Kastelle et al.: Anoplopoma fimbria age validation 



299 



as 3.5 years (Shannon et al., 1970) and as low as 

 1.4 years (Bacon et al., 1976). Pb-210 is incorporated 

 into particulate matter whereby it is removed from 

 the water column and deposited into sediments. To 

 correct for Pb-210 incorporation, we measured the 

 initial ratio, R*, with young fish (1 yr olds). By ana- 

 lyzing very young splitnose rockfish, Bennett et al. 

 (1982) found that the initial ratio of Pb-210/Ra-226 

 was between 0.1 and 0.2. Similar values for the ini- 

 tial ratio were also found in redfish (Sebastes 

 mentella) (Campana et al., 1990), orange roughy 

 (Hoplostethus atlanticus) (Fenton et al., 1991), and 

 blue grenadier (Macruronus novaezelandiae) 

 (Fenton et al., 1990). These results are comparable 

 with the R*=0 we used. The initial ratio cannot be 

 estimated from older age categories because their 

 true ages are uncertain. Therefore, R*=0 for older 

 age categories was assumed. 



Assumption 3 states that the specific activity of 

 Ra-226 incorporated into the otolith core be constant 

 over the time span that the otoliths are receiving the 

 radioisotopes. Assumption 3 is not required if only 

 the otolith core is used and individual Ra-226 mea- 

 surements are made for each age category. In sable- 

 fish the core of 1-year-old fish was appropriate be- 

 cause it is large, and there is a strong possibility 

 that fish migration prior to maturity might cause a 

 change in Ra-226 uptake. The likelihood ratio x 2 test 

 suggested that Ra-226 measurements differed 

 among age categories. Therefore, ratios calculated 

 by using individual Ra-226 measurements were pre- 

 ferred over those calculated with the Ra-226 mean. 

 Campana et al. (1990), using cores composed of the 

 first 5 years of growth, and a mean of 5 whole otolith 

 samples (13 g per sample), found no significant dif- 

 ference in Ra-226 activity between the cores and 

 whole otoliths. This suggested a constant rate of Ra- 

 226 uptake. The differences we found between the 

 Ra-226 activity measured in the four age categories 

 were considerable (Table 3). Because the factors 

 controlling Ra-226 uptake are complex and not well 

 understood, the observed differences may well be real. 



The use of whole otoliths requires modeling the 

 mass growth rate of the otolith over the life of the 

 fish and making assumptions concerning the uptake 

 of Ra-226 and Pb-210 each year. Bennett et al. 

 (1982) modeled the otolith's mass growth rate and 

 assumed that the uptake of Ra-226 over the life 

 span of the fish was proportional to otolith size (i.e. 

 they assumed a constant rate of uptake). In whole 

 otoliths from blue grenadier, the rate of uptake of 

 Ra-226 changed over the life of the fish (Fenton et 

 al., 1990). Therefore, the assumption of a constant 

 rate of uptake of Ra-226 was violated. In whole 

 otoliths from orange roughy, the Ra-226 specific 



activity increased with age (Fenton et al., 1991). 

 Therefore, Fenton et al. (1991) used two Ra-226 

 averages: one for young age categories and a second 

 for old age categories. 



The conservative approach is to use the otolith 

 core and individual category Ra-226 measurements 

 making Assumption 3 unnecessary. Also, by using 

 cores the closed system considered under Assump- 

 tions 1 and 2 is reduced. The behavior of otolith 

 material deposited later in a fish's life need not be 

 considered. The different approaches (otolith core, 

 whole otolith, larger multi-gram samples, or any av- 

 eraging of different age ranges) have trade-offs which 

 should be evaluated in light of a species' biology. 



Sources of error in ageing methodologies 



Differences between the three sets of burnt cross- 

 section ages are explainable. First, storage of bro- 

 ken-and-burnt otoliths in ethanol between readings 

 may cause a fading of growth patterns. Second, 

 variations in application of the otolith interpretation 

 criteria of Beamish and Chilton (1982) and Beamish 

 et al. (1983) could also lead to differences. 



Even with the high specific activity (compared 

 with other species) found in sablefish otoliths, the 

 generally low activity levels of Po-210 and Ra-226 

 found in otoliths makes it important to carefully 

 evaluate reagent blanks and background activities. 

 Therefore, activity levels for reagent blank and 

 background measurements reported in four previous 

 studies (Bennett et al., 1982; Campana et al., 1990; 

 and Fenton et al., 1990, 1991) were compared with 

 those found here (Table 2). Considering the magni- 

 tude of errors, the Po-210 background we found was 

 similar to other reported values (Table 2). The lit- 

 erature showed a much greater range of activities 

 measured in the reagent blanks. Like Bennett et al. 

 (1982), we found a nonsignificant activity level in 

 the Po-210 reagent blank. The Po-210 reagent blank 

 of 0.0103 ±0.0027 dpm reported by Fenton et al. 

 (1990, 1991) seems high compared with the other 

 findings. Also, we were the only study to report a 

 nonsignificant Ra-226 reagent blank activity. Except 

 for the Ra-226 reagent blank result, our background 

 and reagent blank measurements for both Ra-226 

 and Pb-210 are in the same range as those reported 

 in previous applications of radiometric ageing to fish. 



The Ra-226 (or equivalent Rn-222) activity levels 

 were low and difficult to measure. The signal to 

 background ratio can be increased by using a 

 greater sample weight. Fenton et al. (1991) argued 

 that measuring Ra-226 by using barium co-precipi- 

 tation with alpha spectroscopy (Sill, 1987) could also 

 produce a better signal to background ratio. How- 



