As previously mentioned, compositional information (age-structure or size-structure information) form very valuable demographic data for making inferences about total mortality rates. Compositional information has also been used to make inferences about the relative cohort strengths, and more recently, information on the relative abundances of juvenile fish is now being used to improve the short-term forecasts of stock abundance. Other potentially useful types of compositional information include mean body weight, mean length, or mean age information. These simple measures of population composition are usually easier to obtain and are more cost-effective in terms of the information they provide relative to the cost of obtaining detailed age composition information.
Most often, composition information is obtained by randomly sampling the catch from commercial or recreational fisheries. Individual fish are then measured using hard bony structures (fin rays and more commonly otoliths or ear-bones). These aging structures lay down annual growth rings (annuli) much like the rings in a tree and the fish can be aged by simply counting the number of rings. Age composition can also be inferred directly from size composition data, where a length-frequency distribution is converted to an age-frequency distribution using an age-length key.
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