■]h- 



point and base for the comparison of the effects of various factors on the 

 ecosystem (Laevastu and Larl<ins 198l)- Figure 1 illustrates the changes in 

 the species groups biomasses from their input until equilibrium is reached 30 

 years after the start of the run. The biomasses change again immediately 

 following equilibrium when the equ i 1 i br iat ion procedure is removed and the input 

 food selectivity table is replaced by the one adjusted at equilibrium. This 

 replacement of the input food composition table is not necessary to the running 

 of the model and is inadvisable when the input food composition table is 

 estimated with confidence. 



After equilibrium the density dependent influences on growth come into play 

 and maintain the biomasses at a fairly constant level (years 1 to 9, Fig. 2). 

 In the tenth year following equilibrium two severe and arbitrary adjustments 

 were made to the biomasses to study the maintaining effect of the density 

 dependent processes. In Figure 2 the biomass of the silver hake species group 

 was reduced by 75 percent at the start of year ten. In the subsequent ten years 

 all biomasses fluctuate until approximately the original equilibrium distribution 

 of biomasses is regained. In Figure 3 the fishing pressure on the silver hake 

 species group was raised fourfold at the start of the tenth year following 

 equilibrium. Again equilibrium was regained in the subsequent ten years, but 



in this instance a new equilibrium distribution of biomasses was reached, 



2/ 

 reflecting the continuing effect of the increased fishing pressure.— 



Sample output data are given in Table 1. These data are the annual mean 

 values at equilibrium. 



2/ The ordinate scale varies for each species group on these graphs. The 

 ~ biomass of benthos was reduced by a factor of 10 for plotting. 



