FISHERY BULLETIN: VOL. 75, NO. 3 



30 



in 



c 

 o 



25 - 



e 20 



in 

 T3 

 c 

 a 

 in 



o 



CO 



o 



Q 

 < 



15 



10 



5 - 



O C 3 = 1.26, C 4 = 0.37, 



C 5 =0.88, C 6 = I 49 

 • C 5 =40,C 3 =C, = C 6 =0 

 A C 6 = 40,C 3 = C 4 =C 5 =0 



1945 



1950 



1955 



I960 



1965 



FIGURE 9. — Simulated landings of yellowtail flounder with all 

 fishing effort in the third or fourth quarter of the year and with 

 c 3 -c e as assumed to have occurred for 1943-65. 



TABLE 9. — Comparison of simulated catches of yellowtail floun- 

 der with various values of the seasonal effort factors (03, C4, C5, 

 C6>- Percentage changes in yield are relative to the simulated 

 yield with C3, C4, c.5, and cq as in the first line of the table. 



have a very significant impact on the yield of the 

 fishery. There was little change in yield indicated 

 when fishing mortality was assumed uniform 

 throughout the year. The simulations showed that 

 yield of the simulated fishery would have been 

 reduced if all fishing mortality occurred during 

 the first quarter of the year. If all fishing mortality 

 were applied during the second quarter, yield of 

 the fishery would have been lower during the 

 first few years of the simulation, but little differ- 

 ence in total yield is indicated over 23 yr. The 

 expenditure of effort during the third quarter also 

 tended to reduce the early catch, but in the long 

 run appeared to result in the highest yield. By 

 restricting fishing mortality to the fourth quarter 

 of the year, some initial increase in catch was 

 indicated and long-term yield was also increased. 



These results reflect the facts that spawning 

 occurs during the second quarter and growth of 

 fish is limited to the third and fourth quarters of 

 the year according to the model. Clearly, to obtain 

 a short-term gain in yield, it is most advantageous 

 to harvest at or near the end of the growing season 

 (Table 9). Long-term gains were obtained when 

 egg production was optimized by harvesting just 

 after spawning (third quarter). By concentrating 

 effort during the fourth quarter, an increase in 

 yield was indicated for all years of the simulation. 

 Fishing during the first quarter appears to be 

 particularly detrimental because it crops fish just 

 prior to spawning. 



The seasonal pattern of effort exhibited by the 

 fishery in the past includes intense fishing during 

 the first quarter and the fourth quarter of the 

 year. Apparently these balance, resulting in 

 yields similar to the case where fishing is uniform 

 through the year. In recent years, the annual 

 catch quota for the United States (established by 

 the International Commission for the Northwest 

 Atlantic Fisheries (ICNAF)) was divided equally 

 among the four quarters. The result is that fishing 

 mortality was probably distributed nearly uni- 

 formly through the year. There may be some ad- 

 vantage to increasing the portion of the annual 

 quota captured during the second half of the year. 

 It is important to note that the long-term gains 

 obtained by concentrating fishing just after the 

 spawning season will not be realized if recruit- 

 ment is independent of spawning stock size ( Equa- 

 tion (18)). 



Several experiments were conducted with the 

 model in order to determine to what degree the 

 yield of the fishery could be stabilized or increased 

 by regulating the annual expenditure of effort 

 and ultimately F. For a fishery in which recruit- 

 ment is linearly related to stock size, in the long 

 run it is advantageous to reduce fishing effort 

 (and mortality) in order to increase egg produc- 

 tion. Therefore, the fishery was simulated with 

 effort at 80% of observed values (Figure 10). The 

 short-term decrease in yield was rather minor. 

 Considerable long-term advantage was predicted; 

 but even with a reduced level of effort, the simu- 

 lated fishery declined during the late 1940's and 

 early 1950's. However, the recovery when condi- 

 tions became favorable was more rapid at the 

 lower level of effort for this particular case. 



The Beverton and Holt YPR equation (Brown 

 and Hennemuth see footnote 2) indicates less than 

 a 5% increase in catch with a 20% decrease in 



478 



