A GENERAL LIFE HISTORY EXPLOITED POPULATION SIMULATOR 

 WITH PANDALID SHRIMP AS AN EXAMPLE^ 



WiLUAM W. Fox, JR.2 



ABSTRACT 



A general life history population simulator, GXPOPS, with provisions to exploit any age 

 sector of the population is presented. In addition to the usual fish life history pattern, the 

 model allows simulation of random mating, sex-differential maturation, and stepwise growth 

 found in many crustacean and some fish populations. GXPOPS provides for monthly calcu- 

 lations and all rates may be as general as month specific. The generality of the model is 

 illustrated with an examination of closed season management strategies for a pandalid 

 shrimp population. 



The advent of the digital computer has allowed 

 simulation modeling to become the current 

 vogue within nearly all fields of science; fisher- 

 ies is no exception. The computer has freed 

 mathematically inclined scientists from being 

 able to examine only equilibrium or cursory 

 transitional states and from the need for ex- 

 pressing relations in neat closed analytical 

 forms. Numerical integration schemes and the 

 speed of computation have made possible the 

 examination of large systems of differential 

 equations with feedback mechanisms, both from 

 deterministic and stochastic standpoints. Even 

 the simplest self-regenerating population mod- 

 el of Beverton and Holt (1957), which may be 

 regarded as a simulation model in that it has a 

 feedback mechanism, is largely intractable for 

 looking at transitional states and parameter 

 variability without the aid of a computer. 



Paulik (1969) gives a good review of simula- 

 tion modeling in fisheries. He divided the mod- 

 eling of fishery systems in an admittedly arbi- 

 trary way into two categories, management 

 models and scientific models, depending on the 

 manner in which a model is utilized. Manage- 



' Submitted as part of a Ph.D. thesis. University of 

 Washington, Seattle, WA. 98195. 



- Southwest Fisheries Center, National Marine 

 Fisheries Service, NOA A, La Jolla. CA 92037. 



Manuscript accepted April 1973. 



FISHERY BULLETIN: VOL. 71, NO. 4, 1973. 



ment models are employed for decision making 

 in evaluating or planning strategies for manage- 

 ment purposes, while scientific models are con- 

 structed to help elicit certain basic knowledge 

 of a biological system. The actual implementa- 

 tion of models in the two categories, however, 

 should be similar in utilizing the model for 

 planning, alternative evaluation, organization, 

 and identifying sensitive areas of the system. 

 There are at least two fishery simulation mod- 

 els currently in existence which were designed 

 to be generally applicable to most exploited 

 populations. The most comprehensive is 

 GAMES (Gales, 1972). This simulator has sev- 

 en interacting sectors from the fish stock to 

 the marketing of the final fish product, is modu- 

 lar in structure, and uses the most updated 

 means of input/output. Unfortunately, GAMES 

 is still in a state of development. The other 

 simulator, POPSIM, was written by Walters 

 (1969). POPSIM's major feature is an optimiza- 

 tion routine for planning harvesting strategies 

 for a number of harvesting periods. Both of 

 these simulation models are easily adapted to 

 the life history characteristics of fish popula- 

 tions which have been traditionally studied in 

 fisheries science. However, the FORTRAN IV 

 computer simulation model, GXPOPS, was 

 written to allow additionally for the life history 

 pattern of many crustaceans and some fishes 

 that require actual coj^ulation in reproduction 

 and a sex-differentiated maturation schedule. 



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