FISHERY BULLETIN; VOL. 76. NO. 4 



also possible that some stock(s) which are avail- 

 able to surface fishing are never available to the 

 longline fishery. Since significant tagging efforts 

 have begun only recently in the Atlantic and the 

 results of these studies have not been published, 

 data are not available to evaluate the availability 

 of yellowfin tuna to both gears. 



However, there is evidence from the Pacific that 

 yellowfin tuna are not equally available to long- 

 line and surface gears. With the permission of W. 

 H. Bayliff of the Inter-American Tropical Tuna 

 Commission (lATTC), we examined yellowfin 

 tuna tag return data from the eastern Pacific dur- 

 ing 1963-66 in an attempt to evaluate the avail- 

 ability offish to both gears in that area. We tabu- 

 lated the number of tag returns for fish larger than 

 100 cm at return by 10-cm size groups (Table 3). 

 All of the fish hadbeenatliberty for at least 10 mo. 

 Although all of the tagged fish were measured 

 when released, not all were measured when recov- 

 ered. Bayliff recommended the relationship 



167 (1 - e 



-0,fii/-0.833i - 



Again at the suggestion of Bayliff, we estimated 

 the expected return of tags from longline-caught 

 fish when all fish are equally available to both 

 gears. Assuming tag recoveries were independent 

 of each other, recovered tags were reported at the 

 same rate by both components of the fishery, and 

 tagged fish were equally available to both gears: 

 then the expected returns of tagged fish of size / by 

 gear / in year k is given by 



E(R„,) =R,.,N„,JN,^k (1) 



'1, when size is between 101 and 110 cm 



,6, when size is between 151 and 160 

 . _ fl, when fish are caught by surface gear 

 l2, when fish are caught by longline gear 

 '1, when fish are caught in 1963 

 , 2, when fish are caught in 1964 



3, when fish are caught in 1965 

 ,4, when fish are caught in 1966 



estimated by Davidoff ( 1963) for growth of yellow- 

 fin tuna in the eastern Pacific as the best equation 

 to estimate the size of unmeasured fish. All of the 

 returns were surface-caught fish, even though 

 longliners captured a considerable number of yel- 

 lowfin tuna in the eastern Pacific (east of long. 

 130"^ W) (Kume and Joseph 1969). In fact for many 

 of the 10-cm size groups, the longliners caught 

 more yellowfin tuna than the surface gear 

 operators (Table 4). 



Table 3. — Number of returns of tagged yellowfin tuna from the 

 eastern Pacific Ocean by size interval and year iW. H. Bayliff, 

 pars, commun.). 



Size interval (cm) 



1963 



1964 



1965 



1966 



101-110 

 111-120 

 121-130 

 131-140 

 141-150 

 151-160 



16 

 7 

 

 

 

 



where /?,;/; = number of returns and 

 N ,ji; = number of fish caught. 



A dot in the position of a subscript signifies sum- 

 mation of the variable over the subscript, e.g., X, .^ 



2 



= 1 X, 



./ = 1 



,jh  



Forty fish were returned by the surface gear 

 during 1963-66 (Table 3). Using the statistics of 

 Tables 3 and 4 and the three assumptions, a return 

 of 5.4 of these tags would have been expected 

 from the longline fishery and 34.6 from the sur- 

 face fishery. The chi-square value, corrected for 

 discontinuity, for the observed and expected re- 

 turns (Equation 1) is 5.13, with probability 

 slightly less than 0.025. The power of the test of 

 the hypothesis of independence, equal reporting 

 rate, and equal availability was reduced because 

 we combined the year and size strata to avoid 



Table 4. — Catch of yellowfin tuna from the eastern Pacific Ocean (east of long. 130°W) in hundreds of 



fish by size and gear iKume and Joseph 1969). 



Size interval 

 (cm) 



1963 



1964 



1965 



1966 



Surface 

 gear 



Longline 

 gear 



Surface 

 gear 



Longline 

 gear 



Surface 

 gear 



Longline 

 gear 



Surface 

 gear 



Longline 

 gear 



101-110 

 111-120 

 121-130 

 131-140 

 141-150 

 151-160 



653 

 473 

 508 

 237 

 240 

 212 



336 

 455 

 390 

 751 

 541 

 144 



4,082 

 2.245 

 720 

 448 

 320 

 102 



173 

 465 

 1.078 

 804 

 469 

 104 



3.386 



2.211 



1.895 



905 



498 



194 



30 

 93 

 444 

 758 

 466 

 205 



2.926 



2.044 



1.312 



718 



536 



204 



54 

 116 

 304 

 515 

 575 

 200 



810 



