the animals mentioned above. The catches in- 

 chided some animals smaller than 1 cm. or larger 

 than 10 cm., but they probably contributed less 

 than 5 percent of the total volume. Micronekton 

 and zooplankton overlap in catches in plankton 

 nets — for instance, euphausiids occur in both. 



The reasons for measuring the standing crop of 

 micronekton and each of its main components were 

 as follows : 



( 1 ) Because tunas feed on micronekton, a knowl- 

 edge of its distribution might help to explain the 

 variable distribution of tunas in the eastern trop- 

 ical Pacific. 



(2) Comparisons of net-caught and tuna-caught 

 micronekton (the latter from tuna stomachs) 

 might be of value in the study of feeding behavior 

 of tunas, especially in the matter of possible selec- 

 tion of organisms. 



(3) Food-chain relations in the ocean have had 

 much physiological and statistical study between 

 the producer and herbivoi-e trophic levels, but com- 

 jiaratively little study has been made between 

 those levels and the carnivore levels. This defi- 

 ciency seems to reflect a shortage of data on stand- 

 ing crops of oceanic carnivores, especially primary 

 carnivores such as small fish and cephalopods 

 ^vhich are eaten by secondaiy carnivores; where 

 such data are available, they generally refer to a 

 few species for which there are commercial fisher- 

 ies (e.g., herring, Clupea harengus). Because 

 good programs of measurement of phytoplaonkton 

 and zooplankton were already operating in the 

 eastern tropical Pacific, it seemed worthwhile to 

 measure micronekton as well. 



This paper presents a summary of most of the 

 micronelrton data obtained before 1964 in the east- 

 ern tropical Pacific and analyzes them in reference 

 to distribution and relation to contents of stom- 

 achs of tuna. Statistical analysis in reference to 

 food-chain relations has been made, in part, else- 

 where (Blackburn, 1966a). 



MATERIAL AND METHODS 



The net-caught micronekton obtained in the 

 eastern tropical Pacific and adjacent waters was 

 taken in tlie following three ways: 



(1) In standard (identically made) night hauls 

 of a net called the 1.5-m. (or 5-foot) net (de- 

 scribed below ) , at a ship speed of 5 knots. 



(2) In nonstandard night or day hauls of the 

 same net. 



(3) In hauls of a net called the higli-speed net, 

 described below, made at the ordinary cruising 

 speed of the vessel (which ranged from about 9 

 to 12 knots, for the different vessels used). 



The most useful quantitative data are those from 

 standard night hauls, and this paper is concerned 

 almost entirely with them. They were obtained on 

 the following cruises: TO-58-1 (or SCOT), 

 April-June 1958; TO-59-1, January-February 

 1959; TO-59-2, August^September 1959; TO- 

 60-1; May 1960; TO-60-2 (or STEP-1), Sep- 

 tember-December 1960; and TO-61-1, March- 

 April 1961 (see figs. 1 and 2 for areas 

 covered). The total number of standard night 

 hauls from all these cruises was 131 (see table 

 1). In addition, brief mention is made of 

 38 standard hauls (19 night, 19 day) made 

 on cruise TO-62-1 (or TEMPO), August 1962, 

 off Acapulco, Mexico. The hauls in this series (see 

 table 2) are separated from the main data l^eca.use 

 they were made close together in space and time; 

 they were not comparable with the other 131 hauls 

 which were much more widely distributed in space 

 and time in the eastern tropical Pacific. 



Nonstandard hauls, occasionally made by day 

 or night, are not discussed. The high-speed hauling 

 method, and some data from it, are briefly de- 

 scribed and evaluated. 



1.5-M. NET: DESCRIPTION, OPERATION, 

 AND PERFORMANCE 



Figure 3 shows the 1.5-m. net. It is in the form 

 of an elongated pyramid; the base (the mouth of 

 the net) is 1.5 m. square, and tlie measurement 

 from the center of the base to the apex is 5.8 m. 

 Tlie base or mouth is surrounded by a narrow 

 selvage laced to a square frame of galvanized 

 iron, to which the towing bridles and depressors 

 are attached. The apex is open and the opening 

 contains a brass fitting set in canvas, by which a 

 cod end (not included in the 5.8-m. length) can 

 be attached. Tlie material and mesh of the net 

 (excluding selvage and cod end) are uniform 

 throughout: Marion Textiles 467-pattern nylon 

 netting,^ with meshes approximately oblong and 

 measuring about 5.5 mm. by 2.5 mm. The long 



^ Mention of manufacturer does not imply endorsement of tiie 

 liroduct. 



72 



U.S. FISH AND WILDLIFE SERVICE 



