Kastelle et al.: Anoplopoma Fimbria age validation 



293 



and Chilton, 1982; Beamish et al., 1983). The age 

 range considered by Beamish and Chilton (1982) 

 was from to 43 years, but their study had few 

 individuals from the upper end of the age range. The 

 OTC method can validate only incremental growth 

 zones after injection, leaving interpretation of ear- 

 lier growth in any one fish questionable. Ideally, the 

 age range of OTC-injected fish spans all ages. 

 Younger OTC-injected fish can then be used to in- 

 fer that incremental growth zones seen prior to an 

 OTC injection on older fish are annuli. With these 

 limitations, the procedures used by Beamish and 

 Chilton (1982) and Beamish et al. (1983) confirmed 

 that ages counted on the burnt cross-section were 

 accurate. 



Our goal was to use radiometric dating techniques 

 to validate the break-and-burn ageing criteria used 

 at the AFSC for sablefish aged to approximately 35 

 years. This validation used the measured ratio of 

 Pb-210/Ra-226 in the otoliths to provide estimates 

 of total age, thereby complementing previous OTC 

 validation work which only confirmed incremental 

 growth. 



In the first application of radiometric ageing to 

 fishes, Bennett et al. ( 1982) used the ratio of Pb-210/ 

 Ra-226 to validate ages up to 80 years for splitnose 

 rockfish (Sebastes diploproa). More recently, these 

 radioisotopes were used by Campana et al. (1990) 

 and Fenton et al. (1990, 1991) in age validation and 

 longevity studies in a variety of fish species. Addi- 

 tional radioisotope pairs such as Th-228/Ra-228 

 have also been used to age fish (Smith et al., 1991). 



The isotopes Ra-226 and Pb-210 are part of the 

 naturally occurring decay chain of U-238: 



L/-238 > Ra-226 > Rn-222- 



4.5xl0 9 3T KSOOvr 



Pb-210 



3.8 d 



Pb-210- ->Po-210 >P6-206 



22.3 vr 138 d 



(1) 



where the dashed lines indicate short-lived interme- 

 diary nuclides that are not shown. 



Both Ra-226 and Pb-210 are found naturally in 

 seawater. Ra-226 is a calcium (Ca) analogue which 

 accompanies Ca through the food chain and is de- 

 posited in fish tissue, particularly calcified struc- 

 tures, along with Ca (Swanson, 1985; Porntep- 

 kasemsan and Nevissi, 1990). The otoliths of teleo- 

 sts consist of an acellular organic protein matrix 

 mineralized with aragonite, a form of calcium car- 

 bonate in which the radioisotopes are deposited 

 (Mugiya, 1977; Campana and Neilson, 1985). Pb-210 

 is also accumulated by the biota through the food 

 chain (Shannon et al., 1970; Heyraund and Cherry, 

 1979). In fish, Pb-210 is preferentially deposited in 



the bone or liver 2 (Swanson, 1985). Its initial activ- 

 ity in the otolith must be measured (later as R*) for 

 the application of radiometric ageing. 



When Ra-226 is deposited in otoliths, like Ca it 

 remains immobile, and a disequilibrium is created 

 between Ra-226 and all of its progeny. With time, 

 the activity of shorter-lived daughter products like 

 Pb-210 will increase. In the pair of radioisotopes 

 used here, Ra-226 and Pb-210, the difference be- 

 tween their half-lives is great (Eq. 1). Therefore, 

 after about 100 years in a closed system the activ- 

 ity of both Ra-226 and Pb-210 will become equal, 

 establishing a so called "state of secular equilibrium" 

 (Faure, 1986). A chronometer is started when Ra- 

 226 is first deposited in the otolith, and the activity 

 ratio Pb-210/Ra-226 is a function of the time elapsed 

 since deposition. 



Radiometric dating applied to fish otoliths relies 

 on three basic assumptions (Faure, 1986; Smith et 

 al., 1991): 



1 The otoliths are closed with respect to the loss or 

 gain of any radioisotopes in the decay chain. 



2 The initial activity ratio of Pb-210/Ra-226 in the 

 otoliths should be much smaller than one, ideally 

 close to zero, and known or measured. 



3 The specific activity [disintegrations per minute 

 per gram (dpm/g)] of the radioisotopes in the 

 material incorporated into the otoliths must be 

 constant. 



These assumptions will be considered in detail 

 later in the "Discussion" section. But first, it is im- 

 portant to consider Assumption 3 because it explains 

 why we did not use whole otoliths. Assumption 3 is 

 the most problematic of the three assumptions when 

 applied to whole otoliths because it also requires 

 assuming a mass-growth rate for the otoliths (Smith 

 et al., 1991). Campana et al. (1990) used otolith 

 cores in their application of radiometric dating to 

 fish. When using otolith cores and individual mea- 

 surements of Ra-226 for each sample being radio- 

 metrically aged, Assumption 3 becomes unnecessary. 

 However, if measurements of Ra-226 are averaged 

 over the different samples being radiometrically 

 aged, Assumption 3 requires that the different core 

 samples have the same activity levels. Our study 

 followed the procedures of Campana et al. (1990) 

 and used otoliths cores. But unlike Campana et al. 

 ( 1990), we used individual Ra-226 measurements for 

 each sample being aged so that Assumption 3 was 

 unnecessary. 



Noshkin, V. E., K. M. Wong, R. J. Eagle, T. A. Jokela, and J. 

 A. Brunk. 1988. Radionuclide concentrations in fish and inver- 

 tebrates from Bikini Atoll. Lawrence Livermore National Labo- 

 ratory, Livermore, Ca., UCRL-53846, 53 p. 



