

ZOOPLANKTON ABUNDANCE EST THE CENTRAL PACIFIC 



119 



cal hauls, and 124 percent for horizontal hauls; 

 and they concluded that oblique hauls gave more 

 reliable and consistent results than vertical or 

 horizontal hauls. The more erratic nature of the 

 surface haul, as compared with the oblique, is evi- 

 dent from figure 4, showing the results of cruise 2, 

 on which both types of haul were used. 



That the oblique and surface hauls made on this 

 cruise were not greatly different, however, is in- 

 dicated by a comparison of the volumes (cc./m. 3 ) 

 obtained at 12 pairs of stations (table 1, A and 

 C) , the members of each pair occurring at approx- 

 imately the same latitude, but at a different longi- 

 tude (fig. 1, A). It may be shown that the mean 

 difference (0.00575 cc./m. 3 ) between the paired 

 hauls does not differ significantly from (P=0.4) . 

 Therefore, the few surface hauls made on cruise 2 

 have been included in this report but have been 

 omitted from the statistical analysis dealing witli 

 sources of variation and correlations with environ- 

 mental factors. 



On cruises 2, 5, and 8, stations were visited 

 consecutively regardless of the time of day or 

 night. Because of this practice, the effects of 

 diurnal migration must be considered in an evalu- 

 ation of zooplankton abundance at any place and 

 time. Initially we had hoped, through the use 

 of the oblique tow to 200 meters' depth, to nullify 

 to a large extent differences in the samples caused 

 by vertical movements of the plankton. That we 

 did not succeed is indicated by close perusal of 

 the tables of data, particularly for cruise 5 (table 

 2), and cruise S (table 4), comparing successive 

 day and night hauls. It will be seen that usually 

 the night hauls produced a higher volume than the 

 day hauls. This tendency will be discussed later 

 in the section on sources of variation in zooplank- 

 ton volumes. 



Another problem that adds to the difficulty of 

 estimating zooplankton abundance is the uneven 

 distribution of plankton organisms. This has 

 been referred to by many planktologists and is 

 emphasized by Haeckel (1890), Herdman (1923), 

 Gardiner (1931), Hardy (1936), Wilson (1942), 

 Riley and Bumpus (1946), Sears (1950), and 

 others. By our method of straining a large vol- 

 ume of water, averaging over 1,000 cubic meters 

 per haul, and of sampling in uniform fashion from 

 the surface to 200 meters' depth, it is assumed that 

 the variation in catch due to the uneven distribu- 

 tion of organisms is minimized. 



The amount of water strained during each haul 

 was measured by a flow meter suspended in the 

 mouth of the net. Each flow meter was calibrated 

 by towing it over a measured course at approxi- 

 mately the same speed used in making the plank- 

 ton hauls. The flow meters were calibrated be- 

 fore and after each cruise, and the average of these 

 calibrations was used to compute the volume of 

 water strained in cubic meters for each haul dur- 

 ing that cruise. Within a limited range of tow- 

 ing speeds the number of revolutions registered 

 by the meter indicates the length of the water 

 column passing through the net ; multiplying this 

 length by the area of the mouth of the net gives 

 an estimate of the water volume strained. 



There has never been evidence of clogging on 

 any of the tows, possibly because of the relatively 

 coarse mesh used in the nets and the general pau- 

 city of plankton. 



In making the tow, the net and a 75-pound 

 streamlined weight were attached to the cable, 

 which was paid out slowly at uniform speed. As 

 the net was lowered, the length of wire out and 

 the angle of stray were recorded at 2-minute 

 intervals. As soon as a calculated depth of 200 

 meters was reached, the net was retrieved at a 

 slow, uniform speed. The wire angle and the 

 length of wire out were again recorded at 2-minute 

 intervals. At a towing speed of about •_' knots, 

 an oblique tow to a depth of 200 meters and return 

 required about 30 minutes. A graph of depth 

 reached plotted against time (fig. 3) for 3 tows 

 made on cruise 5, shows that, for practical pur- 

 poses, equal amounts of time are spent at all 

 depths; i. e., assuming the towing wire represents 

 a straight line in the water, the net strains approxi- 

 mately the same amount of water for each meter 

 of depth passed through. It is recognized that 

 the towing wire does not actually describe a 

 straight line during the tow, but the error caused 

 by a slight curve is small. 



When the net reached the surface at the end of 

 a tow, it was lifted out of the water, suspended 

 vertically from a boom, and washed down with a 

 hose. The plankton bucket was then detached, 

 and its contents were washed into an enameled 

 pan. Next, the sample was transferred to a 

 1-quart fruit jar, and sufficient formalin was 

 added to approximate a 10-percent solution. The 

 formalin was neutralized with borax. A com- 

 pleted label was placed in the jar. 



