PLANKTON CONCENTRATION — EASTERN & TROPICAL PACIFIC 381 



that characterize the areas (with some overlapping of sparse numbers) 

 are the hyperiid amphipods (T. Bowman M.S.) and the euphausiids 

 (B. Boden, M.W. Johnson and E. Brinton, M.S.). 



South of 35 °N the southern extension of the more abundant 

 northern species is commonly 150 miles offshore in the California cur- 

 rent in the deeper layers. Apparently they do not readily survive the 

 northward flowing counter currents and eastern portions of eddies that 

 often prevail inshore south of Point Conception. 



In the offshore area the picture is not so clear since the distribution 

 there involves more oceanic and warm water species which extend into 

 the warm surface waters to the north outside the California current, 

 and in the south, tend to impinge more closely upon the coast of Baja 

 California. 



The plankton volumes obtained by the Scripps Institution "Shell- 

 back Expedition" 1952 have been studied by Milner B. Schaefer and 

 Leo Berner, who find an offshore continuation of low plankton all the 

 way to the equatorial current system. 



Tropical Pacific 



It has long been known that in certain offshore oceanic areas in 

 the region of the equatorial current system, there occurs an increased 

 abundance of fish, birds and larger invertebrates. More recent studies 

 have shown that there is also an increase in the plankton abundance, 

 thus providing an explanation for the impressive display of higher 

 forms. The equatorial crossings reporting increased plankton within 

 the current system are Graham (1941) for the Carnegie crossing at ap- 

 proximately 140°W; King and Demond (M.S.) for Pacific Oceanic 

 Fisheries Investigations crossings in the central Pacific between lon- 

 gitudes 155° and 175°W. King and Demond report similar findings by 

 Takashi Tokioka (1942) along 134°E. 



In the present paper is given the results of plankton hauls taken 

 aboard the Scripps Institution vessel "Horizon" in connection with the 

 "Capricorn Expedition" in 1952-53. The area covered extends over 

 much of the tropical Pacific mainly below the equator but w^ith cross- 

 ings of the equatorial current system at about 169°E and about 124°W. 

 (Figs. 3 & 4) 



Methods: Two types of nets were employed: (1) a 1-meter net, of the 

 type described above, for the general larger zooplankton, and (2) a small 

 truncated net of No. 20 bolting silk to sample the microplankton. This 

 second net had a filtering cone one meter long attached to a ring 28 

 cm. in diameter. The truncated portion had a mouth opening of 18 cm. 



