Abundance and Distribution of 

 Zooplankton in Hawaiian Waters, 1955-56 



By 



EUGENE L. NAKAMURA, Fishery Biologist 



Bureau of Commercial Fisheries Biological Laboratory 

 Honolulu, Hawaii 96812 



ABSTRACT 



Methods and results of a study of zooplankton are described. Sampling was 

 conducted on eight cruises by vessels of the Bureau of Commercial Fisheries 

 Biological Laboratory, Honolulu, and on nine cruises by the motor vessel Makua 

 of the Hawaii Division of Fish and Game. Sampling included oblique hauls 

 with a single open net at to 60 m. and to 200 m.. and three nets towed simul- 

 taneously at three levels: an open net at to 60 m. and closing nets at 70 to 130 

 and at 140 to 200 m. (estimated depths). 



The catches from the three-net hauls revealed a greater abundance of zoo- 

 plankton in the uppermost layer than in deeper water regardless of the time of 

 sampling. Differences existed between windward and leeward areas of the island 

 of Oahu at certain times, but one area did not have consistently greater volumes 

 of zooplankton than the other area throughout the seasons. The significance of 

 seasonal differences was masked by the possibility of annual fluctuations. 



Composition of plankton varied by depth, season, and area. Relative abun- 

 dance was comparatively stable with depth and season. Decapod crustaceans 

 were consistently more abundant in the windward than in the leeward area. 

 Halosphaera viridis , a planktonic alga, was sometimes very numerous. Various 

 groups of zooplankton exhibited conspicuous diel movement. 



INTRODUCTION 



Results of studies of Hawaiian zooplankton 

 collected in 1950-52 and in 1953-54 have been 

 reported by King and Hida (1954, 1957). This 

 paper presents results from sampling in 1955- 

 56. Spatial and temporal variations in the dis- 

 tribution and abundance of zooplankton are em- 

 phasized, but unusual occurrences of phyto- 

 plankton are also discussed. 



MATERIALS AND METHODS 



Table 1 lists the ships, cruises, and dates on 

 which samples of zooplankton were collected. 

 Detailed data for each cruise are presented in 

 tables in the appendix. 



Gear 



Two types of sampling gear were used in the 

 present study: The three-net series described 



by King, Austin, and Doty (1957) and the single 



open net (of the same construction as the upper 



net in the three-net series) described by King 



and Demond (1953). The middle and lower nets 



of the three-net series had opening and closing 



devices. All nets had a mouth diameter of 1 m. 



and were equipped with calibrated flowmeters. 



The hauls lasted about one-half hour. The nets 



were towed at a ship speed of about 2 knots. 



Late in 1956, the silk grit gauze in the main 



body of the nets (30XXX) and in the rear section 



and cod end (56XXX) was replaced with nylon. 



Widths of apertures in the silk and nylon were 



nearly the same: 0.65 and 0.31 mm. for silk 



1 / 

 and 0.656 and 0.308 mm. for nylon^^ 



— E. H. Ahlstrom has informed me that the two 

 nets have different apertures while being towed 

 and also have different straining properties. 

 Unfortunately, records of the type of gauze in 

 the nets, by station and date, are not available. 



