FISHERY BULLETIN: VOL. 70, NO. 3 



Appendix Figure 2. — Latitudinal distribution of longline-caught fish on long 150°W during January-February 

 1953, a period of poor fishing success. A. Number of fish per 100 hooks. B. Weight (in kilograms) per 100 hooks. 



weight of fish life at a given trophic level and 

 compare that weight with other properties of the 

 biosphere, e.g., standing crop of zooplankton. 

 Uniform sampling of a given trophic level, espe- 

 cially if it comprises several species, is difficult. 

 In the longline, however, we have a tool that 

 appears to sample a reasonably restricted trophic 

 level. All of the principal species taken are 

 definitely carnivorous, as shown by their attrac- 

 tion to fish baits, so this characteristic defines 

 and restricts the "trophic level." In the present 

 instance the trophic level is composed principally 

 of tunas, sharks, and billfishes. 



Additional information that aids in defining 

 the trophic level stems from studies of the food 



habits of the fishes. King and Ikehara (1956) 

 have shown that the yellowfin and bigeye tunas 

 in the central Pacific essentially share a common 

 diet of small to medium-sized invertebrates and 

 fishes. Recent studies suggest that the same 

 statement applies to the skipjack tuna and al- 

 bacore. Most pelagic sharks taken on the long- 

 line in the central Pacific also seem to share the 

 tuna forage (Strasburg, 1958) . Large billfishes, 

 however, appear to represent a somewhat higher 

 trophic level. The smaller items in their diet 

 approximately coincide with the large items in 

 the diet of the tunas and sharks. Thus, there 

 are good grounds for considering tunas and 

 sharks as representatives of a restricted trophic 



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