Age determination in fish

The most commonly used techniques involve counting natural growth rings on the scales, otoliths, vertebrae, fin spines, eye lenses, teeth, or bones of the jaw, pectoral girdle, and opercular series.

[1] 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.

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.

[5] 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.)

Shortly after Hoffbauer's findings were published, structures other than scales were examined for utility of aging fish.

Johannes Reibisch, working for the Commission of Scientific Investigation of German Seas at Kiel, attempted to use Hoffbauer's techniques to age plaice (Plueronectes platessa) but found it difficult to accurately discern annuli.

[5] A fellow scientist also with the German Commission at Kiel, Friedriche Heincke, also frustrated with difficult scale annuli, further studied other structures to age fish.

Focusing on fish scales, Hjort developed an extensive aging program collecting statistics on birth rate, age-distribution and migration.

[6] Hjort's research elicited debate from the biomathematician D’Arcy Wentworth Thompson, who later rescinded his criticisms.

His research otherwise received glowing praise and would lead to fundamental changes in the way fish populations were studied and managed.

Such bony structures used for age estimation are vertebrae, opercula, fin rays, pectoral spines, among others.

Some bony structures such as fin rays and pectoral spines may be harvested without sacrificing the specimen, unlike otoliths.

[11] Preparation for bony parts involves first cleaning by soaking the structure in bleach or boiling to remove soft tissues.

Age structure analysis can be performed by the above methods which are the most direct, through estimates of length and weight, or a combination of both.