AHLSTROM: FISH LARVAE IN EASTERN TROPICAL PACIFIC 



meshed net when hauled obliquely, but was used 

 alone for taking surface hauls. The finer- 

 meshed net was 0.5 m in diameter at the mouth, 

 constructed of 333 /^ Nitex cloth, with approx- 

 imately an 8 to 1 ratio of effective straining 

 surface to mouth area. The third net, used for 

 collecting micronekton, had a 5-ft square mouth 

 opening and was constructed of mesh measuring 

 approximately 5.5 X 2.5 mm; this net could 

 not be operated from the research vessel Rock- 

 away on EASTROPAC I but was employed from 

 the other three vessels. 



Usually four zooplankton collections were 

 made at each "biological" station: an oblique 

 collection and a surface collection with the 1-m 

 net, an oblique collection with the 0.5-m net, and 

 an oblique collection with the micronekton net. 



In taking oblique plankton hauls, the 1-m net 

 was paired in an assembly frame with the 0.5 

 m net. The assembly of nets was fastened to 

 the towing cable by a bridle about 5 m above 

 a 100-lb weight. The assembly was lowered to 

 depth by paying out 300 m of towing cable at 

 the controlled rate of 50 m of wire per minute. 

 The assembly remained at depth for 0.5 min and 

 then was retrieved at a uniform rate of 20 m 

 per min. Total towing time was about 21.5 min. 

 Towing speed was ca. 2 knots. The depth 

 reached by the net was estimated from the angle 

 of stray (departure from the vertical) of the 

 towing cable. We sought to maintain an angle 

 of stray of 45°, which lowered the assembly 

 to a depth of approximately 210 m. Our con- 

 cern was to sample the upper 200-m stratum. 

 The average depths of hauls taken by the four 

 research vessels are summarized in Table 1. 

 Over 80 % of the hauls made on EASTROPAC I 

 were lowered to depths of 200 m or more, and 

 nearly 95 ''r reached depths of 180 m or greater. 

 However, two hauls were exceptionally shallow 

 (71-90 m) , and nine additional hauls were taken 

 to depths of less than 150 m. 



Usually four paired net-assembly hauls were 

 taken per day, spaced at about 6-hr intervals. 

 Although the four hauls were planned to be taken 

 at about midnight, dawn, noon, and sunset, the 

 timing of hauls was not coordinated between 

 research vessels. The middle-of-the-night hauls 



Table 1. — Depth of paired oblique plankton hauls taken 

 by the four research vessels on EASTROPAC I. (Net 

 lowered by paying out 300 m of towing cable) 



were all taken before midnight (2201-2400) 

 on Rockaway, for example, while on Argo 

 most hauls, were made after midnight (be- 

 tween 0001 and 0400 hr). The time of day 

 of occupancy of stations (based on the midtime 

 of each haul) is summarized by hourly intervals 

 in Table 2. At least some hauls were taken 

 during every hour of the day, although fewer 

 than 10 (2-8) were obtained during six of the 

 hourly intervals. Fewest hauls were obtained 

 between 0901 and 1000 hr (2 hauls) and be- 

 tween 2101 and 2200 hr (4 hauls), whereas 

 the largest number of hauls were taken between 

 2201 and 2300 hr (59 hauls) and between 1001 

 and 1100 hr (53 hauls). Hauls were made 

 with equal frequency during the four periods 

 of the day on Argo, Jordan, and Rockaway; 

 most plankton hauls were taken near midnight 

 or noon from Alaminos. 



The numbering system for observations em- 

 ployed on EASTROPAC cruises made use of five 

 digits divided into two groups, as 11.022, 12.002, 

 etc. The outer digit preceding the period is the 

 cruise number common to all vessels participat- 

 ing in a given EASTROPAC cruise; for EAS- 

 TROPAC I, this number is 1. The other digit 

 preceding the period is the identifying number 

 given to each research vessel, with the lowest 



