MICRONEKTON OF THE EASTERN TROPICAL PACIFIC OCEAN: FAMILY 

 COMPOSITION, DISTRIBUTION, ABUNDANCE, AND RELATIONS TO 

 TUNA 1 ^ 



By Maurice Blackburn, Research Biologist 



Institute of Marine Resources, Scripps Institution of Oceanography 



University of California, San Diego, Calif. 92037 



ABSTRACT 



The taxonomic composition and distribution of 

 micronekton (fishes, crustaceans, and cephalopods 

 about 1 to 10 cm. in largest dimension) were studied 

 from catches of night net hauls in the upper 90 m. 

 In most parts of the eastern tropical Pacific Ocean. 

 One type of haul (net 1.5 m. square at mouth, uniform 

 mesh size throughout, hauled obliquely at ship speed 

 of 5 knots) contributed most of the data and is con- 

 sidered to be superior to any other existing type of 

 haul with a net or trawl of comparable size for quanti- 

 tative work on micronekton. 



Ten families (Myctophidae, Gonostomatldae, 

 Galatheidae, Euphausiidae, Penaeidae, Squillidae, 

 Portunidae, Sergestidae, Enoploteuthidae, and 

 Cranchiidae) and one suborder (Apodes: leptocephall) 

 contributed 93.4 percent of the volume of the total 

 catch. Some of these groups are localized, and others 

 are widely distributed geographically; abundance varies 

 according to the distribution of physical phenomena 



which are responsible for eutrophic conditions. Agree- 

 ment in family composition was poor for fishes between 

 the net catches and the stomach contents of yellowfin 

 tuna (Thunnus albacares) and skipjack tuna (Euthyn- 

 nus pelamis) from the same areas; agreement between 

 catches and stomach samples was fair for crustaceans. 

 The reasons for the differences are discussed. The com- 

 monly held opinion, that tunas are opportunistic 

 feeders, within their sensory limitations, remains 

 tenable. 



To the extent that the net hauls sample kinds of 

 micronekton which are Important as food for tunas, 

 they can be used to compare quantities of tuna prey in 

 different areas. This comparison shows that the richest 

 tuna forage is off western Baja California and that an 

 area west of Ecuador and northern Peru with practical- 

 ly no surface fishing probably has about as much forage 

 as some areas which support commercial surface 

 fishing. 



Comprehensive biological-oceanographic inves- 

 tigations in the eastern tropical Pacific Ocean be- 

 gan witli expeditions Eastropic (1955) and Scope 

 (1956). Mucli attention ^yas given on these expedi- 

 tions to distrilnitions of primary productivity, 

 standing crop of cliloi-ophyll a. and standing crop 

 of zooj^hankton (Hohnes, Schaefer, and Shimada, 

 1957; Hohnes and others, 1958); these observa- 

 tions were intended to contribute to tlie under- 



• Contribution from the Scripps Institution of Oceanography, University 

 of California, San Diego. 



- Tliis work was financed by the Bureau of Commercial Fisheries under 

 Contract Nos. 14-19-008-9354. 14-17-0007-1, 14-17-0007-28, 14-17-0007-70, 

 14-17-0007-139. and 14-17-0007-221, with funds made available under the Act 

 of July 1, 1954 (68 Stat. 376), commonly known as the Saltonstall-Kennedy 

 Act. It was also supported by the Atomic Energy Commission through 

 contract AT(ll-I)-34, Project 99, with the Institute of Marine Resources, 

 University of Califoniia. It was part of the research of the STOR (Scripps 

 Tuna Oceanography Research) Program. 



Some preliminary results of this research were given in a symposium at 

 the Tenth Pacific Science Congress of the Pacific Science Association, held 

 at the University of Hawaii in .\ugust and September 1961. The symposium 

 was entitled "Factors affecting the behavior of predaceous marine fishes, 

 especially sharks." 



Published August 1968 



FISHERY BULLETIN: VOL. 67, NO. 1 



standing of the ecology of the tunas of the region 

 and to oceanographic knowledge. Much of this 

 work was done by the Scripps Institution of 

 Oceanography, University of California. In 1957 

 the Institution's work in this field was put on a 

 continuing basis in the STOR Program, with sup- 

 port from the Bureau of Commercial Fisheries. 

 As a result, several more cruises were made in the 

 eastern tropical Pacific, which differed from 

 Eastropic and Scope in that measurements of an 

 additional biological property, namely standing 

 crop of micronekton, were made routinely. 



Micronekton, a term occasionally found in ma- 

 rine biological literature (e.g., Mai-shall, 1954), is 

 here defined as the assemblage of actively swim- 

 ming fishes, crustaceans, and cephalopods, ranging 

 from about 1 cm. to 10 cm. in greatest dimension. 

 In this paper it means all fishes, crustaceans, and 

 cephalopods caught by a net designed to sample 



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