FISHERY BULLEITN: VOL. 76. NO. 4 



80 100 120 140 



FORK LENGTH (cm) 



Figure l. — Estimates of size- specific fishing mortality of Atlan- 

 tic yellowfin tuna as a function of porportion of catch ((/>) by 

 longline fishery that comes from stock(s) exploited by surface 

 fishery. 



surface fishery. The relative values of yield per 

 recruit within a hypothesis are not significantly 

 affected by the portion of the longline catch that 

 comes from the stock(s) exploited by the surface 

 fishery (Figure 2). Therefore, the three hypothet- 

 ical stock structures do not seem to have much 

 bearing on decisions concerning minimum size 

 regulations. 



Estimates of yield per recruit were also plotted 

 as functions of fishing effort (mortality), size at 

 recruitment, and portion of longline catch that 

 comes from stock(s) exploited by the surface 

 fishery. Again the relative values of the results are 

 not significantly influenced by the stock structure 

 (Figure 3a, b). We note that Figure 3 is in agree- 

 ment with the conclusion of Fox and Lenarz 

 (1974), ". . . that the Atlantic yellowfin fishery is 

 approaching or has obtained a plateau where sub- 

 stantially increased sustainable average yield of 

 yellowfin tuna will not be obtained by increasing 

 fishing effort without some concomitant change in 

 the constitution of the fishery. . . ." They used the 

 production model approach under the alternative 

 assumptions that either the longline or surface 

 gear exploits the same or separate stock(s). 



The effect of the surface fishery on the longline 

 fishery was examined by estimating yield per re- 



5 4r 



9 



i 5.0 



q: 

 o 

 111 

 a: 



4.6 





4.2 



3.8 



J L 



J L 



J. 



J L 



32.5 52.5 72.5 92.5 112.5 132.5 



FORK LENGTH AT RECRUITMENT (cm ) 



Figure 2. — Yield per recruit ( kilograms) ofAtlantic yellowfin to 

 surface and longline gear as a function of size at recruitment and 

 proportion of catch (</>) by the longline fishery that comes from 

 stock! s ) exploited by surface fishery . The vector of fishing mortal- 

 ity is equal to the value at the time of study. 



cruit to the longline fishery in the presence and in 

 the absence of a surface fishery (Figure 4). The 

 results suggest that if the two gears exploit the 

 same stock(s), the surface fishery reduces the po- 

 tential yield per recruit to the longline fishery by 

 about twofold at the position of the fishery during 

 the study period (i.e., multiplier of effort = 1) and 

 about fivefold for a threefold increase in effort. The 

 same procedure was used to examine the effect of 

 the longline fishery on the surface fishery (Figure 

 5). The results indicate that at the level of fishing 

 effort at the time of study, the yield per recruit to 

 the surface fishery would be increased by 25% if 

 the longline fishery ceased. 



Although the presence of each fishery reduces 

 the yield per recruit of the other, the yield per 



04 08 12 16 20 2'» 

 MULTIPLIER OF EFFORT 



04 08 12 16 20 24 28 

 MULTIPLIER OF EFFORT 



Figure 3. — Yield per recruit of Atlantic yellowfin tuna as a 

 function of fishing effort and proportion of catch i4>) by longline 

 fishery that comes from stock(s) exploited by surface fishery: (a) 

 size at recruitment is 32.5 cm, (b) size at recruitment is 77 cm. 



812 



