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Fishery Bulletin 101(3) 



significant efTects on the limit of detection of Sr. As a re- 

 sult, higher beam currents (>20 nA) were appropriate for 

 measuring Sr/Ca ratios in spite of beam damage observed 

 at higher beam power densities. Beam diameters between 

 7 and 10m provide the best temporal resolution (i.e. cover- 

 ing fewer daily increments). The lower CV of Sr/Ca ratios 

 observed with the 7-m beam diameter was likely due to the 

 lower temporal variation afforded to smaller beam diam- 

 eters (compared to larger beam (liamctcrs) and lower error 

 related to specimen damage (com[)ared to the 1-m beam). 

 The lower CV of Sr/Ca ratios at the 7-pm beam diameter 

 suggested that in spite of beam damage, the Sr/Ca ratio 

 was not dramatically affected by beam damage. However, 



the increase in CV for the smaller diameters suggested that 

 there are limits to usable beam densities. 



Greater precision of Sr/Ca measurements is critical to 

 understanding life history of some species (Markowitz et al., 

 2000) or in situations where differences in environmental 

 Sr/('a raios are less than those observed between ocean wa- 

 ter and freshwatcrs (Rieman et al., 1994; Volk et al., 2000). 

 Volk et al. (2000) found that timing of freshwater entry 

 and length of freshwater residence by summer steelhead 

 (O. mykiss) and spring chinook salmon had efTects on oto- 

 lith core or primordia Sr/Ca levels. Summer steelhead and 

 spring chinook enter freshwater and stay for up to several 

 months before spawning. Volk et al. (2000) suggested that 



