As with billfish, catches of tunas increased 

 rapidly to a peak then fluctuate about some sUghtly 

 lower level. 



ANALYSIS AND RESULTS 

 Geographical Distribution 



The average quarterly catch rate of billfish is 

 shown by species within 5-degree areas in Figures 7 

 through 12, for the years 1956-1970. Catch rate is 

 expressed in numbers of fish caught per 1,000 

 hooks. Such figures provide a useful basis for exam- 

 ining the quarterly average distribution of the 

 longline-caught billfish. If catchability is assumed 

 to be constant, then shifts in centers of billfish 

 abundance^ can be utilized to infer migratory be- 

 havior. 



Striped Marlin 



The catch and effort statistics show striped mar- 

 lin to be widely distributed throughout the eastern 

 Pacific Ocean during all seasons of the year, occur- 

 ring from about lat. 35°N to 40°S to the coastline of 

 the Americas (Fig. 7). The areas of high relative 

 density which occur near shore are variable among 

 quarters. 



During the first quarter highest hook rates are 

 near the Revillagigedo Islands, the tip of Baja 

 California, and in the Gulf of California. These ex- 

 pand southward and westward during the second 

 quarter and third quarter. By the end of the third 

 quarter the area of high relative abundance has ex- 

 tended southward to lat. 10°N and westward to 

 long. 130°W. It is during this period when the high- 

 est densities of striped marlin in the eastern Pacific 

 are encountered, centered in the area bounded by 

 long. 125°-110°W and lat. 15°-25°N. During the 

 fourth quarter the area of high marlin abundance 

 diminishes and the striped marlin appear to be 

 found nearer shore. A slight displacement north- 

 ward of the high hook rates from about lat. 25°N to 

 30°N is observable during the third and fourth quar- 

 ters, most likely associated with the movement of 

 warmer water northward. 



In the southern area of the fishery striped marlin 

 are relatively abundant as far south as lat. 35°S dur- 

 ing the first quarter of the year. This period repre- 

 sents the southern summer, when warmer waters 

 are displaced southward. As southern waters begin 

 to cool at the onset of the southern winter, the 



^Throughout the report "abundance" refers to "apparent 

 abundance." 



southern boundary of the distribution is displaced 

 northward. This continues through the third quarter 

 during which relatively few striped marlin are found 

 below lat. 20°S. During the fourth quarter as the 

 southern summer begins, striped marlin commence 

 to move southward and are again found below lat. 

 30°S. 



Also during the first quarter of the year striped 

 marlin appear to be abundant around the Galapagos 

 Islands and off Colombia and Ecuador. There also 

 seems to be an area of high concentration in the 

 offshore region bounded by lat. 10°-15°S and long. 

 85°-100°W. During the second quarter there appears 

 to be a general decrease in abundance. However 

 there is a suggestion of a southerly shift in the high- 

 est concentration offish that continues through the 

 third quarter at which time the highest concentra- 

 tions south of the equator are in the general region 

 of long. 95°-115°W and lat. 15°-25°S. By the fourth 

 quarter the areas of greatest abundance are again 

 around the Galapagos and off South America. This 

 pattern of changes in the distribution of marlin sug- 

 gests a shoreward migration during the southern 

 summer and an offshore migration during the 

 southern winter. 



In an earlier publication Kume and Joseph 

 {1969a) compared the available data from subsis- 

 tence and sport fisheries with those from the long- 

 line fisheries. They concluded that the seasonal dis- 

 tribution of striped marlin as reflected by sport and 

 subsistence fisheries corresponds well with that in- 

 ferred from commercial longline catches. 



As already noted, striped marlin are found 

 throughout the Pacific Ocean but their abundance is 

 much greater in the eastern half than in the western 

 half of the Pacific. An area of high abundance also 

 occurs in the region bounded by lat. 15°N to 25°N 

 and long. 130°W to 175°E. Whether more than one 

 subpopulation of striped marlin exists is not known. 

 On the basis of their analysis, Kume and Joseph 

 (1969a) noted that it seemed unlikely that striped 

 marlin in the eastern Pacific are comprised of more 

 than a single stock. They did not comment, how- 

 ever, on the relationship of the stock in the eastern 

 Pacific to those farther west. 



Blue Marlin 



Blue marlin are captured by longline vessels in 

 the eastern Pacific between approximately lat. 30°N 

 and 35°S (Fig. 8). In the northern areas the distribu- 

 tion of blue marlin does not appear to fluctuate sea- 

 sonally but in the southern areas there appears to be 



315 



