cruises 20 and 21 were of longer duration and 

 provided synoptic observations of both insular 

 and oceanic waters throughout the Hawaiian area. 

 On Gilbert cruise 16, a short series of hauls 

 was made to determine the difference in catch 

 of zooplankton and tuna larvae in 0- to 100- 

 meter and in 0- to 200-meter tows during day 

 and night hours. 



The approximate locations of the plankton 

 stations are shown in figures 1 to 4. More 

 exact positions together with date and hour of 

 hauling, amount of water strained, and the zoo- 

 plankton volumes for each station are given in 

 the Appendix, tables 6 to 11. Data collected on 

 earlier cruises in 1950-52 and published in a 

 previous report (King and Hida 1954), are also 

 utilized in this study. 



METHODS 



All of the collections were obtained with 1- 

 meter (mouth diameter ) nets of 30XXX silk grit 

 gauze (apertures averaging 0.65 mm. in width), 

 equipped with flowmeters. Oblique hauls of ap- 

 proxinnately 30 minutes' duration to a depth of 

 about 200 nneters were employed at most 

 stations. At station C, one of the "permanent" 

 stations visited on a number of cruises, hauls 

 were made to 40 or 50 meters because of the 

 shallow depth of water (50 to 100 meters); on 

 Gilbert cruise 16 a short series of oblique hauls 

 was made to 100 meters depth. A more connplete 

 description of the net used and of the method of 

 hauling plus the calculation of sampling depth 

 and amount of water strained have beenprovided 

 in an earlier report (King and Demond 1953). 



The method of processing the samples in 

 the laboratory was essentially as employed by 

 King and Hida (1954). Following the usual pro- 

 cedure at the POFI laboratory, the volume of all 

 organisms less than 2 cm. in length plus the 

 volume of organisms 2 to 5 cm. in length that 

 might be considered of significant nutritional 

 value, were combined to give a single volume 

 nneasurement for each sample. This figure was 

 divided by the estimated amount of water passing 

 through the net, as determined fronn the flow- 

 meter reading, to obtain the volunne of zooplank- 

 ton, as food, per unit of water strained. 



The volunnes from all standard hauls were 

 adjusted to remove the effects of diurnal varia- 

 tion by the method described by King and Hida 

 (1954). Briefly, this consists of calculating the 

 regression of plankton volume on solar time of 

 day, the latter expressed as a sine function. 

 The zooplankton volumes are increased or re- 

 duced dependent upon the hour of hauling and 



adjusted to 0600 or 1800 hours when the 

 corresponding sine value is 0. Solar time is 

 presently used in the calculations. The method 

 provides a reasonably good correction for day- 

 night differences as judged by the significance 

 of the "t" values and the night/day ratios for the 

 adjusted volumes (table 1). 



The data from the 7 cruises listed in table 1 

 were adjusted by individual cruise. A pooled 

 regression coefficient (b = 0.1110) calculated 

 from the connbined data of these 7 cruises, n-iost 

 of which provided rather thorough coverage of 

 the Hawaiian area and during which stations were 

 visited consecutively regardless of time of day 

 or night, was used in adjusting the few volumes 

 of cruises 11, 12, 13, and 16 of the Gilbert and 

 cruises 22, 23, 24, and 26 o f the Snnith. On 

 these latter cruises sampling was not conducted 

 around the clock, or there were too few data to 

 be adjusted by individual cruise. Throughout 

 this report we have ennployed the adjusted 

 volumes, except when noted for shallow hauls. 



VARIATION WITH DEPTH 



A short series of experimental hauls made 

 on Gilbert cruise 16 provided an opportunity to 

 compare the difference between the standard, 

 200-meter, oblique haul and a 100-meter, 

 oblique haul in the catch of zooplankton during 

 both day and night periods. A special effort was 

 made to obtain an accurate count of the tuna 

 larvae and other fish larvae in these samples. 

 The results, summarized in table 2, demon- 

 strate the superiority of night over day and of 

 shallow over deeper hauls in the capture of zoo- 

 plankton, tuna larvae, and other fish larvae. 

 Disregarding depth of hauling, the night samples 

 averaged 2.5 times as great in bulk as the day 

 samples. Inrespect to depth, disregarding time 

 of hauling, the 100-meter hauls averaged 1.6 

 times as great as the deeper 200-meter hauls. 



Although the data are very limited they 

 provide some other interesting conclusions. For 

 zooplankton as a whole the difference in catch 

 rate between 0- to 100- and 0- to 200-nneter 

 hauls is slightly less in the daytime than at night, 

 resulting, we believe, from a downward migra- 

 tion and general dispersal of the plankton 

 throughout the upper layers during the day, and 

 its greater concentration near the surface at 

 night. The tuna and other fish larvae were 

 generally more abundant near the surface at 

 both times, but the vertical differences in their 

 distribution were not quite as great at night as 

 in the day. Also in respect to the fish larvae, 

 there was a higher night/day ratio for the 200- 

 meter hauls than for the 100-meter hauls. Both 



