298 



Fishery Bulletin 92(2), 1994 



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10 15 20 25 30 35 40 45 50 



Burnt Cross-section Age (yr), Reader 2 



-5 5 10 15 20 25 30 35 40 45 50 



Burnt Cross-section Age (yr), Reader 3 



Figure 2 



Comparison of burnt otolith cross-section ages 

 (from Readers 1, 2, and 3) with adjusted radiomet- 

 ric ages (4.5 years were subtracted for the time 

 between capture and analysis I. Range in adjusted 

 radiometric ages is 1 standard deviation, and the 

 range in burnt otolith cross-section ages are the 

 maximum and minimum in each age category (see 

 Table 1 ). Ranges of burnt cross-section ages for 

 Readers 2 and 3 are wider than for Reader 1 be- 

 cause age categories were defined by Reader 1 

 ages. 



1982; Swanson, 1985; Fenton et al., 1990). Addition- 

 ally, feeding rate, metabolic rate, and calcium depo- 

 sition rate could all affect the specific activities 

 found in otoliths. Sablefish are one of the fastest- 

 growing epipelagic juvenile fishes (Shenker and 

 011a, 1986; Kendall and Matarese, 1987). Therefore, 

 the higher Ra-226 and Pb-210 activity levels seen 

 in sablefish otolith cores could be related to rapid 

 uptake of Ra-226. 



Assumptions for radiometric dating 



We discussed three assumptions which must be sat- 

 isfied for the radioisotope ageing of sablefish to be 

 valid. Assumption 1, that otolith cores are closed 

 with respect to loss or gain of any radioisotopes in 

 the decay chain, has not been tested. Considering 

 the decay chain containing Ra-226 (Eq. 1), Rn-222 

 is a source of concern. For Ra-226 to decay to Pb- 

 210, it must first become Rn-222 (half-life=3.82 

 days) which is a noble gas. It is conceivable that Rn- 

 222 could migrate in the otolith. A loss of Rn-222 

 would lead to underestimation of the true age. How- 

 ever, the calcium carbonate crystalline structure of 

 the otolith probably prevents radioisotopes from 

 migrating. 3 



Welden (1984) measured Pb-210 activity in the 

 calcified cartilage of vertebral centrum from sharks, 

 applying a procedure similar to the dating of sedi- 

 ments. The calcified cartilage allowed the Pb-210 to 

 migrate in the vertebra so the assumption of a 

 closed system did not hold true. Previous research 

 by Goreau and Goreau (1960), Moore et al. (1973), 

 Dodge and Thomson (1974), and Veeh and Burnett 

 (1982) confirmed that calcium carbonate in coral 

 acted as a chemically closed system. In Rn-222 dif- 

 fusion experiments, Moore et al. (1973) could not 

 detect migrations in coral. 



For our study, Assumption 2 required that in the 

 core of otoliths, initial ratios of the two radioisotopes 

 be measured or known, and ideally be near zero. 

 More Ra-226 than Pb-210 may be encountered by 

 fish since Ra-226 is not in equilibrium with Pb-210 

 in seawater (Bacon et al., 1976). The environmen- 

 tal residence time of Ra-226 in seawater has been 

 reported to be as high as 950 years (Szabo, 1967) 

 and as low as 0.7 to 5.5 years (Shannon and Cherry, 

 1971). The environmental residence time of Pb-210 

 in surface waters has been reported to be as high 



3 At the "CSIRO, International workshop on otolith chemistry" 

 2-6 March 1992, in Hobart, Tasmania, Australia the possibil- 

 ity of Rn-222 migration was discussed with some enthusiasm. 

 G. Fenton suggested that the otolith is relatively impermeable, 

 and that Rn-222 migration is not a major problem 



