294 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



production rates. (Some ecologists consider the 

 three — numbei-s, biomass, and energy — as different 

 concepts of the same pyramid.) The concept of 

 the pyramid of numbers was, perhaps, first given 

 definite form by EUon (1927: p. 69). The pyra- 

 mid of biomass was described for some Wisconsin 

 lakes by Juday (1942) and similar studies have 

 been made in other areas. The pyramid of pro- 

 duction rates was introduced by Petersen (1918) 

 in his calculations of the quantitative food rela- 

 tions in coastal waters where eelgrass, Zostera sp., 

 is the main source of primary food. He postulated 

 that 10 plant units were needed to produce 1 unit 

 of herbivorous animal weight, and in turn, 10 

 herbivorous units were required for each unit 

 weight of primary carnivore produced. The 

 pyramid of energy, expressed in terms of effi- 

 ciency or productivity rates for the various trophic 

 levels, has been determined for lake environments 

 by Lindeman (19-12) and Juday (1942), and for 

 a marine area — Georges Bank — by Clarke (1946). 



A table of conversion factors, the pounds of feed 

 needed to produce 1 pound of fish, is given in 

 Bi-own (1957 : vol. 1, p. 386) ; they range from 2.3 

 to 7.1 for various species. Petersen's lOX factor, 

 however, is the one most frequently quoted. 



The Honolulu Laboratory staff has obtained 

 estimates of the biomass, or standing crop, at three 

 trophic levels in the central Pacific : zooplankton, 

 forage organisms, and tunas. Information on rate 

 of production has also been obtained for the pri- 

 mary producers, i.e., the uptake of C^* by phyto- 

 plankton. Such data are not available for the 

 other trophic levels but may be calculated from the 

 information on hand. 



Employing the data used in figure 13, we cal- 

 culate that the average catch of the 6-foot trawl 

 in 78 hauls M-as 1.86 ml./ 1,000 m.^ of water 

 strained, and the average catch of the 10-foot 

 trawl in 157 hauls was 1.65 ml./l,000 m.^ For the 

 same general area, zooplankton hauls averaged 

 29.64 ml./l,000 m.^, or roughly 16 to 18 times the 

 catch of the trawls. Although we are dealing 

 with two standing-ci'op measurements, the rela- 

 tion is of the same order of magnitude as the 10 X 

 factor calculated by Petersen (1918) for the dif- 

 ference in production rate between two adjacent 

 trophic levels. 



Over the range of latitude sampled (49° N. to 

 19° S.) , variations in zooplankton abundance were 



SEC 



SEC ECC NEC 



NPC 



AC 



S-LATITUOE-N 



Figure 21. — Variation with latitude in the average volume 

 of travil catch per cubic meter of water strained by the 

 6- and 10-foot Isaacs-Kidd trawls, as compared with the 

 zooplankton catch. (Zooplankton data for areas south 

 of latitude 30° N. are from King and Hida, 1957a and 

 1957b; for areas to the north of 30° N. from files of 

 Bureau of Commercial Fisheries Biological Laboratory, 

 Honolulu.) 



closely paralleled by the trawl-catch volumes 

 (fig. 21) , except in the region of the Aleutian Cur- 

 rent, where there was a marked increase in zoo- 

 plankton catch that was proportionally much 

 greater than the increase in trawl catch. Both 

 measures of the fauna show the influence of i- 

 richment in the region of the equatorial upwel' ig 

 and to the north in the Aleutian Current. 



Although the 6-foot trawl caught at a higher 

 rate per unit of water strained than the 10-foot 

 trawl in five of the six areas, the close correspond- 

 ence in catch rates between the two trawls is of 

 interest and provides further evidence that the 

 trawls were about equal in efficiency. If the dif- 

 ference between the two trawls is of any signifi- 

 cance, however, it may indicate that the larger 

 net was sampling at a slightly different trophic 

 level than the smaller net. This possibility is also 

 indicated by the difference in size of organisms 

 sampled by the two nets (table 7). 



TRAWL CATCHES AS A MEASURE OF TUNA FOOD 



One of the major aims of the midwater trawling 

 progi-am was to obtain a quantitative measure of 

 the abundance and distribution of potential tuna 

 food. Several reports have been published, others 



