FOOD OF BIGEYE AND YELLOWFIN TUNA 



67 



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YELLOWFIN BIGEYE 



FifiiRE 0. — Comparative importance, in volume, of the major food elements. 



centage by occurrence and percentage by volume 

 for the fish famihes prominent in the diet exhibited 

 little variation with the size of the tuna. 



Variation in Food with Depth of Capture 



Figure 7 is a diagram of one unit (a basket) of 

 POFI longline gear, showing the arrangement of 

 hook-bearing dropper lines and the general lay of 

 the line with respect to the surface. Although an 

 attempt is made to set the line at each station in a 

 standard fashion, with an average distance be- 

 tween buoys of about 900 feet, the actual depth 

 of fishing is quite variable depending upon the 

 amount of sag in the main line, which is gi-eatlv 

 influenced by wind and current conditions. 



FiriiRK 7. — .Arrangement of a unit (ba.sket) of POFI 

 standarfi loMKlinc gear showing the float lities, main line, 

 hook-bearing dropper lines, and the general lay of the 

 line with respect to the surface. 

 :{887:)4 ( ) — Sft 2 



Murphy and Shomura (1953a) have calculated 

 that the ma.ximuin possible depth for hooks 1 and 

 6, with a 900-foot buoy interval, is 310 feet; for 

 hooks 2 and 5, it is 450 feet; and for hooks 3 and 

 4, it is 530 feet. These maximum depths are 

 seldom achieved, however, because of the rela- 

 tively strong surface currents generally prevailing 

 in this region. The miniminn depths are even 

 more uncertain; therefore it is difficult to define a 

 depth range for each of the hooks. We postulate 

 that liooks 1 and 6 may fish at depths of 1.50 to 

 300 feet, hooks 2 and 5 at depths of 250 to 400 

 feet, and hooks 3 and 4 at depths of 300 to 500 

 feet. Despite this variation and the imcertain- 

 ties involved, it is worthwhile, without attempt- 

 ing to designate actual fishing depths, to make 

 comparisons between tlie shallow (hooks 1 and 6), 

 intermediate (hooks 2 and 5), and deep (hooks 3 

 and 4) levels of capture with respect to dift'erences 

 in stomach contents. Because of tlie rather slight 

 differences in composition of tlie food associatetl 

 with the size of the tuna, the two size groups 

 (<!l40 cm. and !>140 cm.) were combined for liiis 

 study. 



Table 3 shows the variation in composition of 

 stomach contents with <le])th of capture; the varia- 

 tion in the two general categories, squid and fish, 



