SISSK.WVINK COMPAKTMF.NTAI.IZKI) SIMULATION MODi L 



30 



to 



S 25 



E 20 



o 



CO 



o 



Q 

 < 



OBSERVED EFFORT 



80% OF OBSERVED 



EFFORT 



1945 



1950 



1955 



I960 



1965 



FIGURE 10. — Simulated landings of yellowtail flounder with 

 observed level of fishing effort and with 80% of the observed 

 level. 



fishing mortality (for 0.6 =£ F =s 1.5 and age at 



first capture between 1.75 and 3.0). Therefore, 

 most of the increase in yield indicated in Figure 10 

 must result from improved recruitment at lower 

 levels of F. 



Since recruitment and growth appear related to 

 temperature, the possibility of using this environ- 

 mental variable to predict an appropriate level 

 of effort was considered. The model is such that 

 growth and recruitment are proportional to T g 

 and T r , respectively. Therefore, the following 

 relationship between fishing effort and T g and T r 

 was utilized: 



fi=c l5 - {(TPi-i+OVi-J. 



(20) 



Effort for year i was based on the growth- 

 temperature factor for the year i — 1 since Equa- 

 tion (20) is of little value unless effort can be set 

 in advance. The recruitment-temperature factor 

 from 2 yr prior (i - 2) was used since recruitment 

 lags spawning by about 2 yr. A 3-yr lag could have 

 been used. T g and T r could have been weighted in 

 Equation (20) since the latter is usually more 

 important in determining equilibrium yield, but 

 this would have introduced another parameter. 

 Initially, c 15 was estimated as 1,870 days of 



fishing, which yields about the average level of 

 effort for 1943-65 when T g and T r equal 1. A 

 value higher and lower than 1,870 days was also 

 considered. Simulated catches for each value of c 15 

 are shown in Figures 11-12, and the simulated 

 catch per unit of effort is shown in Figure 13. 



For 1943-65, c 15 = 2,200 days resulted in a 

 decrease in relative abundance while Ci 5 = 1,540 

 days permitted the relative abundance to in- 

 crease. The value of c 15 (1,870 days) corresponding 

 to the average effort during 1943-65 best stabi- 

 lized the relative abundance of the fishery, but 

 was only slightly more effective than the volun- 

 tary actions of the fishermen who probably re- 

 sponded to fluctuations in fishing success (U). It 

 appears that a function even more sensitive to 

 temperature than Equation (20) is required to 

 better stabilize the population. Since T r is more 

 sensitive to temperature than T g , weighting of 

 these factors (in favor of the former) might result 

 in a function more effective in maintaining the 

 population size during the early 1950's. Never- 

 theless, the yield of the simulated fishery (with 

 the linear recruitment function) could have been 

 substantially increased if fishing effort were reg- 

 ulated by a simple function such as Equation (20) 

 with c 15 considerably less than 1,870 days. 



c 

 o 



e 



to 



C 



o 



o 



CO 



z 



a 



z 

 < 



1945 



I960 



FIGURE 11. — Simulated landings of yellowtail flounder with ob- 

 served effort and with effort set by Equation (20) using c 15 = 

 2,200 or 1,540. 



479 



