FISHERY BULLETIN: VOL. 76, NO. 4 



models of availability of yellowfin tuna to the two 

 types of gear in the Atlatnic Ocean do not have 

 much effect on decisions concerning minimum size 

 regulations. If fish are equally available to both 

 gear types, yield per recruit is higher to a longline 

 fishery than to a surface fishery, but is higher for 

 the combined gears than to either gear fishing 

 alone. 



We also note that there is considerable evidence 

 that large females of all commercially important 

 Thunnus are caught in fewer numbers than large 

 males. The effect of this phenomenon on yield per 

 recruit and relative stock fecundity was examined 

 for Atlantic yellowfin tuna. When plotted against 

 fishing effort, yield per recruit is more dome- 

 shaped when the sex ratio is 1:1, as is usually 

 assumed, than when the sex ratio is as observed. 

 Changes in size at recruitment also have a greater 

 effect on yield per recruit when the sex ratio is 1: 1 

 than when the sex ratio is as observed. Competi- 

 tion between longline and surface fishing is more 

 intense when the sex ratio is 1:1 than otherwise. 

 The fishery has a greater effect on stock fecundity 

 if the sex ratio is 1:1 instead of that observed. 



Tagging studies of yellowfin tuna in the eastern 

 Pacific indicate that movements are fairly slow 

 compared with more highly migratory species 

 such as albacore and bluefin tuna and have not 

 produced any clear-cut evidence of a definite mi- 

 gration pattern. Size composition of the catch 

 suggests that recruitment to the fishery occurs 

 mainly along the coast of Central America. A 

 simulation model was developed for the eastern 

 Pacific to examine the interrelationships of pat- 

 terns of movements of fish, patterns of recruit- 

 ment, and fishing strategy. It was assumed that 

 movements were random and recruitment occur- 

 red either along the coast or throughout the east- 

 ern Pacific. The results indicate that either pat- 

 tern of recruitment could allow the increased 

 catch observed in the Pacific as the surface fleet 

 moved offshore. However, the pattern of recruit- 

 ment does affect the potential yield per recruit of a 

 nearshore surface fishery relative to a surface or 

 longline fishery that exploits the entire area. Both 

 choices of recruitment models resulted in an as- 

 ymptotic relationship between yield per recruit 

 and effort for a longline fishery over the range of 

 effort examined. Overfishing in a yield per recruit 

 sense did not occur, even though catch per effort 

 decreased by 80%. Approximately the same re- 

 sults were obtained for an inshore surface fishery. 

 However, curves of yield per recruit plotted 



against effort for a surface fishery that exploits the 

 entire area are dome-shaped. 



The study reveals several biological and be- 

 havioral parameters which, because of lack of 

 knowledge or information, are rarely considered 

 but do appear to have a significant effect on some 

 aspects of the dynamics of yellowfin tuna fisheries. 

 Tagging and fecundity studies are suggested in 

 order to fill these gaps. Perhaps as important, 

 other aspects of the dynamics of yellowfin tuna 

 fishing appear to be insignificantly affected by the 

 examined parameters. 



ACKNOWLEDGMENTS 



This study was stimulated by Brian J. 

 Rothschild, who has had a long interest in the 

 interactions between longline and surface 

 fisheries for tunas. The work benefited from dis- 

 cussions with Rothschild and William W. Fox, Jr. 

 of the Southwest Fisheries Center La JoUa 

 Laboratory; William H. Bayliff and Patrick K. 

 Tomlinson of the Inter-American Tropical Tuna 

 Commission, La Jolla, Calif.; Allan Fonteneau of 

 the Centre de Recherches Oceanographiques, 

 Abidjan, Ivory Coast; and Frank Mather III of the 

 Woods Hole Oceanographic Institution. Many of 

 the computer runs were made by Atilio Coan and 

 Kenneth Brenneke of the Southwest Fisheries 

 Center La Jolla Laboratory. We thank William H. 

 Bayliff, Jerry Wetherall of the Southwest 

 Fisheries Center Honolulu Laboratory and John 

 P. Wise, National Marine Fisheries Service, 

 Washington, D.C., for reviewing the paper. We 

 owe considerable thanks to an unnamed referee 

 who made many useful suggestions and to Lor- 

 raine Prescott for her patient and excellent typ- 

 ings of several versions of this paper. 



LITERATURE CITED 



Bayliff, w. h., and B. J. Rothschild. 



1974. Migrations of yellowfin tuna tagged off the southern 

 coast of Mexico in 1960 and 1969. [In Engl, and 

 Span.] Inter-Am. Trop. Tuna Comm. Bull. 16:1-64. 

 BEARDSLEY, G. L. 



1971. Contribution to the population dynamics of Atlantic 

 albacore with comments on potential yields. Fish. Bull., 

 U.S. 69:845-857. 



Calkins, T. P. 



1965. Variation in size of yellowfin tuna (Thunnus alba- 

 cares) within individual purse-seine sets. [In Engl, and 

 Span.] Inter-Am. Trop. Tuna Comm. Bull. 10:461-524. 



Calkins, T. p., and b. m. chatwin. 



1971. Geographical catch distribution of yellowfin and 

 skipjack tuna in the eastern Pacific Ocean, 1967-1970, 



824 



