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The Ears tell all

The age of fish is biological information necessary to estimate their population, to facilitate the structure of piscicultural environment and to protect species. This knowledge is often the result of studying scales, bones, the rays of the fins or the otoliths. Scientists are all ears ! By studying the calcified tissues (asseochronometry) fish biologists discover the age of a fish, its history and its environment. This operation demands first of all a reading of the basic structures of the calcified tissue especially its kernel and the growth rings, which leads to an interpretation relating to the biological cycle of the fish (hatching, reproduction, development, migration) or to periodical cycles connected to vital needs (temperature, food). The accumulation of all this information reveals the precise life history of the fish concerned. Today the otoliths provide the best source of information for the scientists. The internal ear of the fish has three pairs of otoliths. These mineralized tissues which allow the fish to hear and to keep its balance are interesting because they begin to form at the embryonic stage of growth. Their shape which is uniform according to the species is a hollow surface with rings. It is easy to observe the alternate concentric zones or annuli from which the age of the fish can be calculated (in the same way as with the growth rings of a tree).

Each year both a light and a dark zone are produced. The first year the daily concentric rings are a real indicator of the development of the fish from larva to juvenile. The thickness and the number of zones visible as well as the chemical composition indicates with precision the various stages in the growth of the fish and its feeding habits : the biggest rings represent for example a period of increased growth.