468 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



47°^8° N. McGaiy, Jones, and Graham state: 

 "The frontier of this wanning layer apparently 

 offers optimum conditions for a phytoplankton 

 bloom followed by an increase in zooplankton 

 abundance." 



As such a "frontier" with an associated trophic 

 level (zooplankton, for example) moves north- 

 ward, one might expect it to be followed by other 

 trophic levels, each successively exploiting the one 

 preceding it. In this case (fig. 10) it is postulated 

 that zooplankton are exploited by forage orga- 

 nisms which are most abundant south of an ad- 

 vancing frontier of high zooplankton abundance. 

 The forage organisms (sauries and squid, not nec- 

 essarily those captured by midwater trawling) are 

 in turn exploited by the albacore, and the largest 

 stomach volumes are found to the south of the 

 area of highest trawling volumes. 



The consumption of food by albacore captured 

 by troll and longline from 120° W. to 140° W. and 

 from 140° W. to 180° during summer and fall in 

 the temperate North Pacific is compared with the 

 abundance of zooplankton in these two areas in 

 figure 11. Such a comparison provides another 

 estimate of the utilization of forage by albacore, 

 although the zooplankton is usually considered 

 two trophic levels removed from the albacore. 



100 



WZ^ ZOOPLANKTON 

 ■IH STOMACH CONTENT 



(108) 



(59) 



(53) 



I40°W-I80° LONGiTura 



30°N-49°N. LATITUDE 



Figure 11. — Stomach content volumes of albacore cai)- 

 tnred by troll and longline and zooplankton volumes 

 from the temperate North Pacific. Plankton data from 

 McGary, Jones, and Austin, 1!).56, and Norpac Com- 

 mittee, 1960. (Numbers in parentheses refer to sample 

 sizes.) 



Albacore captured at 120° W.-140° W. had more 

 food in their stomachs than albacore captured 

 at 140° W.-180°, even though the data upon which 

 figure 11 is based favor the latter. This bias oc- 

 curs because most of the fish captured at 120° 

 W.-140° W. were examined in the field and only 

 stomach content volumes of 5 cc. or greater were 

 recorded. In the construction of figure 11, any 

 field-examined stomach with less than 5 cc. was 

 considered empty, while values from to 5 cc. were 

 recorded for stomachs examined in the laboratory. 

 It appears, therefore, that albacore captured at 

 120° W.-140° W. were utilizing the larger amount 

 of food available to them, as indicated by the 

 higher zooplankton volumes recorded at 120° W.- 

 140° W. 



East-west variations in stomach content volumes 

 of longline-caught albacore in the central equator- 

 ial Pacific are compared with zooplankton volumes 

 in figure 12. These differences may reflect the 

 east-west variation in the equatorial circulation 

 and tend to support the hypothesis advanced by 

 King and Iversen (1962) that decreasing zoo- 

 plankton abundance from east to west in the equa- 

 torial Pacific may be related to predation by an 

 expanding population of forage organisms. The 

 latter in tum are eaten by climax predators, such 

 as albacore. This is indicated by the high stomach 

 content volumes recorded near 180°. As newly 

 upwelled water from the eastern Pacific is carried 

 westward, the inorganic phosphate present de- 

 creases, the temperature increases, and the ther- 

 mocline deepens (Austin, 1958). The decrease in 

 inorganic phosphate presumably indicates an 

 increase in organic production by expanding pop- 

 ulations of phytoplankton and zooplankton. 

 However, such an increase in the abundance of 

 zooplankton from east to west is not shown by 

 the data (fig. 12), and it is to explain this phe- 

 nomenon that the importance of predation by for- 

 age organisms has been suggested. King and 

 Iversen (1962) have reported the amount of forage 

 organisms captured by midwater trawling to be 

 higher at 140° W.-166° W. than at 110° W.-140° 

 W., indicating a westward increase in such pre- 

 dators, many of which depend upon zooplankton 

 as food. 



The volumes of albacore stomach contents also 

 parallel somewhat the east-west variation in 

 stomach volumes reported by King and Ikehara 

 (1956) for the bigeye tuna of the equatorial 

 Pacific, which like the albacore also inhabits the 



