taceans in the diet than smaller skipjack in 

 some areas, but Waldron and King (1963) 

 found no significant difference in the repre- 

 sentation of total crustaceans, fishes, or cepha- 

 lopods in skipjack under and over 60 cm. 



The prey of skipjack tuna form part of the 

 assemblage of active pelagic fishes, cephalo- 

 pods, and crustaceans about 1 to 10 cm in size, 

 which we call micronekton. Micronekton 

 collections were taken during EASTROPAC 

 cruises, and the organisms known or expected 

 to be prey of skipjack, based on the above-cited 

 food habit studies, were sorted out and mea- 

 sured volumetrically. This report describes the 

 distribution of the total of these skipjack-forage 

 organisms as a contribution to skipjack ecology. 

 Other aspects of the study of EASTROPAC 

 micronekton samples, including organisms 

 that are not eaten by skipjack, are described 

 elsewhere (Blackburn et al., 1970). 



MATERIAL AND METHODS 



Seasonally repetitive occanographic observa- 

 tions were made in the eastern tropical Pacific 

 between lat 20 N and 20 S, bounded in the 

 east by the American coast and in the west 

 usually by long 119 W. During EASTROPAC, 

 seven cruises were made of approximately 2 

 months each: February-March 1967, April- 

 May 1967, .June-July 1967, August-September 



1967, October-November 1967, December 

 1967-January 1968, and February-March 



1968. Figures 1(A, B) through 7(A, B) show 

 the extent of the coverage in the eastern tropi- 

 cal Pacific during the successive periods. Most 

 of the data were collected west and south of the 

 existing skipjack fishing areas near the Ameri- 

 can coast. 



We sampled micronekton on these cruises in 

 oblique hauls of a large net, 1.5 m square at the 

 mouth, with a uniform mesh size that retains 

 animals >: 1 cm but not smaller (Blackburn, 

 1968; Jerde, 1967). The micronekton samples 

 were taken from the surface to a depth of ap- 

 proximately 200 m, with ship speeds during 

 the hauls ranging from 4.0 to 6.4 knots. The 

 volumes of water strained were estimated as 

 described by Blackburn et al. (1970). The dis- 

 placement volume of the total skipjack forage 



was measured and standardized in ml/ 1000 

 m' of water strained for each haul. Generally, 

 two micronekton hauls were made during each 

 24-hr period, one usually close to local apparent 

 noon and the other usually close to local mid- 

 night. The day-time catches were usually about 

 one-tenth the size of the night catches, by 

 volume. Similar day-night differences have 

 been observed in other micronekton studies and 

 have been shown to be due to net avoidance and 

 vertical migration of organisms (Pearcy and 

 Laurs, 1966). Because of their larger size, the 

 night samples may give a better representation 

 of the total amounts of skipjack forage present. 

 However, it was considered important to 

 sample in the daytime also because skipjack 

 probably do much of their feeding during hours 

 of daylight (Nakamura, 1962). In addition, 

 two hauls per dicl period were expected to give 

 better resolution of the positions of maxima 

 and minima of forage, than one haul. An esti- 

 mate of sampling variability for night catches 

 is given by a series of 1 1 catches made during 

 one night at approximately the same geogra- 

 phical position. The coefficient of variation of 

 the total micronekton volume was 20% . 



The following components of the micronek- 

 ton net catches are regarded collectively as skip- 

 jack forage: Vincigiicrria spp., all epipelagic 

 fishes, Pleuroncodes spp., all other crustaceans, 

 and all cephalopods. The following other 

 components were not considered part of skip- 

 jack forage: leptocephali, all mesopelagic fish 

 except Viticigucnia, and large watery plankters 

 such as salps and medusae. Fishes with photo- 

 phores were called mesopelagic, most others 

 epipelagic. Leptocephali are transparent and 

 would probably be hard for skipjack to see, and 

 there is no evidence that skipjack eat them. 

 Mesopelagic fishes mostly occur near the sur- 

 face at night, and with the very important ex- 

 ception of Vincigucnia. they are seldom an im- 

 portant item in skipjack stomach contents in 

 the eastern or central tropical Pacific (Alver- 

 son, 1963; Waldron and King, 1963; Naka- 

 mura, 1965). The epipelagic fishes, crusta- 

 ceans, and cephalopods have not yet been com- 

 pletely sorted to families. They may therefore 

 have included some families which Alverson, 

 Waldron and King, and Nakamura did not find 



