FOOD OK BIGEYE AND YELLOWFIN TUNA 



77 



^YELLOWFIN 



BIGEYE 







160 

 I 

 u 

 < 

 Z 

 O 120 



ui 80 



P 40 



UI 



Z 



y- 

 z 



1^120 



CD 



3 100 

 O 



CRUSTACEA 



i 



_a 



SQUID 





I 



80 - 



60 



TOTAL FISH 



i 



HOOKS 



CONV CC CONV 



, IL _ _^ 



186 2 85 



CC 



SHALLOW 



INTERMEDIATE 



CONV CC 



384 

 DEEP 



i 



4 



I 



«| 



J 



I 



i 



J 



i 



CC 



ALL DEPTHS 

 COMBINED 



FioiRE 12. — Variations in average volume per stomach 

 of the major food categories with depth of capture of 250 

 yellowfin and 105 bigeye tuna taken by longline in the 

 C'ountercurrent and the convergent zone. 



the increase with deptli in Gempyhdae as noted 

 for yellowfin. In the convergent zone there is a 

 similar increase with depth in the utilization of 

 Crustacea and Bramidae. 



The major foods are of about equal importance 

 in both areas. There is no indication that the 

 tunas have one set of foods in the CC and another 

 in the converfient zone. Tlie main difference 

 between the two species is the much greater 



consumption of Crustacea by the bigeye in both 

 the CC and tlie convergent zone. If we may con- 

 sider the longline catch rate as an index to abun- 

 dance, it would appear that the bigeye responds 

 in a different manner than the yellowfin to the 

 more favorable foraging conditions whicli, we 

 hypothesize, e.xist in tiie convergent zone. 



OTHER VARIATIONS IN VOLUME OF STOMACH 

 CONTENTS 



^\^len the stomach-content volumes are clas- 

 sified according to an arbitrary scale, the results 

 (table 9) indicate for both species a rather low 

 percentage of empty or near-empty stomachs; the 

 average stomach contained a relatively small 

 amount of food. This may mean that feeding is 

 almost continuous, as contrasted with an irregular 

 or spasmodic feeding habit, and that these fish 

 have a high rate of digestion. For instance, it is 

 hard to believe that a food volume of less than 

 100 CC, which was found in more than 50 percent 

 of the stomachs (table 9), constitutes a daily or 

 even semidaily ration for these large active fish. 

 Unfortunately, our food studies provide no in- 

 formation on rate of food consumption or digestion. 



In longline fishing, the gear is ordinarily set at 

 daybreak and is hauled in during the afternoon. 

 The time of landing is known, but not the time 

 that the fish took the hook. On some cruises, 50 

 percent or more of the tuna are dead when landed. 

 One might assume that these fish hail been on 

 the line for a longer period of time than the fish 

 that were landed alive. On the basis of this 

 hypothesis we examined the records from certain 

 cruises for which we had the greatest number of 

 observations supplying information on condition 

 when landed. These data, as summarized in 

 table 10, seem to indicate that the fish that were 

 dead when landed contained larger volumes of 

 food, on the average, than those that were landed 

 alive. Although we cannot satisfactorily explain 

 this difi"erence, we believe that it may be related 

 to the tendency for more dead fish to occur on 

 the deep hooks than on the liooks fishing at 

 shallow and intermediate depths; and in the 

 yellowfin, at least, we have found an increase in 

 volume of stomach contents with depth of capture 

 (table 3). A combination of these factoi-s might 

 produce the results shown in table 10. 



