A THEORETICAL EXAMINATION OF SOME ASPECTS OF 



THE INTERACTION BETWEEN LONGLINE AND SURFACE FISHERIES 



FOR YELLOWFIN TUNA, THUNNUS ALBACARES 



William H. Lknarz' and James R. Zweifel^ 



ABSTRACT 



This paper explores several aspects of a dual fishery (surface and longline) on yellowfin tuna, Thunnus 

 albacares. The work is exploratory in nature and results, though indicative, are not conclusive for any 

 specific fishery. Our results indicate that the yield per recruit is higher for the longline fishery than for 

 surface gear if all fish are available to both gears and higher for the combined gears than for either gear 

 fishing alone. Theeffect of fishing by one gear on yield to the other gear and the effect of the fishery on 

 stock fecundity is shown to be greater for the often assumed 1:1 sex ratio than for the ratios usually 

 observed. A simulation model was used to examine the interrelations of pattern of movement offish, 

 pattern of recruitment, and fishing strategy. It was assumed that movements were random and 

 recruitment occurred either only along the coast or throughout the fishing area. The results indicated 

 that either of these patterns of recruitment could allow for increased catch as the surface fleet moved 

 offshore. However, location or pattern of recruitment is shown to be important when measuring 

 natural mortality and for examining the potential of a localized fishery, primarily on younger fish, 

 relative to a fisherj' exploiting the full range of the stocks or to one taking primarily older fish. Tagging 

 and fecundity studies are suggested for further investigation of the questions examined in this paper. 



An unsolved problem common to many of the tuna 

 fisheries of the world is the nature of the interac- 

 tion between longline and surface (i.e., seining, 

 pole and line, and occasionally trolling and shal- 

 low handline) fisheries for the same species. 

 Fisheries for yellowfin tuna, Thunnus albacares; 

 albacore, T. alalunga: bluefin tuna, T. thynnus; 

 southern bluefin tuna, T. maccoyii; and bigeye 

 tuna, T. obesus, are prosecuted by both types of 

 gear in the Pacific, Atlantic, and Indian Oceans. 

 Although there can be considerable overlap of 

 sizes of fish taken by the two types of gear, in 

 general, longline gear takes larger (older) fish. 

 Exploitation of a tuna stock by the two types of 

 gear presents management with the problems of 

 determining the effect of various combinations of 

 fishing effort by the two gears on both yield per 

 recruit to the two gears and recruitment to the 

 stocks. In order to make these determinations, it is 

 necessary to estimate 1 ) availability of the stock at 

 each age to each of the two gears [The available 

 portion of the stock is subject to both other mortal- 



' Southwest Fisheries Center La Jolla Laboratory, National 

 Marine Fisheries Service, NOAA, La Jolla, Calif; present ad- 

 dress: Southwest Fisheries Center Tiburon Laboratory, NMFS, 

 NOAA, 3150 Paradise Drive, Tiburon, CA 94920. 



^Southwest Fisheries Center La Jolla Laboratory, NMFS, 

 NOAA, P.O. Box 271, La Jolla, CA 92038. 



Manuscript accepted Mav 1978. 



FISHERY BULLETIN: VOL. 76. NO. 4. 1979. 



ity (any mortality not caused by gear of concern) 

 and fishing mortality caused by the gear of con- 

 cern. The unavailable portion of the stock is sub- 

 ject only to other mortality.], 2) fishing mortality 

 of the available portion of the stock caused by each 

 gear, 3) natural mortality, 4) growth, 5) fecundity, 

 and 6) the relationship between egg production 

 and recruitment. 



The aim of this paper is to examine the interac- 

 tions between longline and surface fisheries for 

 yellowfin tuna and to determine the impact such 

 interactions may have on the assumptions often 

 made in assessment of yellowfin tuna fisheries and 

 thus on the assessment calculations themselves. 

 The paper is divided into three major sections. The 

 first section examines the relationship between 

 availability of the stock(s) of yellowfin tuna to 

 surface and longline fishing and yield per recruit 

 to the two gears. This is an important, and to our 

 knowledge unexamined, aspect of all tuna fish- 

 eries exploited by both types of gear; the subse- 

 quent sections examine two asepcts of the biology 

 of tuna that can affect the catch by each type of 

 gear. The second section examines the effect of age 

 specific sex ratios of yellowfin tuna on yield per 

 recruit to the two types of gear and on egg produc- 

 tion. The third section examines the effect of 



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