10°S, blue marlin are generally the dominant species 

 of billfish. Their eastward extension into the east- 

 ern Pacific reaches to about long. 105°W during the 

 first quarter, decreasing to about long 1 10°W during 

 the second quarter and to long. 120°W by the end of 

 the third quarter. During the fourth quarter, blue 

 marlin appear to become dominant again in a more 

 easterly direction. They are never the dominant 

 species near shore in the eastern Pacific. When 

 compared with tuna, bigeye generally replace the 

 blue marlin as the dominant species in this offshore 

 area. 



In the intervening area, which is by far the 

 largest, striped marlin are generally the dominant 

 species, although shortbill spearfish occasionally 

 are dominant. Striped marlin therefore appear to 

 separate the inshore sailfish stock from the offshore 

 blue marlin stock. When compared with tuna, 

 striped marlin remain as the dominant species north 

 of about lat. 15°N, but in the central and lower 

 latitudes are generally replaced as the dominant 

 species by bigeye and albacore tuna. 



In the southeastern, inshore area, swordfish are 

 dominant. From a small area off northern Peru in 

 the first quarter, their dominance appears to extend 

 in a southwesterly direction. By the third quarter 

 they are the dominant species of billfish to as far 

 south as lat. 40°S and west to long. 105°W. This 

 area begins to contract to the northeast during the 

 fourth quarter. When tuna are included with bill- 

 fish. bigeye appear to replace swordfish as the dom- 

 inant species. 



Trends in Relative Apparent Abundance 



Because of the wide distribution of fishes and the 

 fact that they cannot be observed in the sea it is 

 impossible to estimate their real abundance by 

 counting them. In order to detect relative changes 

 in the abundance of marine fishes, the catch per unit 

 of effort exerted is used as an index of such abun- 

 dance. For billfish the index of abundance used in 

 this analysis is the catch by species per 1 ,000 hooks 

 set. Two important factors can affect the use of 

 catch per unit as an index of abundance. First it is 

 influenced by changes in the availability of the fish 

 themselves and changes in their vulnerability to 

 capture. Secondly, competition of the fish for the 

 hook can bias estimates of abundance in a 

 multiple-species fishery such as the longline 

 fishery. 



With respect to the first source of error, if one 



examines a series of data sufficiently long, the var- 

 iability in availability and vulnerability tends to bal- 

 ance out. We have not attempted to correct for the 

 latter source of error. Catch per effort by quarter, 

 year, and area are discussed below for striped mar- 

 lin, blue marlin, sailfish and swordfish. 



To facilitate the analysis of catch rates, Kume 

 and Joseph (1969a) divided the eastern Pacific east 

 of 130°W into areas based on the geographical ex- 

 pansion of the fishery. These areas have been re- 

 numbered for the present analysis and are shown in 

 Figure 1. 



Striped Marlin 



The overall catch rate for striped marlin in the 

 eastern Pacific trended upward from 1956 to about 

 1965; it decreased during the following 2 yr, but 

 during 1968 increased to its highest level. During 

 1969 and 1970 it decreased to slightly below the 

 1966 and 1967 levels. 



In order to examine in more detail these trends in 

 the abundance of striped marlin we have grouped 

 data into areas in which effort has been consistently 

 expended for an extended time period. We show 

 trends in catch rates for three such areas (Fig. 15). 



The lower panel of Figure 15 shows the catch per 

 thousand hooks for the older, equatorial marlin 

 grounds which include areas 9, 11, and 12 of Figure 

 1. The fishery for striped marlin in this area de- 

 veloped during 1958 and has continued since. Catch 

 rates during the early years were low, less than 2 

 fish/1,000 hooks. These increased progressively 

 until about 1965 when they reached a high of about 

 5.5 fish/1,000 hooks. Since then they have exhibited 

 a downward trend to a level of about 2 fish/ 1,000 

 hooks during 1969-1970. A great deal of quarterly 

 variability is evident but it does not appear to ex- 

 hibit any consistent pattern. Though effort does 

 vary among quarters, there again does not appear to 

 be any consistent pattern; the same general levels of 

 effort have been exerted during recent years. 



Catch rates for areas 3, 5, and 6, the northern 

 inshore marlin grounds, are shown in the middle 

 panel of Figure 15. The fishery for striped marlin in 

 this region began during 1963. At that time hook 

 rates were quite high, about 14 fish/1,000 hooks. 

 During 1964-1965 they decreased to about 10.5 

 fish/ 1,000 hooks. This was followed by an increase 

 to about 12 fish/ 1,000 hooks, and catch rate has 

 remained at about that level. The magnitude of var- 

 iability in the quarterly catch of striped marlin in 



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