Ten families (Myctophidae, Gonostomatidae, 

 Galatheidae, Euphausiidac, Penaeidae, Squillidae, 

 Portunidae, Sergestidae, Enoploteiithidae, and 

 Cranchiidae), and one suborder (Apodes; lepto- 

 cephali only), which was not sorted to family, 

 made up 03.4 percent of the total volume of micro- 

 nekton collected in 131 standard night hauls. The 

 areal distribution of standing crop was broadly 

 the same for most of the gi'oups ; crops were higher 

 onshore than offshore and highest in onshore areas 

 where physical processes that enrich the upper 

 water layer are known to operate. Three groups 

 of crustaceans (Galatheidae, Portunidae, and 

 adult Squillidae) were confined to relatively small, 

 mainly onshore, areas. Tlie data were insufficient 

 for comparison of standing crops at different sea- 

 sons, except in the Gulf of Tehuantepec, where the 

 crops appear to fall a few months after enricliing 

 physical processes decline. 



The comix)sition of the net-caught micronekton 

 was compared with the composition of stomach 

 contents of yellowfin and skipjack tunas as pub- 

 lished elsewhere, for certain areas of the eastern 

 tropical Pacific. As compared with tuna, the nets 

 caught different fishes, but broadly similar crus- 

 taceans. The differences are attributed to : tlie fact 

 that the nets catch large quantities of night-rising 

 mesopelagic fishes which do not occur in the tunas' 

 habitat by day, and are probably not seen by the 

 timas at night; leptocephali are probably not per- 

 ceptible to tunas in the daylight, since they are 

 translucent ; and the more active epipelagic fishes 

 are more efficiently caught by tunas than by nets. 



Families which made up at least 5 percent of 

 the stomach contents of yellowfin or of skipjack 

 tuna in any ai"ea were considered to be potential 

 prey for tlie si>ecies. To the extent that the net 

 sampled these families, comparisons could be made 

 of potential tuna prey in ml. per 1,000 m.^ in dif- 

 ferent areas (with and without a commercial fish- 

 ery), for wliicli net data were available. The com- 

 parisons suggested that one virtually uiifished area 

 supports a crop of iK>teiitial prey of yellowfin 

 tuna which is higher than that in one of the well- 

 fished yellowfin tuna areas; and also a crop of 

 potential skipjack tuna prey which is nearly as 

 liigh as that in one of the well-fished skipjack tuna 

 areas. This area, which lies west of the existing 

 fishery area for surface tuna along the coasts of 

 P^ciiador and northern Peru, might, therefore, con- 



tain yellowfin or skipjack tuna in commercial 

 quantities. Crops of potential tuna prey are much 

 higher off western Baja California than else- 

 where ; the principal prey species there is the gala- 

 theid pelagic crab Pleuroncodes planipes, which is 

 in part, a herbivore (giving the tunas a more effi- 

 cient food chain than in most other areas) and 

 extremely abundant. 



ACKNOWLEDGMENTS 



The following staff membei-s of the Scripps In- 

 stitution and Institute of Marine Kesources were 

 par'ticularly associated with the micronekton 

 work: A. Dougall Keith (design and operation of 

 net), Thelma Brockuian (sorting fishes), Charles 

 W. Jerde (sorting crustaceans), and John A. 

 McGowan (sorting cephalopods). Alan R. Long- 

 hurst and Milner B. Schaefer read and made 

 comments on the manuscript. 



LITERATURE CITED 



Ahlstrom, Elbert H., and Robert C. Counts. 



1958. Developmont and distribution of Vincigticrria 

 liicefia and related .species in the eastern Pacific. 

 U.S. Fish Wildl. Serv., Fish. Bull. 58 : 363-416. 



Alverson, Franklin G. 



1960. Distrilnition of fishing effort and resulting 

 tuna catches from the eastern tropical Pacific by 

 quarters of the year, 1951-1958. Inter-Anier. Trop. 

 Tuna Comni., Bull. 4: 321-446. [English and 

 Spanish.] 



1961. Daylight surface occurrence of myctophid 

 fishes off the coast of Central America. Pac. Sci. 

 15: 483. 



1963a. The food of yellowfin and skipjack tunas in 



the eastern tropical Pacific Ocean. Inter-Amer. 



Trop. Tima Comm.. Bull. 7: 295-396. [English and 



Spanish.] 

 1963b. Distribution of fishing effort and resulting 



tuna catches from the eastern tropical Pacific Ocean, 



liy quarters of the year. 1959-1962. Inter-Amer. 



Trop. Tuna Comm., Bull. 8 : 319-379. [English and 



Spanish.] 

 Aron. William. 



1959. Midwater trawling studies in the North Pacific. 

 Limnol. Ooeanogr. 4 : 409-418. 



1962a. The distribution of animals in the eastern 

 North Pacific and its relationship to physical and 

 chemical conditions. J. Fish. Res. Bd. Can. 19 : 

 271-314. 



1962b. Some aspects of sampling tJie macroplankton. 

 Cons. Perma. Int. Explor. Mer, Rapp. Proc.-Verb. 

 R6un. 153:29-38. 



MICRONEKTON OF THE EASTERN TROPICAL PACIFIC OCEAN 



113 



