FISHERY BULLETIN. VOL. 72, NO. I 



Table 10. — Optimum size (cm) at recruitment for 441 combinations of multipliers of effort by 



bait boats and longliners. 



composition of both small and large purse sein- 

 ers are included, the attempts to achieve some 

 reason in the minimum size regulation based 

 on ma.ximum yield per recruit can become quite 

 unwieldly. 



Dispersion of Gear and Yield Per Recruit 



The second assumption could be important. 

 For example, in the eastern Pacific yellowfin 

 tuna fishery effort has expanded farther off- 

 shore. Evidence suggests that larger fish were 

 farther offshore and were not previously fully 

 available to the fishery. A possible consequence 

 of this phenomenon is a change in yield per 

 recruit. Upon analysis of the data, the Inter- 

 American Tropical Tuna Commission concluded, 

 however, that the possible increase is minor 

 (Joseph, pers. commun.). The surface gears have 

 been fishing quite close to shore in the Atlantic. 

 The possibility of offshore dispersal of the sur- 

 face fleets and the effects of such a change on 

 yield per recruit are unknown. 



Interaction Between Minimum Size and 

 Catch Quota Regulations 



If recruitment is not constant, then the inter- 

 action between minimum size and catch quota 

 regulations should be examined. Catch quotas 



are frequently based on assessments of the 

 maximum sustainable average yield (MSAY), 

 usually through a production model type analy- 

 sis. The shape of the total yield curve, however, 

 may be strongly dependent on the age at re- 

 cruitment, t^.'. Therefore, the interaction be- 

 tween the two types of regulation should be 

 examined before a singular action is taken. As 

 an illustration, consider a population consisting 

 of six age-groups with the growth curve and 

 natural mortality coefficient (M = 0.8) similar 

 to that of the Atlantic yellowfin tuna fishery, 

 and assume also that recruitment is knife-edged 

 at 19 mo. Figure 19 (lower curve) shows the 

 total annual yield as a function of fishing mor- 

 tality with an assumed arbitrary stock-recnait- 

 ment function. Assume further that the fishery 

 is operating at an F = 1.0. The yield per recruit 

 a.t F = 1.0 and t^' — 19 mo is 5.39, but the maxi- 

 mum yield per recruit is 6.11 at t ' = 27 mo. 

 If singular action were taken to increase t ' to 

 27 mo, the upper total yield curve in Figure 19 

 would result. Not only did the yield per recruit 

 increase, but so did the total yield at F = 1.0. 

 In addition, the MSAY increased, but occurs 

 at a much higher value for F. A phenomenon 

 such as this may have occurred inadvertantly 

 in the eastern tropical Pacific with the introduc- 

 tion of purse seiners which gave a better yield 

 per recruit than the existing bait boats (Joseph, 

 1970). 



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