PARRISH: MARINE TROPHIC INTERACTIONS BY DYNAMIC SIMULATION 



Figure 14. -Effect of differential exploitation on the biomass 

 ratio, B2/B3, of two equally competing fish species: predictions of 

 dynamic simulation (DS) and graph theory (GT). The coefficient 

 of instantaneous fishing mortality for species 3 is always F3 = 

 0.1. 



tors were exploited. The ratio B.^/B^ from the 

 simulation at a given E,/F^ ratio was slightly less 

 extreme than in the P12000 web. However, the 

 difference was too slight to permit a meaningful 

 check against graph theory calculations. 



The top predator in this system could also be 

 exploited using low values of F. The results were 

 qualitatively similar to the last case above. The 

 ^2/53 ratio changed only very slightly in this par- 

 ticular system to values intermediate between 

 those of the last case above and those obtained 

 with the P12000 web. At even the lowest F values 

 for the competitors in Figure 14, if exploitation of 

 the top predator was carried above about F = 0.2, 

 the top predator was lost from the system. The 

 same result occurred with F<0.2 on the top preda- 

 tor if slightly higher lvalues than those in Figure 

 14 were applied to the competitors. The two lower 

 trophic levels persisted stably. This vulnerability 

 of the top predator represents another limit to the 

 stability of the larger system. Although it has not 

 been explored, it appears to have implications for 

 possible effects of exploiting real multispecies 

 fisheries. 



The particular combinations used in this brief 

 investigation were far from optimum for explor- 

 ing a large range of exploitation intensities in 

 multilevel webs (The choices were made primarily 

 for similarity to other cases studied previously). 

 The low permissible levels of predation and 

 exploitation that the 3-level system would 

 tolerate, together with the high natural mortality, 



made for difficulty in comparing results with those 

 of trophic systems previously examined; e.g., by 

 graph theory. However, the considerable 

 similarity of the predictions in Figure 14 and Ta- 

 ble 3 by these very different approaches seems 

 highly suggestive. 



AVAILABILITY OF MODEL AND 

 COMPUTING DETAILS 



Written descriptions of various portions of the 

 model and their sources in somewhat more detail 

 are available from the author. The basic computer 

 software package used (IBM 1969) and a more ad- 

 vanced version (IBM 1971) are described in the 

 manufacturer's literature with enough detail in 

 the former case for ready use by the reader. The 

 CSMP package is sufficiently user-oriented that no 

 further interface program is required; the model is 

 written directly into the CSMP structure using 

 simplified FORTRAN-like statements. Program 

 listings and card decks for sample trophic models 

 are available from the author, together with tables 

 of input values used and a glossary of code names 

 of variables and parameters. 



ACKNOWLEDGMENTS 



Part of this work was performed while the 

 author was the recipient of a National Institutes 

 of Health Fellowship, 5F01GM48175-02, General 

 Medical Sciences. This work was based on part of a 

 dissertation submitted in partial fulfillment of the 

 requirements for the Ph.D degree at the Univer- 

 sity of Rhode Island (Graduate School of 

 Oceanography). S. B. Saila provided general coun- 

 sel and encouragement and reviewed earlier ver- 

 sions. W. H. Krueger and N. Marshall also read 

 earlier versions of the manuscript. R. Sternberg 

 was helpful in the solution of a differential equa- 

 tion. 



LITERATURE CITED 



Adelman, H. M., J. L. Bingham, and .J. L. Maatch. 



1955. The effect of starvation upon brook trout of three 

 sizes. Prog. Fish-Cult. 17:110-112. 

 Bagenal, T. B. 



1957. The breeding and fecundity of the long rough dab 

 Hippoglussoides p/afe.s.so/rfe.s (Fabr.) and the associated 

 cycle in condition. J. Mar. Biol. Assoc. U.K. 36:339-375. 

 1967. A short review of fish fecundity. In S. D. Gerking 



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