Ahrenholz et al.: Otolith ageing of Brevoortia tyrannus 



21 



marked group were altered: otoliths were dissected 

 from each fish without bleaching (i.e. without a 59c 

 sodium hypochlorite solution to remove tissue) and 

 serial sectioning was used for the transverse sections 

 rather than single cuts with dual blades. Two sec- 

 tioning orientations were used: a transverse section, 

 taken with the primordium (and focus) as the 

 centerline, on a proximal-distal plane 90° to the an- 

 terior-posterior axis, and an oblique-transverse sec- 

 tion on a proximal-distal plane from a posterior and 

 dorsal position through the focus to the anterior 

 ventral(most) portion (Fig. 1). (Some otoliths were 

 also examined on an unsectioned, sagittal plane.) 

 Transverse sections were taken on the right sagitta 

 and oblique sections were taken on the left, with the 

 exception of the 1992 ALC-marked material for which 

 selection of the right or left sagitta was randomized. 

 Resulting sections were then ground and polished 

 according to the methods of Epperly et al. (1991) and 

 Secor et al. (1992). For otolith terminology and in- 

 crement interpretation we followed Pannella ( 1980) 

 and Campana ( 1992). 



Oxytetracycline and alizarin complexone marks 

 were located with blue light epifluoresence on pre- 

 pared otolith sections and viewed directly on a com- 

 pound microscope or on a video image analysis sys- 

 tem. The OTC-marked increment(s) fluoresced yel- 

 low-green when illuminated with blue light (Fig. 2) 

 and the ALC-marked zone fluoresced orange. The 

 position of each fluorescing mark was fixed with the 

 aid of an ocular micrometer (scope viewing) or with 

 a pointer on the viewing screen; increment counts 

 were made with white or polarized light. Otolith sec- 

 tions viewed on a video monitor were magnified to 

 l,500x. Since only a fraction of a section would fill 

 the screen at this magnification, increment count- 

 ing was done stepwise between distinguishable fea- 

 tures or "landmarks," and counts were summed when 

 interpretation was complete. An additional series of 

 increment counts was performed with the microscope 

 at l,000x and by tallying counts blindly on a hand- 

 held counter. Agreement between the two methods 

 on enumeration generally was better than 95%. Fi- 

 nal counts were means from the two enumeration 



Figure 1 



A whole sagittal otolith from a 39.0-mm (0.481-g) juvenile Atlantic menhaden, Brevoortia tyrannus, is shown to 

 demonstrate the orientation of sectioning for this study. Transverse (dotted lines) and oblique-transverse (dashed 

 lines) sections are displayed. The otolith is oriented with the dorsal edge up and the anterior to the right. 



