in skipjack stomachs. However, we have no 

 reason to think that any member of these fami- 

 Ues would not be eaten since tunas are oppor- 

 tunistic feeders within their sensory and be- 

 havioral limitations (Blackburn, 1968). 



Catches of micronekton by large nets and 

 small midwater trawls give imperfect estimates 

 of the availability of tuna forage (King and 

 Iversen, 1962; Laurs and Nishimoto^). Highly 

 mobile epipelagic fishes and cephalopods may 

 avoid the net. Some small components of the 

 forage, such as euphausiids, may pass through 

 the meshes of some nets, although not the net 

 used in this study. Nevertheless, our measure- 

 ments of standardized displacement volume of 

 potential skipjack forage no doubt validly dis- 

 tinguish areas where forage is rich and poor 

 and are the best relative estimates of potential 

 skipjack forage that can be produced at this 

 time. 



RESULTS 



Figures 1(A)-7(A) and 1(B)-7(B) show the 

 distribution of potential skipjack forage as in- 

 dicated by night and day micronekton samples, 

 respectively, collected on the seven EASTRO- 

 PAC cruises. Contours represent concentra- 

 tions of the forage organisms as indicated by 

 the collections; the night values are greater 

 than the day values on similar contours by a 

 factor of 10, e.g., 80 ml/1000 m' (night) and 

 8.0 ml/1000 m'' (day). Heavy dashed lines in 

 the figures indicate positions of zonally (lati- 

 tudinally) oriented maxima. Where the posi- 

 tion of such a line is not clearly justified by the 

 contours, it is based on differences in actual 

 concentration within the same contour interval. 



It is convenient to mention first the features 

 of forage distribution for the area west of long 

 92 W. Most of this part of the EASTROPAC 

 area is not presently fished for skipjack, and 

 the positions of forage maxima may indicate 

 where skipjack are most abundant. The area 

 east of long 92 W is discussed secondly. 



^Laurs, R. M., and R. N. Nishimoto. Food of troll- 

 caught Pacific albacore, Thunnus alalunga (Bonna- 

 terre ) , and an evaluation of midwater trawl catches as 

 a measure of potential albacore food. National Marine 

 Fisheries Service, Fishery-Oceanographic Center, La 

 Jolla, Calif. 92037. Manuscript. 



Area West of Long 92° W 



The most conspicuous maximum occurs in 

 a zonal band between lat and 5 N. It is 

 evident on at least two adjacent sampled merid- 

 ians in each figure except 6(B). It appears to 

 be well developed for both night and day forage 

 at all seasons. Another maximum sometimes 

 occurs between lat 2 and 8 S. Figures 1(A), 



1(B), 4(B), and 6(A) show it on at least two 

 adjacent meridians. It generally has lower con- 

 centrations over a smaller area and is less con- 

 tinuous zonally than the tnaximum just north 

 of the equator. The data do not justify any 

 statement about seasonal changes in its devel- 

 opment. A zonal maximum also occurs with 

 some regularity between 6 and 14 N. It is 

 evident on two or more adjacent meridians in 

 each figure except 4 ( A ) , 5 ( A ) , and 6(B). This 

 maximum is generally more evident in the 

 charts of the distribution of day forage than in 

 the charts of night forage. There appears to be 

 no great seasonal change in its development, 

 and forage concentrations are generally lower 

 than in the northern equatorial maximum. 



High concentrations of forage appear in 

 several of the charts in the northwestern part of 

 the area, approximately north of lat 14 N and 

 west of long 107 W [Figs. 1(A), 2 (B),4(A), 

 4(B), 5(B), and 7(A)]. For some cruises they 

 are more conspicuous in the night forage than 

 in the day forage; for others, the reverse. Other 

 maxima appear irregularly in generally smaller 

 areas. Forage concentrations in the southern 

 part of the area, and in the northwestern part 

 with the exceptions rhentioned above, tend to 

 be low. 



Area East of Long 92° W 



In the area east of long 92 W, which was 

 sampled less frequently than the more offshore 

 area, the main maxima are coastal. They vary 

 in position, however, not only between cruises 

 but also between night and day forage distri- 

 butions for the same cruise. High values are 

 usually observed off Peru and/or Ecuador, and 

 moderate values sometimes occur in the region 

 of the Panama Bight. Except near Peru, the 

 maximum concentrations near the coast are no 



