Age determination of mysticete whales using210Pb/226Ra disequilibria

Abstract

Accurate age determination is fundamental to the study of population structure and individual growth rates of mysticete whales. Here the disequilibrium between210Pb and226Ra in the tympanic bullae of two mysticete whale species was investigated for use as a chronometer. Radiometric ageing depends upon accumulation of the naturally occurring radionuclide226Ra (exclusive of other238U decay-chain members) in the bullae and subsequent retention of its progeny210Pb. Ages are determined from the210Pb/226Ra activity ratio. Samples were obtained from five gray whales (Eschrichtius robustus) with lengths of 4.5 (a neonate), 7.8, 8.7, 10, and 11.5 m, and two bowhead whales (Balaena mysticetus) with lengths of 12.9 and 17.4 m. In gray whales, radiometric ages were estimated in the three largest whales. In the neonate, the210Pb/226Ra ratio was above one and was not usable. The 7.8-m gray whale was used to determine the initial210Pb/226Ra ratio required for age determination. We propose a theory of gray whale bullae growth starting at the fetal stage with an open system (with a210Pb/226Ra > 1 and a fast growth rate), which transitions by 1 year old to a closed system (with a210Pb/226Ra << 1 and a slow growth rate). In both bowhead whales, radiometric age could not be estimated because the210Pb/226Ra ratio was above one. The excess210Pb in these bullae samples was likely accumulated from the whales' environment via prey, or in the case of the neonate gray whale, across the placental boundary. Our results indicate that the underlying assumptions of the210Pb/226Ra radiometric ageing method may not hold true in bowhead whales. Successful application of this method to bowhead whales is therefore doubtful.