■12- 



in SKEBUB is presented in more detail below. The function relating the composition 

 of the diet to the biomass of the prey is modified to allow the modeller to 

 control the dependence of the final food composition of each predator on either 

 the input table of select ivit ies or on the allowable composition of each species. 



SIMULATIONS OF FEEDING IN SKEBUB 



At the beginning of each monthly time step the growth of each biomass and the 

 food required for the maintenance and growth of this biomass are computed. The 

 food required by each predator of each prey is then determined through the input 

 food selectivity table (CF^ , ) . The total requirement for all predators from 

 each prey type is computed and compared with the amount of each prey biomass 

 designated available. The input food selectivity table is now adjusted by a 

 function of the ratio of available to required food for each prey biomass, 

 (FCOCj^) : 



f^^M ^ = CF, , [(1 + Ae"^ / (1 + Ae"^^''"^k))] 

 N ,k N,k 



where FC , is the adjusted percentage food composition, and A and B are constants. 



This equation is a modification of the logistic equation, forcing the point 

 of inflection to be at x = y = 1 (i.e. when the food required from a prey type 

 is equal to the amount available, (FCOC, = 1 ) , no adjustment is made 

 (FC|^ , = CF ,)). The maximum upward adjustment of the percentage food requirement 

 from any prey item is equal to: 



MaxUp = 1 + Ae , 

 and the maximum downward adjustment: 



MaxDown = MaxUp/1 + A 

 The rate of change is specified by B, although a value of 1.5 has been found 

 reasonable in practice. The effect of increasing A is a parallel increase in 

 the maximum upward and downward adjustments. 



