grow very rapidly, and are highly fecund. They 

 most likely compete for the same kinds of food and, 

 to some extent, living space and the billfishes prey 

 on tuna. Tunas and billfishes are caught in the same 

 areas at the same time; it is not unusual to capture 

 five or six different species of tuna and billfish on 

 the same set of the longline gear. 



Because of the close relationship of these species 

 it is important to understand their community struc- 

 ture. Such information is a necessary antecedent to 

 the rational utilization of billfish resources. In order 

 to examine the community structure of tunas and 

 billfish in the eastern Pacific, as reflected by long- 

 line catches, in terms of the dominant species in the 

 catch we have prepared Figures 13 and 14. For the 

 purposes of this examination the species exhibiting 

 the highest hook-rate in each time-area stratum is 

 considered the dominant species in the catch. We 

 have prepared two sets of data. The first set (Fig. 

 13) includes all of the tunas and billfishes as a com- 

 munity, and the dominant species is whichever one, 

 tuna or billfish, exhibits the highest hook rate. The 

 dominant species are shown by quarter of the year 

 and 5-degree area. 



In the second set of data only the billfishes are 

 considered as members of the community. In this 

 case the dominant species is that species of billfish 

 which exhibits the highest hook rate within a time- 

 area stratum. The dominant species of billfish in the 

 catch is shown by 5-degree area and quarter (Fig. 

 14). 



Tunas and Billfishes 



Of the eight species examined in Figure 13, 

 bigeye tuna appears to dominate throughout all four 

 quarters of the year. In each quarter they are domi- 

 nant between about lat. 10°N and 10°S and eastward 

 to about long. 100°W. East of long. 100°W they are 

 dominant generally between about lat. 5 C N and 5°S 

 to the mainland. These limits appear to vary some- 

 what seasonally. During the southern summer, 

 bigeye appear to be displaced farther south along 

 with an associated displacement of warmer water. 

 A pocket of bigeye seems to persist in the area 

 bounded by approximately lat. 15°-30°S and long. 

 75°-95°W through the year. 



The next most dominant species in terms of ex- 

 tent of distribution, but not necessarily in terms of 

 catch, is albacore. Except in a few rare instances 

 albacore are taken by longline in the western Pacific 

 only south of about lat. 5°S, probably in waters of 



the South Equatorial Current. This species is con- 

 sistently dominant in the catch south of lat. 15°S 

 and west of long. 105°W. During the first and fourth 

 quarters of the year, the southern summer, when 

 warm waters extend farther south, the northern 

 edge of the albacore distribution is displaced south- 

 erly to about lat. 15°S. During the southern winter 

 (second and third quarter) their distribution extends 

 more northerly to beyond lat. 10°S. 



Yellowfin tuna are the next most important dom- 

 inant species of tuna in terms of extent of dis- 

 tribution. This species is the second most important 

 tuna captured in the eastern Pacific in terms of 

 weight landed. The extent of their distribution in 

 terms of dominant species is much more restricted 

 than bigeye and albacore. Yellowfin are dominant 

 in a narrow band throughout the year between lat. 

 5°N and 15°N. They also appear sporadically as the 

 dominant species in the southern hemisphere off 

 northern Peru. 



Very small quantities of southern bluefin are cap- 

 tured in the eastern Pacific Ocean, and in only two 

 areas do they appear as the dominant species (Fig. 

 13d). Their occurence as the dominant species at 

 about lat. 40°S is to the south of all other species of 

 tuna and billfish shown in the figures. 



The billfishes are generally more dominant in the 

 inshore areas than are the tunas, especially north of 

 the equator. 



Of the billfishes the striped marlin is the most 

 dominant. During the first and second quarters they 

 are dominant in the north, in the area west of long. 

 105°W and north of lat. 15°N. This area of domi- 

 nance appears to expand in all directions in the third 

 and fourth quarters. In the southern area they are 

 more dominant during the first and second quarters, 

 occuring in the waters off Ecuador and northern 

 Peru as far as long. 105°W. Their dominance di- 

 minishes remarkably during the third and fourth 

 quarters when their few dominant areas appear 

 generally to be farther offshore. 



The sailfish show a very consistent pattern as the 

 dominant species during all four quarters within 

 about 500 miles of the coast between lat 20°N and 

 the equator. 



Swordfish occur as the dominant species in only a 

 single 5-degree area off Baja California during the 

 first and fourth quarters. 



Generally tunas are the more dominant species of 

 the high seas westward of a line paralleling the coast 

 at about 600-1000 miles offshore whereas billfish are 

 dominant to the east of this line. 



323 



