- v - ■» 



4* 





> 



FIGURE 5. — Neighboring area of a bluefin tuna otolith shown in Figure 2 which demonstrates the uneven effects of 



etching. 



The width of each microincrement varied in accor- 

 dance with its position within a major increment. 

 Microincrement width was probably a function of 

 the time of the year when deposited. The widest 

 microincrements were displayed between the 

 ridges. Furthermore, the microincrements formed 

 at the edge of the sagittae were wide and deposited 

 during a time when the fish were fed large 

 amounts of mackerel as part of the sea ranching 

 operations. Observations on microincrement 

 width suggest that wide microincrements were 

 deposited during summer feeding and growth, 

 while finer microincrements were deposited dur- 

 ing the winter. It was these differences in width 

 that accounted for the formation of yearly 

 increments. 



Most fish species investigated for daily age es- 

 timates have been found to possess daily incre- 

 ments in their otoliths (Pannella 1971; Brothers et 

 al. 1976; Struhsaker and Uchiyama 1976; Taubert 

 and Coble 1977; Methot and Kramer 1979; Steffen- 

 sen 1980; Wild and Foreman 1980; Townsend and 

 Graham 1981; Uchiyama and Struhsaker 1981; 

 Radtke and Dean 1982). Thus, it is conceivable 

 that the microincrements displayed in bluefin 

 tuna otoliths are also daily. In tunas, Wild and 



Foreman (1980) studied daily increments in 

 yellowfin and skipjack tuna, and Uchiyama and 

 Struhsaker (1981) also investigated daily incre- 

 ments in yellowfin and skipjack tuna. Yellowfin 

 tuna are found to deposit daily increments in both 

 studies, whereas Wild and Foreman (1980) sug- 

 gested that skipjack tunas have 25% fewer in- 

 crements than would be expected if the increments 

 occurred daily, while Uchiyama and Struhsaker 

 ( 1981) advocated that daily increments did occur in 

 skipjack tuna. In light of the present data, Wild 

 and Foreman (1980) may have not detected incre- 

 ments formed during winter or colder periods. For 

 giant bluefin tuna it is suggested that the micro- 

 increments are formed daily. If bluefin tuna did not 

 deposit microincrements on a daily schedule, it 

 would be expected that fewer daily increments 

 would be detected in each major increment. Since 

 this is not the case, it corroborates the idea that 

 daily increments are formed in bluefin tuna 

 otoliths and groups of daily increments form an- 

 nual increments. 



Otoliths may be the most useful hard structure 

 for aging fish. Vertebrae and other hard structures 

 are much more susceptible to resorption during 

 times of physiological stress, while otoliths are 



438 



