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Fishery Bulletin 92(4), 1994 



utes to remove any residual moisture, coated with a 

 250-300 A (measured by color on brass) coat of car- 

 bon with a sputter coater, and then stored under 

 vacuum until insertion into the probe. 



The procedures used to analyze otolith composi- 

 tion are detailed in Gunn et al. (1992). Damage to 

 the specimens under the electron beam is inevitable. 

 The amount of damage, and hence quality of the data, 

 is proportional to beam-power density (i.e. beam cur- 

 rent x accelerating voltage/ target area). In Gunn et 

 al. (1992), we concluded that beam power densities 

 greater than 3.0 pW pm~ 2 resulted in unacceptable 

 levels of specimen damage, data precision, and ac- 

 curacy. Hence, data for the current study were ac- 

 quired by using the following beam conditions: 25 



nA current, 15 kV accelerating voltage, a 14 pm di- 

 ameter 'defocused' beam (and hence a beam power 

 density of 2.44 pW pnr 2 ' and a total acquisition time 

 of 3 minutes, 42 seconds per point. Comparisons of 

 parallel scan lines (see Fig. 6) included some analy- 

 ses at a 6 pm beam diameter, 5.5 nA current, and 15 

 kV accelerating voltage; despite the small beam di- 

 ameter, beam power density for this series (2.92 

 pW pm -2 ) is within our 'safe' limit. The electron probe 

 microanalyzer used was a Cameca Camebax fitted 

 with three wave-length dispersive detectors. The 

 concentrations (weight-fractions) of Na (sodium), K 

 (potassium), Ca (calcium), S (sulphur), and CI (chlo- 

 rine) were calculated based on the count rates mea- 

 sured for their respective K a lines, and for Sr (stron- 



