VARIATIONS IN MARINE ZOOPLANKTON FROM A SINGLE LOCALITY IN 



HAWAIIAN WATERS 



BY RICHARD S. SHOMURA AND EUGENE L. NAKAMURA, FISHERY BIOLOGISTS 



BUREAU OF COMMERCIAL FISHERIES BIOLOGICAL LABORATORY 

 HONOLULU, HAWAII 95812 



ABSTRACT 



Data on marine zooplankton and hydrography were 

 obtained off Oahu, Hawaii, at monthly intervals from 

 June 1957 through December 1958. 



Samples collected at 3-hour intervals for 48 hours 

 in June 1957 were examined for diel variation. Volumes 

 of zooplankton exhibited the expected sinusoidal vari- 

 ation during the first 24 hours, but not during the 

 second. High volumes during the second morning 

 were attributed to the presence of an unidentified 

 diatom that retarded drainage of moisture during de- 

 termination of volumes of zooplankton. Variations in 

 abundance were sinusoidal for Ostracoda, Euphausi- 

 acea, Pteropoda, and fish larvae. Similar variations in 

 surface temperature and depth from the surface to 

 the top of the thermocline were attributed to solar 



heating and internal waves, respectively. 



The 19 monthly samples showed correlations between 

 volumes of zooplankton and salinities and between 

 volumes of zooplankton and depth to the top of the 

 thermocline. The 19 months were divided into nine 

 successive periods, each with a temperature-salinity 

 curve differing from that of the preceding and follow- 

 ing periods. Volumes of zooplankton and the abundance 

 of Siphonophora, Chaetognatha, Euphausiacea, deca- 

 pod Crustacea, and Pteropoda increased when portions 

 of the temperature-salinity curves greater than 35.0 %„ 

 increased during the nine periods; and, conversely, the 

 volumes of zooplankton and the abundance of these 

 groups of zooplankters decreased as the amount of 

 water with salinity greater than 35.0 %o decreased. 



Once every month from June 1957 through 

 December 1958, members of the Bureau of Com- 

 mercial Fisherie.s Biological Laboratory, Honolu- 

 lu, collected hydrographic and meteorological 

 data from a station located at lat. 21° 10.3' N., long. 

 158° 19.0' W. for the Island Observatory Project 

 of the International Geophysical Year Oceano- 

 graphic Program (Scripps Institution of Ocean- 

 ography, 1965). This position, i-ef erred to as the 

 IGY station, is about 32 km. southwest of Barbers 

 Point, Oahu, where the depth of the water is about 

 3,000 m. 



Samples of zooplankton were also collected at 

 the IGY station for studies of variations in the 

 standing crop and composition of marine zoo- 

 plankton in relation to enviromnental factors at 

 a single locality in Hawaiian waters. The results 

 showed that volumes of zooplankton were high in 

 waters of high surface and subsurface salinities. 



METHODS 



The samples of zooplankton were collected with 

 a net having a mouth diameter of 1 m. ; a flow- 



Published September 1969. 



FISHERY BULLETIN: VOL. 68, NO. 1 



meter was mounted in the center of the opening. 

 The body of the net was constructed of synthetic 

 fiber of 0.656 mm. aperture width ; the rear section, 

 including the cod end, was made of synthetic fiber 

 of 0.308 mm. aperture width. Detailed descriptions 

 of tlie construction of the net and the method of 

 making the tow have been presented by King and 

 Demond(1953). 



The nets were towed obliquely from the surface 

 to a depth of 60 m. and back in one-half hour. In 

 June 1957, such hauls were made at 3-hour in- 

 tervals over a 48-hour period to measure diel 

 variations. In the succeeding months, four half- 

 hour hauls were made each time the station was 

 occupied : two successive daylight hauls starting 

 at about 1600 l.s.t. (local standard time) and two 

 successive night hauls between 2100 and 0300 l.s.t. 



The methods of processing and determining the 

 volumes of the samples of zooplankton have been 

 described by King and Hida ( 1954, 1957a) . Counts 

 of various groups of plankters were made on the 

 night samples. 



For the counts, an aliquot of a sample was 

 first placed in a counting cell (a 15 by 20 by 1.5-cm. 



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