Estimating the age of fish

Knowledge of fish age characteristics is necessary for stock assessments, and to develop management or conservation plans. Size is generally associated with age; however, there are variations in size at any particular age for most fish species making it difficult to estimate one from the other with precision. Therefore, researchers interested in determining a fish age look for structures which increase incrementally with age. The most commonly used techniques involve counting natural growth rings on the scales, otoliths, vertebrate, fin spines, eye lenses, teeth, or bones of the jaw, pectoral girdle, and opercular series. Even reliable aging techniques may vary among species; often, several different bony structures are compared among a population in order to determine the most accurate method.

Aristotle (ca. 340 B.C.) may have been the first scientist to speculate on the use of hard parts of fishes to determine age, stating in Historica Animalium that “the age of a scaly fish may be told by the size and hardness of its scales.” However, it wasn’t until the development of the microscope that more detailed studies were performed on the structure of scales.Antonie van Leeuwenhoek developed improved lenses which he went use in his creation of microscopes. He had a wide range of interests including the structure of fish scales from the European eel (Anguilla anguilla) and the burbot (Lota lota), species which were previously thought not to have scales. He observed that the scales contained “circular lines” and that each scale had the same number of these lines, and correctly inferred that the number of lines correlated to the age of the fish. He also correctly associated the darker areas of scale growth to the season of slowed growth, a characteristic he had previously observed in tree trunks. Leeuwenhoek’s work went widely undiscovered by fisheries researchers, and the discovery of fish aging structures is widely credited to Hans Hederström (e.g., Ricker 1975). Hederström examined the vertebrae of pike (Esox lucius) and concluded that each contained growth rings which could then be used to determine the fish’s age. In 1859, Robert Bell reported that one could use these growth rings to reliably determine the age of all fish after examination of sucker (Catastomus sp.) vertebrae and yellow perch (Perca flavescens) scales that he raised in a pond for two years showed “two rings or circles.”

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Wikipedia