The average volume of zooplankton for the day 

 hauls was 15 cc./l,000 in.'', lower than that for any 

 previous period. 



JUNE TO JULY 1958 



Except for a continued rise in temperature and 

 a shallower isothermal layer, the changes during 

 this period were not very great. The average to 

 60 m. temperature was 25.5° C. The a\-erage to 

 60 m. salinity increased to 34.86%o. The average 

 200 to 300 m. salinity declined further to 35.10%o. 

 The average depth to the top of the thermocline 

 was 49.8 m. 



The night volumes of zooplankton for June to 

 July averaged 41 cc./l,000 in.^ as compared to the 

 eight-period average of 37 cc./l,000 m.^, and the 

 average day volume of 20 cc./l,000 m.^ was the 

 same as the nine-period day average. Siphono- 

 phora, Annelida, Amphipoda, Tlialiacea, and fish 

 larvae were relatively abundant in this period. 



AUGUST TO NOVEMBER 1958 



Changes in temperature, salinity, and depth of 

 isothermal layer continued. Tlie to 60 m. tem- 

 perature reached its 1958 peak in this period with 

 an average of 26.3° C. The average to 60 m. 

 salinity (34.02%o) increased over the pre^-eding 

 period (34.86%o). The average 200 to 300 m. 

 salinity continued to decline to 35.00%o. This 

 period and June to July 1957 had the lowest 200 

 to 300 m. salinities for the nine periods. The depth 

 to the top of the thermocline deepened to an 

 average of 74.0 m. 



The low abundance of zooplankton was similar 

 to the lows of June to July 1957, December 1957 to 

 Marcli 1958, and r>eceml>er 1958. Only tlie averages 

 for Pteropoda and Pelecypoda were above the 

 eight-period average. 



DECEMBER 1958 



This period was characterized by a marked de- 

 crease in temperature, rising salinities, and a pro- 

 gressively deepening isothermal layer. As in 1957, 

 the to 60 m. temperature underwent a large drop 

 between November and December. For 1958 the 

 decline was from 25.9° C. in November to 24.1° C. 

 in December (fig. 6A). The to 60 m. salinity of 

 35.07%o continued to increase from 34.82%o in July 

 (fig. 6B). The 200 to 300 m. salinity (35.06%o) in- 

 creased from the previous period. The depth to 

 the top of the thermocline was 93.3 ra. 



The volume of zooplankton for December 1958 

 was again lower than the nine-period average. 

 Except for seven groups {Halosphaera. Forami- 

 nifera, Ostracoda, Amphipoda, Heteropoda, 

 Thaliacea, and fish larvae) , however, the plankton 

 during December 1958 was more abundant than 

 in June to July 1957, December 1957 to March 

 1958, and August to November 1958. 



COMPARISON BETWEEN PERIODS 



A noticeable feature in the 19 months' observa- 

 tions was the dissimilarity of the hydrographic 

 features for the same months of the 2 years. The 

 200 to 300 m. salinities for June to July were sub- 

 stantially lower in 1957 than in 1958, whereas the 

 to 60 m. and 200 to 300 m. salinities for August 

 to November were liigher in 1957 than in 1958. The 

 depth to the top of the thermocline was shallower 

 in the fall of 1957 than in the fall of 1958. Tlie 

 to 60 m. temperatures reflected the seasonal fluctu- 

 ations of high temperatures in summer and fall 

 and low temperatures in winter and spring and 

 so did not vary greatly between years for the same 

 months. 



Zooplankton was more abundant during the fall 

 of 1957 than in the fall of 1958 (figs. 4 and 9). 

 Previous studies of th& distribution of zooplankton 

 in Hawaii have indicated a lack of consistent sea- 

 sonal change. In 1950 and 1951, volumes were 

 significantly liigher during early summer and 

 midsummer than in late summer and fall (King 

 and Hida, 1954). But in 1956, the standing crop 

 of zooplankton was greatest in January, April, 

 and September (Nakamura, 1967). 



June to July 1957 and August to November 1958 

 were alike. Both had .similar T-S curves (fig. 7), 

 hence their temperature and salinity character- 

 istics were similar (fig. 8). Both of these periods 

 had low volumes of zooplankton (fig. 9). The day 

 volume of zooplankton for June to July 1957 was 

 equal to the nine-period mean while that for Au- 

 gust to November 1958 was low. 



December 1957 to March 1958 and May 1958 

 were also alike. Both had similar T-S curves (fig. 

 7). Both had low day volumes of zooplankton. A 

 night volume was not available for May 1958. 



The two jieriods (Septemlx>r to November 1957 

 and April 1958) that had high night and day 

 volumes of zooplankton (fig. 9) also had high 



VARIATIONS IN MARINE ZOOPIiANK/TON IN HAWAIIAN WATERS 



97 



