also did not diflfer greatly in taxonomic composi- 

 tion (table 2) . These results were expected because 

 all the catches were made in the same body of 

 surface water. They suggest that the catches of 

 the other, more isolated, single hauls listed in this 

 jDaper are reasonably representative, both qualita- 

 tively and quantitatively, of the micronekton pop- 

 ulation sampled. 



AREAL DISTRIBUTION OF 

 MICRONEKTON BY TAXA 



Table 1 gives the actual and standardized 

 volumes of fish, crustaceans, and cephaloiK>ds for 

 each of the 131 standard night hauls of the 1.5-m. 

 net, excluding the special series mentioned in the 

 previous section. It also gives the number, date, 

 and area (see below) of each of the stations at 

 which each haul was made. 



The approximate position of each station is 

 shown in figure 1 or figure 2. The actual positions 

 of the stations are available from the following 

 sources: cruise T0-5S-1, Holmes and Blackburn, 

 1960; TO-59-1 and TO-59-2, Blackburn et al., 

 1962; TO-60-1, Scripps Institution of Oceanog- 

 raphy, 1967; TO-60-2, Scripps Institution of 

 Oceanography, 1961; TO-61-1, Blackburn, un- 

 published. 



The stations were assigned to areas which are 

 shown in figure 5. Areas 1 to 14 are those recog- 

 nized by Alverson (1963a) in a study of the food 

 of tropical tunas taken in the U.S. surface tuna 

 fishery. Areas 15 to 18 enclose stations occupied 

 farther offshore. No stations with micronekton 

 hauls were occupied in areas 7 and 13 ; areas 11 and 

 12 have been combined in this paper; a few stations 

 close to the boundaries of certain areas have been 

 assigned to tliose areas for convenience. 



For the 131 hauls, the actual tota.l volumes were : 

 12,756 ml. of fish; 25,240 ml. of crustaceans; and 

 2,008 ml. of cephalopods; grand total, 40,004 ml. 

 These numbers ai'e given by taxa (including some 

 species) and areas in table 3, which is the most con- 

 venient way of presenting the composition of the 

 whole material. Some of these data are used in later 

 sections (tables 8-15). 



Although many families are listed in table 3, 

 only 10, together with tlie Leptocephali of the 

 suborder Apodes (Pisces) whicli liave not Ijeen 



classified to family, contributed more than 1 per- 

 cent of the grand total ; these 11 groups combined 

 accounted for 93.4 percent (table 4) . The Euphau- 

 siacea of table 3 are all members of the family 

 Euphausiidae. 



Table 5 gives standardized volumes of the prin- 

 cipal fish and crustacean components of the micro- 

 nekton as listed in table 4, for each station. The 

 sum of volumes equals the standardized volumes 

 for total fish and total crustaceans in table 1. 



Figures 6 to 13 show distributions of standard- 

 ized group volumes for the two most extensive 

 cruises, TO-58-1 (SCOT) and TO-60-2 (STEP- 

 1) ; figure 1 identifies the stations; the data are 

 fi'om tables 1 and 5. These cruises were made in 

 the Northern Hemisphere in the northern spring 

 and the Southern Hemisphere in the southern 

 spring, respectively. The two space-time situations 

 were comparable climatically; trade winds were 

 declining in a.verage strength from their seasonal 

 maximum about late winter or early spring. A 

 general similarity should, therefore, exist between 

 the two situations in certain wind-connected 

 upper-ocean features which affect the production 

 and distribution of organisms. For instance, 

 amount of coastal upwelling, mean depth of mixed 

 layer, ami mean velocity of westerly surface cur- 

 rents would all be expected to be declining, as a 

 result of the decrease in the trade winds, in each 

 situation. Because the two cruises were comparable 

 in range of latitude from the Equator, and range 

 of distance offshore, it is reasonable to combine 

 biological data from both in the way that has been 

 done in figures 6 to 13. These figures, then, give 

 the best available picture of regional distribution 

 of the standing crops of vai-ious kinds of micro- 

 nekton over a. large part of the eastern tropical 

 Pacific, under comparable physical conditions. 



Figures 6, 7, and 8 chart the distribution of fish, 

 crustaceans, and cephalopods, respectively (data 

 from table 1 ) . They show clearly that the standing 

 crop of each of tliese three components of the 

 micronekton declines from onshore to offshore. 

 Table 6 shows tlie magnitude of these changes. 



This distribution is rather similar to that of 

 standing crops of chlorophyll a and zooplankton 

 in the same region (Brandhorst, 1958; Bennett, 

 1963; Forsbergh and Josepii, 1964; Blackburn, 

 1966b). Each of the three standing crops — chloro- 



82 



U.S. FISH AND WILDLIFE SERVICE 



