LENARZ and ZWEIFEL: INTERACTION BETWEEN LONGLINE AND SURFACE FISHERIES 

 ENTIRE FISHERY 



ENTIRE FISHERY 



HIGH M 



3 



o 



UJ 



q: 



Q 



-BEH 



-BEH 



-1 — I — r 

 4 8 12 



~ 1 I I I I I I r 

 16 20 24 28 32 



1 

 3 6 



MULTIPLIER OF EFFORT 



Figure 8. — Estimates of yield per recruit of Atlantic yellowfin 

 tuna at size of recruitment at time of the study as a function of 

 fishing effort and sex hypothesis: (a) high Input F, (b) low In- 

 put F. 



yield per recruit. Yield per recruit is closer to the 

 maximum under high Input F than low Input F. 

 The curves are considerably more dome-shaped 

 when a 1:1 sex ratio is assumed than under the 

 other two hypotheses. Under high Input Fand the 

 1:1 hypothesis only a 3^c increase in yield per 

 recruit could be obtained by increasing fishing 

 effort. Under the BEH hypothesis, a 20% increase 

 in yield per recruit could be obtained by doubling 

 the effort. 



Estimates of yield per recruit as a function of 

 size at recruitment are shown in Figure 9. Again 

 the choice of Input F has little effect on the relative 

 values of yield per recruit. A slightly greater de- 

 pendence of yield per recruit on minimum size is 

 obtained when the high Input F is used. Under 

 high Input F, and the 1:1 hypothesis a 107c in- 

 crease in yield per recruit could be achieved by 

 increasing size at recruitment. Under the BEH 

 hypothesis, only a d'^c increase would occur. 

 Eumetric fishing occurs when size at recruitment 

 is raised from the current 32.5 to 82.5 cm under the 

 1:1 hypothesis and 72.5 cm under the BEH 

 hypothesis. 



Estimates of yield per recruit as a function of 

 fishing effort were also calculated for each gear 



H M 



1 — I — 1 — \ r 



J2 5 52 5 72 5 92 5 112 5 132 5 



32 5 52 5 72 5 92 5 112 5 132 5 

 MINIMUM SIZE (cm) 



Figure 9. — Estimates of yield per recruit of Atlantic yellowfin 

 tuna at level of fishing effort at the time of the study for 1:1, BEH 

 and HIGH M hypotheses as a function of size at recruitment: (a) 

 high Input F, (b) low Input F. 



(Figure 10). The results show that the curves are 

 more dome-shaped for the longline fishery than for 

 the surface fishery under all three hypotheses. 

 Furthermore, the longline fishery is more sensi- 

 tive to fishing effort under the 1:1 hypothesis than 

 under the other two. The curves for the surface 

 fishery are dome shaped under the 1:1 hypothesis, 

 but appear to approach an asymptote under the 

 other two. 



We also estimated yield per recruit for each gear 

 when the other gear is not exploiting the stock 

 (Figure 11). A comparison of Figures 10 and 11 

 reveals that yield per recruit to the longline 

 fishery would increase by about 115*^ if surface 

 fishing were eliminated under high Input F and 

 the 1:1 hypothesis and 76*?^ under high Input F 

 and the BEH hypothesis. Yield per recruit to the 

 surface fishery would increase by about 30% if the 

 longline fishery were eliminated under high Input 

 F and the 1:1 hypothesis and 229c under the BEH 

 hypothesis. Thus, the nature of age-specific sex 

 ratio has a greater effect on that of the longline 

 fishery than on the relative success of the surface 

 fishery. The curves for a longline fishery in the 

 presence of a surface fishery are dome-shaped 

 (Figure 10), while the curves in the absence of a 

 surface fishery are not (Figure 11). This again 

 points out the importance of not treating the two 

 fisheries as separate entities unless it is shown 

 that they exploit separate stocks. 



Stock fecundity (egg production per recruit) 

 relative to an unfished stock was estimated as a 

 function of fishing effort. Stock fecundity was con- 

 siderably affected by the choice of fecundity index 



815 



