UCHIYAMA and STRUHSAKER: AGE AND GROWTH OF SKIPJACK AND YELLOWFIN TUNAS 



able the investigator to determine the best path to 

 follow from the core to the edge of the otolith. 

 Eventually, the counts either converge on a value 

 or repeated identical counts are obtained. The 

 number of counts required to determine age is 

 dependent on the readability of the otolith. An 

 average of about 20 counts was made for fish >1 yr 

 old. We found that yellowfin tuna sagittae were 

 easier to read than those of skipjack tuna. In yel- 

 lowfin tuna, the increments are wider and can be 

 examined at a lower magnification (200 x), which 

 provides a broader view and greater depth of field. 

 The increment counts on 15 skipjack tuna and 3 

 yellowfin tuna sagittae were verified by a second 

 reader whose counts were within 9% (5.4% aver- 

 age deviation) of the original counts except for one 

 which was 17% off the original count. 



Statistical Analysis 



The parameters of linear growth stanzas were 

 determined by the use of LINFIT, a computer pro- 

 gram (Kamer^) which used the ordinary least 

 squares procedure to fit two or three straight lines 

 to bivariate data. Join points, also known as break 

 points, are the places where one regime ends and 

 another begins; these are assumed to exist. The 

 range of the explanatory variable in which the join 

 points are expected to occur must be designated. 

 Given the range for these join points, the program 

 performs the ordinary least squares procedure on 

 each possible combination of regimes. The result- 

 ing output indicates the partitioning scheme, the 

 intercept and slope of each fitted line representing 

 a regime, the residual sum of squares for each 

 regime, the combined residual sum of squares for 

 the model, and the values of the dependent and 

 explanatory variables at the join points. The 

 linear growth stanzas used in this paper were 

 selected under the following conditions: 1) 

 minimized combined residual sum of squaires for 

 the model and 2) each join point associated with a 

 partitioning scheme has a value for the explana- 

 tory variable which lies between the last data 

 value in the regime to the left and the first data 

 value in the regime to its right. 



On the assumption that the maximum number 

 of increments approximates the age of the fish in 



*Kamer, G. A computer program for fitting straight lines to 

 regimented data. Manuscr. in prep. Southwest Fisheries 

 Center Honolulu Laboratory, National Marine Fisheries Service, 

 NOAA, Honolulu, Hawaii. 



days, we calculated a growth curve for central 

 Pacific skipjack tuna, eastern Pacific skipjack 

 tuna, and central Pacific yellowfin tuna for com- 

 parison with other studies. The von Bertalanffy 

 growth parameters were estimated on an annual 

 basis using the computer program BGC3 (Abram- 

 son 1971). 



ESTABLISHING THE 

 GROWTH INCREMENT-DAY RELATION 



The only direct evidence we have that a number 

 of growth increments equal the same number of 

 days came from the serendipitous opportunity to 

 examine skipjack and yellowfin tunas under 

 known captive conditions. These fishes were not 

 held under strict experimental conditions and 

 their primary use was not for aging studies; how- 

 ever, a detailed record on the amount of food con- 

 sumed by each fish during the experiment was 

 maintained. The experiment was similar to that 

 used for nehu, S. purpureus (Struhsaker and 

 Uchiyama 1976), and was carried out at the 

 Kewalo Research Facility of the Honolulu 

 Laboratory. 



The stress of being hooked, transported, and 

 confined in the community tanks at the Kewalo 

 Research Facility, as well as the taking of little or 

 no food during the first week of captivity, probably 

 all contributed to the formation of a check mark on 

 the otolith (Figure la, b). When the tuna were 

 sufficiently recovered to feed normally, they re- 

 ceived a daily ration which apparently was 

 adequate to maintain life but inadequate for nor- 

 mal increment formation. Perhaps very thin in- 

 crements were formed during this period, which 

 might have added prominence to the check mark. 

 When these tunas were fed to satiation through- 

 out the day, the widths of the increments increased 

 and became countable. 



Under low magnification (200 x), growth incre- 

 ments formed on the edge of a sagitta during the 

 experiment were not as well defined as those of a 

 tuna captured in the wild. The increments ap- 

 peared thinner than normal when examined 

 under high magnification (400 x). For this reason, 

 a sagitta was first examined under low power 

 (200 X) to locate the check mark and then 

 examined under high power (400 x) to enumerate 

 the growth increments formed during the experi- 

 mental feeding period. Although growth incre- 

 ments occurred all along the periphery of the 

 sagitta, the full array of increments corresponding 



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