18). Although it cannot be assumed that each of 

 tJhe prey groups was captured by the net or by the 

 tunas witli the same degree of efficiency in each 

 area, the standardized vohmies may be compared 

 from area to area (for the prey of each tuna species 

 separately), to indicate broad quantitative differ- 

 ences and similarities among areas in the extent to 

 which they support standing crops of potential 

 tuna prey. Obviously, these volumes do not rep- 

 resent absolute concentrations of tuna prey. 



Standardized volumes of skipjack tuna prey 

 were higher than those of yellowfin tuna prey 

 (table 15) for most areas, because they included 

 euphausiids and squillid larvae. Volumes in area 

 1 were by far the highest, because of tlie great 

 abundance of Pleuroncodes planipes; they are 

 further discussed below. The next highest volumes 

 were in areas 8 and 10, but they are based on only 

 two hauls per area and may, therefore, not be rep- 

 resentative; volumes for areas 9, 14, and 15 are 

 likewise ignored because only three or four hauls 

 were made per area. 



Table 15. — Standardized volumes (ml.jKfim?) of total 

 potential prey of yellowfin and skipjack tuna as taken 

 in standard night hauls of the 1.5-m. net, for the areas 

 shown in figure 5 



[Actual volumes (ml.) from tahle 3. other data from table 1; for further ex- 

 planation see text] 



Comparisons may now be made between volumes 

 for areas 1, 2, 3, 4, 5, 6, and 11-12, which have a 

 significant commercial surface fishing for yellow- 

 fin and skipjack tunas and volumes for areas 16, 

 17, and 18, in which there is no significant fishery 

 (see Alverson, 1960, 1963b, for distribution of the 

 surface fishery). The lowest volume of yellowfin 

 tuna prey in an area which supports a fishery was 



3.7 ml. per 1,000 m.', in area 3; the lowest for 

 skipjack tuna prey was 4.9 ml. per 1,000 m.', in 

 area 5. Volumes for area 16 were slightly below, 

 and those for area 18 well below, these figures ; but 

 in area 17 the volume was higher than 3.7 for prey 

 of yellowfin tuna and only slightly lower than 4.9 

 for prey of skipjack tuna. 



Such small differences among areas should be 

 interpreted very cautiously, for reasons given 

 above. It could be argued, however, that area 17 

 has a sufficiently high standing crop of tuna prey 

 to support a standing crop of yellowfin tuna, and 

 possibly one of skipjack tuna, which might prof- 

 itably be fished. Of course, the presence of an 

 adequate crop of tuna food does not guarantee 

 the presence of tuna, as is shown by the general 

 scarcity of skipjack tuna in areas 3 and 4 in the 

 years in which the micronekton was collected 

 (Alverson, 1960, 1963b) ; high sea temperatures 

 may have been limiting in those years, however 

 (Blackburn and associates, 1962; Blackburn, 

 1963). A more conservative conclusion from the 

 data of table 15 is that some areas west of the 

 existing surface fishei-y might support commercial 

 surface-fishing operations and should be further 

 explored ; and that area 17 is likely to be the best 

 and area 18 the poorest of the three areas consid- 

 ered, as far as abundance of yellowfin and skipjack 

 tunas are concerned. 



A Japanese commercial subsurface hook fishery 

 for yellowfin tuna exists at about the same lati- 

 tudes as area 17; it is located west of area 17 but 

 seems to be extending into that area. Similar fish- 

 ing is carried on west of, and to some extent in, 

 area 16, but the catch per unit of fishing is lower 

 than in area 17. No longline fishery exists in or 

 near area 18 (Suda and Schaefer, 1965). Skipjack 

 tuna are not commonly taken by subsurface 

 hooking. 



The high standardized volumes of potential tuna 

 prey in area 1 warrant comment. Although area 1 

 is eutrophic, its mean rate of primary production 

 appears to be comparable with that of some other 

 areas, such as 5 and 6, in which tuna prey are 

 much scarcer (Holmes, 1958; Hela and Laevastu, 

 1962; Blackburn, 1966b). The special feature of 

 area 1, as far as trophic relationships of yellowfin 

 and skipjack tunas are concerned, appears to be 

 the presence of an abundant herbivore, Phuron- 

 codes planipes^ which is large enough to release 



108 



U.S. FISH AND WILDLIFE SERVICE 



