Radiochemical Dating

Age determination studies based on elemental or isotopic concentrations in the otolith are motivated by the ongoing requirement for accurate age-structured information in support of many fish studies. The objectivity of elemental assays is appealing in light of the subjectivity that can confound or invalidate the interpretation of annuli in otoliths or other structures. As a result, all chemically-based age determination studies to date have focused on the yearly scale, and virtually all have been used to validate more traditional, less expensive methods of age determination.

Radiochemical dating of calcified structures has a long history in corals and molluscs. The same underlying concepts apply to fish otoliths, and are based on well established physical principles governing radioactive decay. Radioisotopes are incorporated into fish otoliths in exactly the same way as are stable isotopes of any given element. Once incorporated into the otolith, the radioisotopes decay into radioactive daughter products, which are themselves retained within the acellular crystalline structure. Since the half-lives of the parent and daughter isotopes are known (and fixed), the ratio between them is an index of elapsed time since incorporation of the parent isotope into the otolith. Secular equilibrium is approached as the rate of loss (through decay) of the daughter comes to equal the rate of loss of the parent. Radioisotope ratios in whole otoliths can be interpreted if some rather problematic assumptions can be met. However, the extracted otolith core reflects elapsed time since core formation, which in turn is very similar to the age of the fish. Since interpretation of the otolith core also avoids the problematic assumptions, it is widely acknowledged to provide more reliable results than would the whole otolith.

Two isotopic pairs have proved popular in ageing problematic fish species: 210Pb : 226Ra and 228Th : 228Ra. When based on analysis of the extracted core, the radiochemical approach appears to be both objective and accurate in the estimation of fish age. Nevertheless, the isotopic concentrations requiring measurement are exceedingly low, resulting in assay precisions which are often less than optimal. Current discriminatory power is on the order of 5 yr for 210Pb : 226Ra and 1-2 yr for 228Th : 228Ra, over age ranges of 0-40 and 0-8 yr, respectively. Therefore, this approach is best suited to species where the candidate age interpretations are widely divergent, such as in Sebastes or Hoplostethus.

Examples of radiochemical dating as applied to age validation in temperate and tropical species are presented in Campana et al. (1990), Smith et al. (1991), and Campana et al. (1993).