Wx 









FIGURE 4. — Microincrements detected on the slope of a major protein ridge from a bluefin tuna. Differences in 



widths cause the yearly increments. 



cessive cross sections which were etched for dif- 

 ferent time periods. This sequential etching made 

 it possible to follow microincrements within the 

 major increments. A difference in etching can be 

 seen in Figures 2 and 5. Although major incre- 

 ments were clear in most etching times, the micro- 

 increments were not. Through the utilization 

 of these techniques it was possible to obtain 

 microincremental numbers for major increments 

 (Table 1). 



The microincrement counts in each major in- 

 crement varied from 273 to 385 with the lowest 

 count being found on the edge of the otolith. The 

 summations of the microincrement counts for each 

 fish were remarkably close and not significantly 

 different (P > 0.05). Also, means of micro- 

 increments for each fish were not significantly dif- 

 ferent (P > 0.05) from the expected of 365 per year. 

 These data increase the credibility of the micro- 

 increments being daily and present a plausible 

 verification of the major increments as being 

 annual. 



Each microincrement is composed of a protein 

 matrix with calcium carbonate crystals, in the 

 aragonite crystal configuration, deposited within 

 the matrix. Etching with EDTA dissolves the 



TABLE 1. — Numbers of microincrements found in the major 

 increments on the outer edge of the rostral lobe of the sagittae 

 of four bluefin tuna, Thunnus thynnus. 



'From counts of major increments by light microscopy. 



aragonite crystals leaving areas with a higher 

 protein content to form discernible increments 

 (Figs. 3, 4). Extended etching (times varied de- 

 pending on the area of the otolith) can cause the 

 protein ridges to collapse and prevent counting of 

 the microincrements. Thus, etching times were 

 critical to the acquisition of viewable increments. 



437 



