ZOOPLANKTON ABUNDANCE IN THE CENTRAL PACIFIC 



141 



many times our maximum value of 0.1025 cc./m. 3 

 The average of 0.208 cc./m. 3 for the entire "West 

 Coast between 25° N. and 47° N. in April and May 

 1949, is just about 10 times our mean for the cen- 

 tral equatorial Pacific. 



Other regions of sparse plankton are found in 

 the Gulf Stream and in the Sargasso Sea of the 

 Atlantic Ocean. These areas apparently have 

 much poorer plankton populations than Atlantic 

 coastal and continental shelf waters (Clarke 1940; 

 Riley 1939b; Riley and Gorgy 1948). 



Table 18 is a summary of representative plank- 

 ton values for various areas of the Atlantic and 

 Pacific Oceans. Part of the variation in these val- 

 ues may be due to differences in mesh size among 

 the nets used by different investigators. The 

 averages for the central Pacific are the poorest of 

 the lot ; in fact they are so low that one is inclined 

 to speculate as to how the pelagic fish populations, 

 particularly the relatively large population of 

 tunas, are supported. 



Table 18. — Comparison of plankton abundance in various areas of the Atlantic anil Pacific Oceans 

 [Values given in wet volume, estimated number, or wet weight] 



Area 



PACIFIC AREA: 



Samoa to the Marshalls via the Equator. 



Java Sea 



Open sea, Palau Is... 



Bikini, just outside reef _ 



West coast, U.S. (offshore stations) 



Central Pacific- . 



ATLANTIC AREA: 



Baltic Sea 



Gull of Maine (10 to 100 m. depth) 



North Atlantic (coastal water) 



Florida Strait. - 



Oulf Stream, off Florida 



Oulf Stream, off Georgia 



North Atlantic continental slope- 

 North Atlantic, coastal- .. 



North Atlantic, coastal 



North Atlantic, offshore. 



Sargasso Sea 



Oulf Stream 



Slope water 



Plankton values 



0.4 to 0.95 cc./m. ! 



Avg. =0.8.3 cc./m.' 



110 to 530, no./m.' 



26.23 to 62.20 no./m.' 



/0.016 to 0.129 cc./m.' 



I Avg. =0.057 CC./m.' 



5 to 109, no./m. 1 Avg. =35- 



0.002 to 0.102 CC./m.' 



Avg. =0.027 cc./m.' 



1.9 to 11.0 cc./m.' 



0.12 to 4.30 cc./m.'-.. 



Avg.=0.5 to 0.8 cc./m.'. 



0.02 cc./m.' 



0.05 cc./m.' ... 



0.07 cc./m.' 



4.3 cc./m.' 



8.1cc./m.» 



/Avg. =0.54 cc./m.' 



I Max. = 15.5 cc./m.' 



/Ave. =0.40 cc./m.' 



(Max. =3.5 cc./m.' 



Avg. =0.045 gm./m.' 



lsta. =0.137 gm./m.' 



2sta. = 0.14 and 1.6 gm./m.'. 



0.33 mm. 

 0.33 mm 

 0.33 mm. 

 0.37 mm. 



0.65 mm. 

 ■0.65 mm.. 



Mesh aperture of net used 



n.33mm -. 



1.25 mm., front; 0.8 mm., middle 



and rear 



29.38 meshes/in., front; 48.54 

 meshes/in., rear — 



0.158 mm. 



10 strands/cm 

 0.158 mm 



Kramer (1906). 

 D.-lsman (1939). 

 Motoda (1940). 

 Johnson (1949). 



California prog. rept. (1950). 

 This report. 



Kramer (1906). 

 Bigelow (1926). 

 Bigelow and Sears (1939). 



Kiley (1939b). 



Clark (1940). 



Riley and Gorgy (194S). 



PRODUCTIVITY 



The practical application of most plankton re- 

 search is to provide data for estimating and com- 

 paring the productivity, or available food, in 

 various areas of the sea. It lias been strongly em- 

 phasized in more recent plankton literature that 

 the '•standing crop" does not give a true measure 

 of the rate of production. Harvey (1934) and 

 Harvey et al. (1935) have shown that the size of 

 the standing crop of phytoplankton is greatly af- 

 fected by the grazing of animal herbivores and 

 therefore at any one time is merely a momentary 

 balance between the processes of production and 

 consumption. In the tropics, steady grazing by 

 predators may keep the zooplankton at a lower 

 level of abundance than in higher latitudes where 

 seasonal features of the environment allow the 

 plankton to "pulse" or bloom and thus increase 

 much faster than the predators. The apparently 



low standing crop may be considerably counter- 

 balanced by a high rate of turnover and nearly 

 uniform production throughout the year. 



It is generally assumed that in water masses 

 where the annual plant production is great the 

 density of the animal population will also be great. 

 This assumption is roughly borne out by general 

 observations (Harvey 1945). Delsman (1939) 

 has stated, "Where no rich plankton can develop, 

 no rich macrofauna, no abundant fish population 

 can either be expected." In the same vein, "The 

 dependence of various elements of the food chain 

 on a preceding one, conditions the distribution of 

 the larger forms" (Hesse, Allee, and Schmidt 

 1951). Also, it is reasonable to believe that the 

 zooplankton population will be the maximum that 

 the plant crop can support. Local situations may 

 not conform to this generality, but when large 

 areas are considered, there is usually found a direct 



