Ottaway and Simkiss (1979) and Ottaway (1978) developed a method of incubating scales in vitro with C14 marked glycine. The scales must be extracted from live fish and incubated immediately with the glycine at temperatures like those in their habitat. The amount of radioactive glycine fixed by the osteoblasts which remain stuck to the scales by the end of a given period provides a basis for determining the rate of growth.
In normal cells, each cell has a fixed quantity of DNA, regardless of individual physiological condition. NRA present in the cell, on the other hand, is a direct anabolic function. The proportion of DNA and RNA is therefore an indicator of protein synthesis and growth (Bulow, 1987).
This method is based on the radioactive imbalance of the Ra226 fixed during otolith formation. The radioactive decay of this element produces Pb210. The proportion of the two isotopes indicates how much time has gone by since the incorporation of the Ra226. The Pb210 isotope is appropriate for age determination as its half-life of 22.3 years is comparable to fish longevity (Bennet et al., 1982; Campana et al., 1990).
The method requires species with a long life-span like Sebastes which can reach the age of 80, in which the changes can be evaluated (Bennett et al., 1982).
Lipofucsin is a lipopigment which forms increasingly with age in the cytoplasm, and is considered one of the clearest indicators of ageing in post-mytotic cells (Nandy, 1985).
The lipofucsin build-up in krill (Euphasia superba) has been used to determine the age composition of the population (Ettershank, 1985). Its presence in cerebral tissues is a function of age and can be used to determine fish age (Hill and Radtke, 1988; Hill and Womersley, 1991).
Growth can be measured directly by capture, marking and subsequent release into the environment (Ikenouye and Masuzawa, 1968). Where a set of initial length values and length at recapture are available, and the time between tagging and recapture is known, then growth can be determined by the method of Gulland and Holt (1959), Ford-Waldford (Ford, 1933) and Munro (1982).
Growth can be observed directly in fish raised in captivity, where length progression can be followed throughout the rearing period. Growth in captivity is not, however, comparable to growth under natural circumstances.
Growth can be established in sedentary populations which can be observed in the same place over long periods of time. This information is obtained by photographs or by drugging the fish and measuring them directly.
The sets of data obtained should make it possible to use the Gulland and Holt method (1959), or other methods (see 8.4).
