LENARZ and ZWEIFEL: INTERACTION BETWEEN LONGUNE AND SURFACE FISHERIES 



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MULTIPLIER OF EFFORT 



MULTIPLIER OF EFFORT 



FIGURE 20. — Yield per recruit, yield per recruit per effort, and average size of catch for hypothetical uniform surface fishery: (a) yield 

 per recruit, (b) yield per recruit per effort, (c) yield per recruit per effort in cells 71 and 85, (d) yield per recruit per effort in cells 69, 84, 

 and 97, and (e) average size of fish in catch. 



fishery (Figure 20). With uniform recruitment, 

 maximum yield per recruit changes from about 1 .4 

 kg for an inshore fishery to about 5.0 kg for a 

 uniform fishery. 



The results of this section indicate that the pat- 

 tern of recruitment is primarily of interest for 

 examining the potential of a nearshore surface 

 fishery to a surface fishery that exploits the entire 

 area or a longline fishery. The presence of some 

 small yellowfin tuna in length-frequency data for 

 offshore areas from the eastern Pacific fishery 

 (Figure 17) reveals that some recruitment occurs 

 offshore. Recruits apparently are not highly 

 available to surface fishing offshore because most 

 yellowfin tuna are caught in schools associated 

 with porpoise. Such schools normally contain only 

 low percentages of small yellowfin tuna. A well- 

 designed tagging study could provide estimates of 

 the exploitation rate by size for yellowfin tuna in 

 the offshore areas. Until the pattern of recruit- 

 ment is determined, it will be necessary to con- 

 tinue estimations of relative production to 



longliners, inshore surface gear, and offshore sur- 

 face gear in an empirical fashion. 



We examined only one reasonable example of an 

 infinite number of possible configurations of the 

 transfer matrix A and time interval. Further use 

 of the model should include a sensitivity analysis 

 of the results to choice of A and number of cycles 

 per year. 



SUMMARY AND CONCLUSIONS 



This paper examines three aspects of dual 

 fisheries (surface and longline) on yellowfin tuna. 

 Models of yellowfin tuna fisheries are developed to 

 evaluate possible effects of unknown components 

 of the biology and behavior on the fisheries. The 

 results, while not conclusive because of in- 

 sufficient knowledge, indicate the magnitude of 

 the effects of those factors which were examined. 



We present evidence that not all yellowfin tuna 

 are equally available to longline and surface 

 fisheries in the Pacific Ocean. We show that three 



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