662 



Fishery Bulletin 88(4), 1990 



Measurements of otolith composition made in regions 

 of the sagittae formed after the completion of yolksac 

 absorption and corresponding with a period soon after 

 first feeding corroborated data that showed the influ- 

 ence of yolk on otolith composition. There were no 

 significant differences in Sr/Ca (unpaired two-tailed 

 ^tests, / = 1.30, P = 0.201), S/Ca (t = 2.02, P = 0.051), 

 or K/Ca {t = 1.09, P = 0.282) ratios measured in the 

 later-formed regions of the otoliths from the two 

 groups of fry and only slightly significant differences 

 for Na/Ca ratios {t = 2.70, P = 0.010). Differences be- 

 tween the Sr/Ca ratio of the sagitta edge and primor- 

 dium were highly significant in the sea-farmed brood- 

 stock progeny (paired two-tailed ^-tests, t = 10.38, 

 P<0.0001), and only slightly significant in the progeny 

 of the freshwater broodstock (t = 3.13, P = 0.003). 



Six electron microprobe transects over a sagitta from 

 a single Oncorhynchus mykiss fry of sea-farmed brood- 

 stock origin were made to show Sr/Ca ratios over the 

 entire otolith including the nucleus and pre- and post- 

 yolk absorption regions (Fig. 3). The outermost points 

 towards the otolith rostrum and posterior show the 

 sagitta Sr/Ca ratios after the completion of yolksac 

 absorption and provide evidence for much reduced 

 otolith Sr/Ca ratios when compared with the otolith 

 material produced during the period of yolksac utiliza- 

 tion. The data also indicate that, in general, high Sr/Ca 

 ratios prevail throughout the region of the nucleus and 

 are not confined to the primordia alone. 



Measurements of egg calcium and strontium com- 

 position show that ova of the sea-farmed rainbow trout 

 contain higher levels of strontium than their freshwater 

 conspecifics. The mean strontium content was 0.054 ± 

 0.013 /iM/g of ova (dry weight) for ova that had devel- 

 oped in seawater and 0.010 ± 0.002 jxMlg for ova from 

 freshwater fish, and the difference between these 

 values was significant (unpaired two-tailed /-test, 

 / = 6.60, P = 0.0006). Mean calcium content of yolk was 

 0.035 ± 0.005 mM/g from sea-farmed broodstock and 

 0.026 ± 0.004 mM/g from freshwater fish and these 

 values were also significantly different (/ = 3.06, 

 P = 0.022). 



Figure 3 



.Six transect.s of otcilith Sr/Ca nitios made with a wavt- 

 length-dispersive electron microprobe on a single sagitta 

 from an Onrorfufnchiis miikina fry of sea-farmed brood- 

 stock origin. The otolith was ground to the level of the 

 primordia in the sagittal plane. Each column in the histo- 

 grams represents an individual mieroprolie measurement 

 (iV = 120). Outermost points show the sagitta Sr/Ca ratios 

 after the completion of yolk.sac absorption. 



Sagitta length (435 |jm) 

 s6 



70 

 60 



Z 40 



^30 



=^ 20 



10 



i   ": ' >'  



^Mm 



100 :(X) m) 400 



Otolith distance (JJm) 



