Clear et aL Validation of annual increments in otoliths of Thunnus maccoyn 



31 



B 



Figure 3 



SEM micrographs showing strontium marks apparent as bright bands on sections taken from sister 

 otohths of specimen OB 102. (A) An oblique longitudinal section through the postrostral axis on 

 which the primordium (P) and postrostrum (PR) are marked. (B) A transverse section on which the 

 primordium and ventral margin (VM) are marked. Scale bars: 1 mm 



scattered electron images of polished sections through 

 appropriate growth planes (e.g. the oblique longitudi- 

 nal section along the PR axis). The visibility of the Sr 

 mark (brightness in the backscattered electron image) 

 was highest in fish that had been relatively small at 

 the time of injection (e.g. 50-55 cm FL). An example 

 is specimen OB 102 (Fig. 3), which measured 49 cm 



at time of release and was injected with 2 mL SrClg 

 solution. This bright band was frequently associated 

 with a local growth interruption immediately before 

 the band, presumed to be a tagging check. This check 

 was sometimes seen on the whole, etched otoliths (Fig. 

 4) but, although apparent in the SEM at high magni- 

 fications, it was not discernible fi-om the other surface 



