The use of the nuclear weapons-generated 14C pulse in the surface ocean waters has been accepted as a reliable method of estimating the ages of many species of surface-dwelling fish. Fish have carbonate structures (otoliths) in their ear cavities that grow more or less radially over the lifetime of the fish, in the process laying down visible zones that are thought to be deposited annually, and hence offer the possibility of allowing the age of the fish to be determined in a manner analogous to tree-ring counting. Observation of the rising edge of the 14C pulse provides a time marker that can be used to confirm the validity of the zone-count method of age measurement. The technique is important because determining the longevity of fish species is an essential tool for the management of economically significant fish stocks.
The bomb-14C method has generally been regarded as unsuitable for use in deep-water fish because of the time-lag in the penetration of the 14C to the depth inhabited by the fish and because of the smearing-out of the advancing 14C front. In fact, earlier studies went to considerable trouble to exclude deep-water otoliths that might contain bomb-14C ‘contamination'. Here, we present a work-in-progress description of measurements made on otoliths of two species, bluenose (Hyperoglyphe antarctica) and rubyfish (Plagiogeneion rubiginosum), that inhabit depths from 800 to 1000m in the adult stage. Oxygen isotope analyses on micro-samples taken along the otolith radii provide the temperature, and hence depth, history of the growing fish. AMS 14C measurements on samples along the otolith radii show i) the pre-bomb 14C level in the carbonate is similar to pre-bomb surface water levels, and ii) a sharp rise in 14C at a discrete position to a level higher than the surrounding DIC levels, not showing significant smearing due to depth.
We suggest that the explanation for these observations is that a significant proportion of the otolith carbon is derived from protein sources transferred directly from surface regions to the fish being studied, possibly by predation on near-surface-dwelling species. The presence of a well-defined 14C marker in the otoliths has allowed us to compare intervals of known duration with corresponding zone-counts, apparently confirming the annual deposition rates in rubyfish and raising questions about the counting of zones in bluenose. These results indicate that the use of bomb-14C can be applied to at least some deep-dwelling fish species.
See more of Poster Session I
See more of The 10th International Conference on Accelerator Mass Spectrometry (September 5-10, 2005)