Testing for loss of 222Rn from Pacific halibut ( Hippoglossus stenolepis) otoliths

Abstract

Radiometric ageing of fish by measuring the disequilibrium of 210Pb/ 226Ra found in otoliths has been used by numerous researchers in over 15 different studies. The ultimate goal of the previous research has been to validate conventional ageing methods. If 222Rn, an intermediary between 226Ra and 210Pb, escapes from the otoliths, the radiometric age would be biased young. The goal of this study was to test for this bias using Pacific halibut ( Hippoglossus stenolepis) otoliths collected in 1960 and analyzed in 1997. If 222Rn loss occurred, the estimated radiometric age would be less than the 37 year span between collection and analysis. In comparison to other fish species, Pacific halibut otoliths were found to have an unusually low activity of 210Pb and 226Ra. Low activity measurements lead to radiometric age estimates with very large estimated errors. All estimated radiometric ages for the Pacific halibut otoliths were much greater than 37 years. This suggests that 222Rn is not escaping from the otoliths and that radiometric ages are not biased low. However, considering the estimated error, the results are not definitive. This study should be repeated using a different species that is known to have a higher activity of 210Pb and 226Ra.