FISHERY BULLETIN: VOL. 76, NO. 4 



4i- 



OC_l I I I I I I I I I I I — I — I — I 1 1 — I 



04 08 12 16 20 24 2 8 3 2 36 



MULTIPLIER OF EFFORT 



Figure 10. — Estimates of yield per recruit of Atlantic yellowfin 

 tuna when both gear fish at size of recruitment at the time of the 

 study as a function of sex ratio hypothesis, fishing effort, and 

 gear: (a) surface gear with high Input F, (b) longHne gear with 

 high Input F, (c) surface gear with low Input F, and (d) longline 

 gear with low Input F. 



and sex ratio hypothesis but only slightly affected 

 by the choice of Input F (Figure 12). At the level of 

 fishing effort at the time of the study under high 

 Input F and 1:1 hypotheses, the relative fecundity 

 is 0.28 when the fecundity index I is used and 0.39 

 when fecundity index II is used. Under the HIGH 

 M hypothesis, relative fecundity is 0.55 when 

 fecundity index I is used and 0.61 when fecundity 

 index II is used. Thus, at the level of fishing effort 

 at the time of the study, the choice of fecundity has 

 a 10 to 30% effect on estimates of relative fecun- 

 dity, while the choice of sex ratio hypothesis has a 

 30 to 50% effect. The two choices, fecundity index 

 and sex ratio hypothesis, also have considerable 

 effect on relative fecundity when plotted as a func- 

 tion of size at recruitment (Figure 13). 



The relationship between stock fecundity and 

 recruitment has not been demonstrated for any 

 tuna. As shown above, one of the difficulties in 



demonstrating such a relationship is obtaining a 

 reasonably accurate estimate of stock fecundity. 

 Even if stock fecundity could be accurately deter- 

 mined, the recruitment process is likely to be so 

 complex that much more research would be re- 

 quired before a reliable predictor of recruitment 

 could be developed. 



It is interesting to note that similar estimates of 

 yield per recruit and relative fecundity are ob- 

 tained under the HIGH M and BEH hypotheses. 

 Thus it appears that research should be directed 

 toward determining whether or not the 1:1 

 hypothesis or one of the other two are valid rather 

 than distinguishing between the HIGH M and 

 BEH hypotheses. This research should be a fairly 

 simple matter. The choice of fecundity index is 

 also of significance for estimating relative fecun- 

 dity. The difference between the two indices is 

 caused mainly by different maturity schedules 

 (Hayasi et al. 1972). The surface-caught fish ap- 

 peared to mature at an earlier age than longline- 

 caught fish, and could be an artifact related to the 

 phenomenon noted by Hisada ( 1973); i.e., mature 

 fish tend to prefer warm water. It should also be a 

 fairly simple matter to determine the cause of the 

 difference between the two indices. 



SIMULATION MODEL OF PATTERNS 



OF DISPERSAL AND RECRUITMENT 



OF YELLOWFIN TUNA 



Factors that could cause groups of tuna to not be 

 available to all components of a fishery include 

 nonrandom movements, random movements but 

 nonrandom distribution of fishing gear or effort, 

 and recruitment that is nonrandom in a geo- 

 graphical sense. 



Extensive tagging experiments have not pro- 

 duced any clear-cut evidence of a definite migra- 

 tion pattern for yellowfin tuna in the eastern 

 Pacific. Bayliff and Rothschild (1974) recently 

 found evidence for both random dispersal and di- 

 rected movements. They were not able to remove 

 the effects on their data of lack of fishing effort in 

 some time-area strata and of the coastal boundary. 

 The evidence for directed movements indicated 

 that such movements were generally parallel to 

 the coast, suggesting that the presence of the coast 

 influenced their results. Fink and Bayliff (1970), 

 in a synthesis of extensive tagging data, proposed 

 that recruitment to the nearshore surface fishery 

 is not random in a geographical sense, but tends to 

 take place off Mexico and in the Panama Bight. 



816 



