SHARP and FRANCIS; ENERGETICS MODEL FOR YELLOWFIN TUNA POPULATION 



Figure 5. — The average biomass estimate of the exploited 

 yellowfin tuna population in the CYRA is shown. The historical 

 fishery label indicates the coastal fishery which operated prior 

 to 1965; the expanded fishery indicates the process of seaward 

 areal expansion which dramatically changed the estimates 

 of exploited biomass from 1966 until approximately 1968. 

 Fishery regulation was implemented in September 1966. The 

 simulation of the unexploited populations yielded estimates 

 of the average biomass for the two cohorts to be S;^ = 282,400 

 metric tons; Sg = 272,700 metric tons; S^ + Sg = 555,100 

 metric tons. Recruitment was assumed to be consistent with 

 recent levels. 



1964 1965 1966 1967 1968 1969 1970 1971 1972 



Figure 6. — The catch in metric tons of yellowfin tuna from 

 the CYRA is shown for the study period. The cohorts and tottd 

 catch are indicated by symbols as in Figure 5. 



-• SEMESTER fl COMOfiTS 

 -X SEMESTER B COHORTS 

 -o TOTAL (SEMESTER A * SEMESTER B) 



FIGURE 7. — Estimates from ENSIM of the kilocalories used per 

 year by yellowfin tuna in the exploited CYRA population for 

 the 1964-72 period. 



growth (areal expansion) on population size esti- 

 mates. From Figures 6 and 7 it is obvious that the 

 catch has great fluctuations (e.g., 1971) but the 

 energy flow seems to have stabilized in the ex- 

 ploited population estimates. This may be artifac- 

 tual but we think it may be significant to attempt 

 interpretation. 



The ratio of yield in weight to gross growth is 

 another interesting indicator (Figure 8). Note the 

 differential rate of exploitation of the semestral 

 cohorts through time prior to 1967. The S^ and 

 Sg cohorts became approximately equally ex- 

 ploited in this respect about 1967 or at about the 

 end of the changes in fishery strategy and when 



1967 (968 



YEAR 



Figure 8. — The ratio of the yield in weight (catch) to gross 

 growth for the years 1964-72. Note the relative similarity of the 

 levels of the cohorts respective ratios in the regulated years as 

 compared to the preregulated years. 



45 



