FOX: POPULATION SIMULATOR 



Table 2. — Effect of season length, beginning subsequent to hatching, on maximum equilibrium yield and allied fishery 



parameters for the simulated pandalid shrimp population. 



decrease in average weight as compared to the 

 annual fishery. The breeding success, defined 

 as the fraction of available eggs which are fer- 

 tilized, declined with the season length until a 

 6-mo season was reached. 



An unknown factor not programmed into the 

 simulation model, however, is the effect of 

 trawling on the behavior of the shrimp during 

 breeding or on the possibility of causing dis- 

 lodgement of the egg clutches and a subsequent 

 higher mortality rate. In view of these uncer- 

 tainties, the adoption of a 6-mo season from 

 hatching to the onset of breeding, as several 

 states have, appears to be a biologically pru- 

 dent approach for a natural population similar 

 to the simulated population. The 6-mo season 

 is only slightly lower in maximum equilibrium 

 yield, catch per unit effort, and average size 

 than the optimal 8-mo season, but is better 

 than the annual fishery in the first two cate- 

 gories. The actual implementation of a 6-mo 

 season on an annual fishery, however, would 

 have to weigh socioeconomic factors because 

 the maximum yield is obtained with twice the 

 amount of fishing effort per month as compared 

 with the annual season. If the fishery were only 

 able, at the time of adopting the closed season, 

 to exert a monthly effort of 1.0, the expected 

 seasonal equilibrium yield is about 10-15% less 

 than that for the annual fishery, but the expect- 

 ed equilibrium mean monthly catch per unit 

 effort would increase 70-80% . The latter should 

 provide a substantial increase in the economic 

 standard of an average fisherman, provided 

 that he has an alternative means of investment 

 for the 6-mo closed season which would provide 

 a sufficient return. The simulation model can 



be used also to evaluate the expected transition- 

 al states from an annual fishery to a seasonal 

 closure fishery as well as the expected equilib- 

 rium results discussed here. A socioeconomically 

 feasible strategy, then, may be determined 

 given the current state of the fishery. 



PROGRAM AVAILABILITY 



A listing and card set-up documentation for 

 program GXPOPS are available on request 

 from the author. 



ACKNOWLEDGMENTS 



Douglas G. Chapman and Gerald J. Paulik 

 of the University of Washington, Seattle, and 

 Brian J. Rothschild of the National Marine 

 Fisheries Service, La Jolla, Calif., read an 

 earlier version of this manuscript and offered 

 suggestions for improvement. I wish to express 

 my gratitude for their time and helpful com- 

 ments. 



LITERATURE CITED 



Berkeley, A. A. 



1930. The post-embryonic development of the com- 

 mon pandalids of British Columbia. Contrib. Can. 

 Biol. 6:79-163. 

 Beverton, R. J. H., AND S. J. Holt. 



1957. On the dynamics of exploited fish popula- 

 tions. G.B. Minist. Agric, Fish. Food. Fish. In- 

 vest., Ser. 2, 19, 533 p. 

 Conway, G. R. 



1969. A basic model of insect reproduction and its 

 implications for pest control. Ph.D. thesis, Univ. 

 California, Davis (Libr. Congr. Card No. Mic. 

 70-25134), 256 p. Univ. Microfilms, Ann Arbor, 

 Mich. (Diss. Abstr. 31:3457B.) 



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