hauls to a depth of about 140 m. (Thrailkill, 

 1963); and on all other operations in the 

 eastern tropical Pacific on which this net was 

 used, oblique hauls were made to about 300 m. 

 The volumes of catches from hauls made to 

 different depths are not comparable, even when 

 standardized to a uniform volume of water 

 strained, for reasons given under the discus- 

 sion of vertical distribution. In the later sec- 

 tions on horizontal and seasonal distribution, 

 therefore, only the catches (milliliters per 

 1,000 m. 3) from oblique hauls to a depth about 

 300 m. have been considered; actually, because 

 the depths reached in these hauls often dif- 

 fered considerably from the desired 300 m., 

 hauls to any depth in the range 201 to 400 m. 

 were included. These hauls were more 

 numerous than any other kind made in the 

 eastern tropical Pacific (except off Baja Cali- 

 fornia where the CalCOFI hauling routine was 

 used), and the greater depth range ( to 300 m.) 

 has the advantage that it probably minimizes 

 diurnal variations. No distinction was made 

 between hauls at different times of day or 

 night. None of the results from operations 3, 

 6, 8, and 13 have been used. 



Most of the data lists distinguish between 

 volumes of small zooplankton and total zoo- 

 plankton; the difference (frequently small or 

 nil) is the volume of organisms longer than 

 5 cm. or with a volume greater than 5 ml. 

 Only volumes of small zooplankton have been 

 used here. 



Micronekton 



This category includes active animals about 

 I to 10 cm. long that were generally collected 

 with the large net and towing routine described 

 by Blackburn and associates (1962) and Black- 

 burn (MS.^). 



A certain technique was used to collect 

 micronekton. An oblique haul was made at 

 night between about 90 m. and the surface, 

 from a ship steaming at 5 knots, with a large 

 net of uniform mesh-aperture about 5.5 mm. 

 by 2.5 mm. Volume of water strained was 

 estimated from mouth-area, distance towed, 

 and an empirically obtained filtration coeffi- 

 cient. Volumes of micronekton (everything 

 taken by the net, except sea snakes and watery 

 planktonic animals such as tunicates and 

 siphonophores) were expressed in milliliters 

 per 1,000 m.'' by Blackburn (MS., see foot- 

 note 2); in data lists published hitherto, only 

 total volumes of micronekton were given. 

 Other kinds of nets and hauls were used for 

 nnicronekton on operations 8 and 18; the re- 

 sults could not be compared with those from 

 the method described above. 



^Blackburn, Maurice. Micronekton of the eastern tropi- 

 cal Pacific Ocean: family composition, distribution, abun- 

 dance, and relationships to tuna. (Scripps Institution of 

 Oceanography, University of California, 1965). 



VERTICAL DISTRIBUTION OF 

 BIOLOGICAL PROPERTIES 



Holmes (MS., see footnote 1) summarized 

 the available information about vertical dis- 

 tributions of chlorophyll a and primary pro- 

 ductivity in the upper 150 m. of the eastern 

 tropical Pacific. Of 108 noon-station profiles 

 of chlorophyll a^ which were based on five or 

 more sampling depths each, he found maxima 

 in the upper part of the thernnocline on 90, 

 at the bottom of the isothermal layer on 5, and 

 elsewhere within the isothermal layer on 13. 

 Maxima appeared to be in the upper part of 

 the thermocline on night-station profiles also, 

 although there were only 18 stations and only 

 3 sampling depths to each. The data did not 

 appear to justify generalization about the 

 relative proportions of active and inactive 

 chlorophyll a in the region of the nnaximum 

 or elsewhere along the water column; neither 

 could the author give reasons for the existence 

 of the maximum at the depths at which it 

 occurs. More recently, Lorenzen (1965) 

 presented data which suggest that maxima 

 near the top of the thermocline or in the 

 isothermal layer are composed primarily of 

 chlorophyll in living cells. 



For 21 profiles of primary productivity 

 incubated in situ in the eastern tropical 

 Pacific, Holmes (MS., see footnote 1) distin- 

 guished four categories: those in which pro- 

 ductivity decreased exponentially with depth; 

 those with two maxima, one at the surface and 

 one lower; those with two maxima, both below 

 the surface; and those with rather uniform 

 productivity in the upper 30 m. The profiles 

 differed little below about 50 m. Holmes did 

 not draw any definite conclusions about the 

 reasons for the existence of such different 

 distributions and observed that some profiles 

 varied considerably from day to day in the 

 same locality. He pointed out that surface 

 inhibition of photosynthesis did not seem to be 

 pronounced, and that production seemed to be 

 appreciable below the mixed layer in sonne 

 parts of the eastern tropical Pacific. 



Holmes (MS., see footnote 1) analyzed the 

 results of zooplankton hauls made at the same 

 24 stations and times of day with three 

 opening-closing nets or samplers, hauled 

 horizontally at particular depths. Hauls in the 

 upper part of the thermocline or the lower part 

 of the mixed layer were nearly always richer 

 than hauls made near the same time deeper 

 in the thermocline. Day and night hauls showed 

 no obvious difference in the distribution of 

 zooplankton. 



Zooplankton hauls are generally made over 

 standard depth ranges, irrespective of thermal 

 structure. At three stations at which opening- 

 closing samplers were used at six depths each, 

 analysis of the resulting profiles of the stand- 

 ing crop of zooplankton showed that over the 

 range to 140 m., an average of 62 percent 



