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Fishery Bulletin 93(2), 1995 



from fish previously aged by Kenchington and Au- 

 gustine (1987). The dimensions of otoliths from 1+ 

 fish (ages assigned by Kenchington and Augustine, 

 1987) were examined and the average linear dimen- 

 sions and weight for a 1+ fish determined. The aver- 

 age otolith dimensions of 1+ fish were selected as 

 the shape of the core to be removed. This core size 

 was chosen 1) because it ensured minimal influence 

 of changing 226 Ra (as reported in Fenton et al., 1990); 

 2) because it represented the minimum core size that 

 could be repeatably isolated (attempts to remove 

 smaller cores, e.g. 0+ dimensions, resulted in the 

 otolith shattering or splitting, and being unusable; 

 and 3) because it fitted the age estimate of 1+ otoliths 

 by Kenchington and Augustine ( 1987) who were able 

 with confidence to validate fish age for individuals 

 under 3 years old. 



Removal of otolith core 



Blue grenadier otoliths chosen for coring were se- 

 lected from females collected from the same site, on 

 the same date, and of similar fish length and otolith 

 weight. Cores were removed from the otoliths by 

 sanding the outer layers away by hand (with respect 

 to the primordia) with wet and dry sand-paper (sili- 

 con carbide paper) in progressively finer grades un- 

 til the desired core dimensions were achieved. The 

 mean core dimensions were compared with the mean 

 whole otolith dimensions for 1+, 2+, and 3+ fish. New, 

 wet and dry sand paper was used for each otolith to 

 eliminate the risk of 210 Po cross contamination be- 

 tween otoliths. The cores of 2 or 3 fish were pooled to 

 produce the 1-g sample necessary for radiometric 

 analysis. 



Radiometric analysis 



Cores were cleaned of adventitious 210 Po by exposure 

 to an alkaline H 2 2 solution for 1/2 hour. The analy- 

 sis of 210 Pb via its alpha-emitting, short-lived daugh- 

 ter proxy 210 Po followed the method we have previ- 

 ously described (Fenton et al., 1991 ), employing high 

 resolution alpha-spectrometry. Polonium-210 was 

 assumed to be in equilibrium with 210 Pb in all 

 samples, and 210 Pb concentrations were corrected 

 back to the date offish collection. Mean 210 Po reagent 

 blank was 0.0071 ±0.0012 disintergrations per minute 

 (dpm-g"" 1 ). Recovery of 210 Po was invariably >90%. 

 Instrumental background counts (for 208 Po and 2I0 Po) 

 were less than 1 count per day. Analysis of 226 Ra was 

 by a direct alpha spectrometry technique (Fenton et 

 al., 1991). The mean activity of the 226 Ra blanks was 

 0.0174 ±0.002 dpm-g- 1 . Both 210 Po and 226 Ra blank 

 values were reduced to lower values than those from 



previous studies because of tighter quality control of 

 reagents and minor improvements in technique. 



Stable element analysis 



The levels of lead (Pb) and barium (Ba) are presumed 

 to act as stable equivalents of 210 Pb and 226 Ra (Fenton 

 and Short, 1992) and, as such, can potentially be used 

 to assess the uptake of the radioactive isotopes and 

 for normalizing the radiometric data. Therefore the 

 concentrations of stable lead, barium, calcium (Ca), 

 and strontium (Sr) in each otolith core sample were 

 measured. An aliquot of the same dissolved otolith 

 core solution used for 210 Pb and 226 Ra analysis was 

 analyzed. Lead and barium were analyzed by induc- 

 tively coupled plasma mass spectrometry (ICP-MS) 

 and strontium and calcium by inductively coupled 

 plasma atomic emission spectrometry (ICP-AES). 



Calculation of fish age 



Fish age was calculated by using the model proposed 

 by Campana et al. (1990) and described in Smith et 

 al. (1991). The 210 Pb/ 226 Ra activity ratio of the otolith 

 core during growth, assuming a constant mass 

 growth model for the period of core formation only, 

 is given by the equation: 



Adl210 _ 



Pb =i-a-R) 



A Ra™ 



XT 



where A = specific activity (i.e. activity concentra- 

 tion); 



R - initial activity ratio (i.e. uptake activity 

 ratio); 



A = decay constant of 210 Pb (year -1 ); 



T = period of core formation (years). 



The activity ratio of the core at any subsequent time 

 in the life of the fish is unaffected by mass growth of 

 the otolith and is therefore given by the equation: 



'/■/, 



A Ra 2M 



226 ^ ' 



i-a-R) 



-IT \ 



XT 



-X(t-T) 



where t= age of the fish (years). 



The fish age is found by solving for t by the 

 Newton-Raphson iterative method. All radiometric 

 age values are given with an error value of ±1 stan- 

 dard deviation. 



