men was taken in the 131 hauls of the 1.5-m. net, 

 but se\'eral, including; larire ones, have been taken 

 at various times in hauls of the liigh-speed net. 

 Under the circumstances in whicli we have used 

 these nets, the hi<rh-speed net is obviously the more 

 efficient (Ijehaving- more like a tuna) for catching 

 exocoetids, in spite of its smaller mouth. This 

 greater efficiency arises from the greater speed of 

 hauling or because we have towed it only in the 

 upper 10 m., where exocoetids are probably most 

 abmidant. 



The apparent scarcity of enoploteuthid and 

 cranchiid cephalopods in tuna stomachs might not 

 be real, because of their inclusion in unidentifiable 

 material, as noted in the fifth point above. This 

 explanation might also apply to leptocephali. 



For some of tliese reasons, it is perhaps surpris- 

 ing that the percentages for some other groups 

 (Galatheidae, Euphausiidae, Squillidae, and Vm- 

 clguerria) agree as well as they do in the net- 

 caught and tuna-caught micronekton ; euphausiids 

 certainly, and probably Pleuroncodei planipes 

 (Longhurst, 1967), and Yinciguerria (Ahlstrom, 

 personal communication), are more abundant 

 near the surface by night than by day. These kinds 

 of micronekton sometimes occur at the sea surface 

 in full daylight, however, and possibly their day- 

 time submergence is not so great or so regular as 

 to make them entirely unavailable to tunas. 



It is possible that records of the percentages of 

 individual species of Crustacea, which are mostly 

 not avaihible at present, would remove some of the 

 similarities which appear to exist between net 

 catches and contents of tuna stomachs — for ex- 

 ample, in the euphausiids. Such records would not 

 affect the galatheids greatly, however, since nearly 

 all were PlruroncodcH planipes in botli the net 

 catches and the contents of tuna stomachs. Vinci- 

 guerria would not be seriously affected either, be- 

 cause all of Alverson's specimens and most of mine 

 were V. hicetia, although my material included a 

 few v. nimbrar/a, and several specimens that were 

 doubtful (see Ahlstrom and Counts, 1958, con- 

 cerning Yinciguerria in the eastern Pacific) . 



These considerations give insufficient reason to 

 doubt the connnonly held opinion that yellowtin 

 and skipjack tuna feed on most kinds of micronek- 

 ton which occur in their habitat, with the qualifi- 

 cation that the prey must occur under conditions in 



which the tuna could perceive them visually. The 

 tunas are opportunistic feeders, within their sen- 

 sory limitations. The only important exception to 

 this generalization is the apparent failure of yel- 

 lowfin tuna to eat euphausiids, which are much 

 used by skipjack tuna. Efficient capture of euphau- 

 siids by large pelagic fish may require special be- 

 havior, however (see Blackburn, 1957, on the 

 gempylid scombriform Thyrsites atwn), and it is 

 possible that these behavior patterns have not 

 evolved in yellowfin tuna. 



COMPARATIVE ABUNDANCE OF 

 POTENTIAL TUNA PREY BY AREAS 



From tables 8 to 14, and the tables of Alverson 

 (1963a) which show percentage composition of 

 tuna stomach contents for other areas, a list was 

 made of families and other groups which made up 

 5 percent or more by volume of stomach contents 

 of yellowfin and skipjack tunas in any one of the 

 areas 1 to 14.'"' Tliese groups were considered po- 

 tential tuna prey in all areas where they occur. 

 Groups which were not represented in the net 

 hauls — the Engraulidae and Polynemidae — were 

 disregarded. Tliis omission left the following 

 groups, which represent kinds of potential tima 

 prey which the net is capable of catching to some 

 extent in standard night hauls : 



Yellowfin tuna prey 

 Fish: 



Gonostomatidae 



Myetophidae 



Exocoetidae 



Thunnidae 



Carangidae 



Stromateidae 



Coryphaenidae 



Ostracidae 



Tetraodontidae 



Crustaceans : 

 Galatheidae 

 Portunidae 

 Sergestidae 

 Squillidae (adults) 



Cephalopods : 



Ommastrephidae 



Skipjack tuna prey 

 Fish: 



Gonostomatidae 



Myetophidae 



Exocoetidae 



Tlimmidae 



Serranidae 



Scomberesocidae 



Trichiuridae 



Crustaceans : 

 Galatheidae 

 Portunidae 

 Euphausiidae 

 Squillidae (larvae) 

 Phrosinidae 

 Unidentified crab 

 megalopa 



The total volume of net-caught potential prey 

 for yellowfin tuna and for skipjack tuna was stand- 

 ardized (as explained above) for each area (1 to 



« stromateidae (table 3, and below) are equivalent to Nomeidae 

 as listed by Alverson. 



MICRONEKTON OF THE EASTERN TROPICAL PACIFIC OCEAN 



107 



