shown in Figure 1. Ten to 15 sections were sawed 

 from each otolith. The number of sections viewed 

 was dependent upon the clarity of the increments. 



Each otolith section was fastened to an alumi- 

 num scanning electron microscope (SEM) stub 

 with 5-min epoxy. The otolith section was highly 

 polished with 0.3 /u.m alumina paste and etched 

 with 67c EDTA (ethylenediaminetetraacetatic 

 acid, adjusted to pH 8 with NaOH) for 1 to 20 min. 

 The otolith sections were washed in water, dried, 

 coated with gold, and viewed on a SEM at various 

 magnifications. Observations and counts were 

 made while the otolith section was in the SEM. 



It was discovered that different areas of the ros- 

 tral lobe of the otoliths were made clear by differ- 

 ent etching times. Sequential etching made it pos- 

 sible to view microincrements in the outer 10 

 major increments. Individual sections were etched 

 for different periods of time, with 15- to 20-min 

 etching times showing the inner increments more 

 clearly. The 10 outermost major increments were 

 clearly visible in all sections and could be followed 

 from section to section regardless of etching time. 

 Each major increment was chosen to be from the 

 center of one ridge to the center of the successive 

 ridge; sequential etching revealed the micro- 

 increments between the ridges. It was not possible 



to count the microincrements from the edge of the 

 otolith inward past the 10th major increment on 

 any individual section. Consequently, sequential 

 cross sections from each otolith were etched for 

 different periods of time, in steps of 1 min, in order 

 to follow the progression of the microincrements. 

 In the present study, a microincrement was de- 

 fined as an unbroken incremental zone with dis- 

 continuous zones as boundaries (Radtke and Dean 

 1982) and was considered to be a daily increment. 



Results and Discussion 



SEM techniques made it possible to view micro- 

 increments in bluefin tuna otoliths from four indi- 

 vidual fish. The most visually distinct increments 

 were found on the rostral lobe of the otolith cross 

 section (Fig. 1). Thus this area was used predomi- 

 nantly for SEM observations. The major incre- 

 ments of the otolith can readily be seen in Figure 

 2. Higher magnification (10,000 x) revealed that 

 the major increments were constructed of smaller 

 increments which in turn were composed of micro- 

 increments (Figs. 3, 4). 



Differential etching caused the problem that 

 not all the increments could be viewed at the same 

 time. This was overcome through the use of suc- 



FIGURE 1. — Cross section of a bluefin tuna otolith showing the area (arrow) studied for microincrements. This area 



is on the rostral lobe of the otolith. 



435 



