have annual rings on them. These marks of growth originate each year during the winter months and are generally clearly visible, thus making it fairly easy to determine the age of the fish. This can be somewhat more difficult with bass of ten years and older: their scales are thicker and the annual rings are located closer to each other due to a slowdown in growth. The annual rings thus also disclose the speed of growth.
Source: Fritsch, 2005
A bass scale is roughly quadrangular shaped and made up of four fields. There is the front field – the one with the most prominent annual rings (annuli) on it – two lateral fields and a rear field. Only the last field is visible, the others being embedded in the skin and covered by the adjacent scales. The four fields are in contact at the starting point of development of the scale, which is called the nucleus.
The front field has two types of stripes that are arranged concentrically around the nucleus: the circuli and annuli. The circuli are fine, dark streaks that are usually very close to each other. They are often discontinuous and their frequency can vary from one scale to another on the same fish. Annuli are formed when a new growing season begins. They correspond to the little calcified area between two circuli, which appears clear and rather thick.
The white arrows point out the annual rings, the red arrows indicate the false rings.
Source: Fritsch, 2005
When counting annual rings, don’t be fooled by so called ‘false’ rings that you might encounter. These (often partial) rings are the result of unusual conditions encountered by the bass such as food shortage or climate ‘accidents’ as sudden colds. Such occassional disturbances manifest themselves, among others, by an additional mark on the scale. False rings are not related to growth slowdowns in winter, and should not be counted when reading age.