FISHERY BULLETIN: VOL. 79, NO. 1 



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Figure l.— Check mark, indicated by 

 arrow, separates the environmentally 

 marked increments from previous 

 growth increments at a) tip of skipjack 

 tuna sagitta rostrum; b) postrostrum of 

 same skipjack tuna sagitta. 



i 



ly- 



t 



\ 



to the feeding period formed primarily on the tips 

 of the rostrum and postrostrum (Figure la, b). 



The highest count attainable corresponded with 

 the number of days the tuna were fed to satiation, 

 thus confirming the growth increment-day rela- 

 tion (Table 1). The number of increments formed 

 after the check mark usually exceeded the number 

 of feeding days because the tunas continued to live 

 beyond the feeding period. During this latter 

 period, the tunas either received a daily ration or 

 starved. Great care was taken to avoid double 

 counting of an increment where the sagitta was 

 thin. The observations on these specimens were 

 not long term and conditions were not fully con- 



trolled. Therefore, these data are considered ten- 

 tative and in need of replication by rigorous ex- 

 perimental methods. 



Wild and Foreman (1980) were able to show a 1:1 

 (day-to-growth increment) relationship for yel- 

 lowfin tuna, 40-110 cm FL. However, their day-to- 

 growth increment relation for skipjack tuna was 

 significantly <1:1. Although an experiment where 

 the fish lived in its natural environment was 

 highly desirable, there was no control over vari- 

 ables and a record of variables which the fish 

 might have encountered was unavailable. 



Variables such as the amount of food a fish con- 

 sumes (Struhsaker and Uchiyama 1976; Methot 



154 



