300 



Fishery Bulletin 92(2). 1994 



ever, our measurements of Ra-226 background were 

 similar to those of Fenton et al. (1991). 



The low activity of Ra-226 and Pb-210 in the 

 otoliths also requires that care be taken to avoid 

 contamination. The otoliths were stored in 50% 

 ethanol at sea. The grade of the ethanol was not 

 ultra-pure which could introduce contaminating 

 radioisotopes. Ethanol may also leach out some of 

 the radioisotopes during storage. Some previous fish 

 age-validation studies using Pb-210/Ra-226 (Fenton 

 et al., 1990) stored the otoliths dry with desiccated 

 adhering tissues. The dry tissue proved very diffi- 

 cult to remove and introduced Po-210 contamina- 

 tion. We relied on ultrasonic cleaning to break up 

 any soft hydrated tissue after which the otoliths 

 were rinsed thoroughly. This appeared to remove 

 any organic material on the surface of the otolith. 

 Contamination by Po-210 in the adherent tissues 

 would increase the estimated radiometric age. In our 

 study, the results from age category 1 indicate that 

 this was not the case. Also, the grinding paper used 

 could also introduce contamination. 



Conclusions 



The goal of this study was to validate the break-and- 

 burn otolith ageing criteria used for sablefish at the 

 AFSC. Although usually lower, the radiometric ages 

 were within two standard deviations of the break- 

 and-burn ages (Table 1; Fig. 2). If a range of pos- 

 sible ages occurred in the break-and-burn method, 

 the older age was usually chosen. Since radiomet- 

 ric ages were consistently lower than burnt cross- 

 section ages (from Reader 1), this method was prob- 

 ably not the best way to interpret the broken-and- 

 burnt otolith. Possible migration of Rn-222 from the 

 otolith, as discussed earlier, could also explain the 

 difference. 



Nevertheless, the break-and-burn method of age 

 determination for sablefish has been validated by 

 this study for fish up to 34 years old. This is the 

 maximum age regularly seen in commercial or re- 

 search catches. We have shown that the break-and- 

 burn ageing criteria applied to sablefish otoliths pro- 

 duces, on average, ages similar to radiometric ages. 



Acknowledgments 



We thank age readers from the Pacific Biological 

 Station, Canadian Department of Fisheries and 

 Oceans, and the Alaska Fisheries Science Center, 

 NMFS, for providing sablefish ages. We thank the 

 Scientific Editor and three anonymous reviewers for 

 comments which led to significant improvements in 

 the paper. 



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