-9- 



where A and B are input parameters which determine the magnitude and rate of any 

 change in food composition, respectively. The effect of this equation is 

 described in more detail in a later section. The mod i f ied food composition 

 table, (FCN) , is now used to recompute the actual food requirement of each 

 predator from each prey group, which is again compared to the amount allowable 

 from that prey group, and if overconsumpt ion of any prey group is still indicated, 

 the excess is removed from the predator's food for that month. The difference 

 between the required and the actual food consumption for any group is then 

 allotted to starvation, (50^,). 



Calculation of Equilibrium Biomasses 

 Equilibrium is attained when the biomasses of the species groups are constant 

 from one year to the next. Monthly fluctuations do occur due to seasonal 

 variation in temperatures, apex predator consumption, and phytoplankton and 

 zooplankton biomasses. At equilibrium the growth in biomass must equal the losses 

 to each biomass from fishing, predation, and natural mortality. Thus, to attain 

 equilibrium, either the growth of the species can be varied, or the sources of 

 loss can be varied, but in an opposite direction. Growth is determined from 

 empirical data, and the mortality coefficients for fishing and natural causes 

 are assumed to remain constant from year to year (before equilibrium). Predation 

 is the logical variable to adjust to reach equilibrium and, rather than adjust 

 the percentage food composition (derived from stomach samples), the input 

 biomasses of the species groups are modified. An iterative procedure is used 

 which adjusts the biomasses at the end of each year's computations: 



BB,, ,0 . = BB., , + BB,, ,„ - BB,, , 

 N,12,b N,l,a N, 12 ,a N,l ,a 



AGA 



