■10- 



where BB,, ,„ , is the new December biomass, BB , is the previous year's January 

 N , I / , b N , I ,a 



biomass. and BB. ,^ is the current December biomass. The iteration constant, 

 ' N, 12 ,a 



AGA, controls the degree of convergence towards the equilibrium and varies from 

 3.0 at the beginning of the iterative procedure to ^.8 at the end of the iterative 



procedure. Thirty iterations, or "years", have been found necessary to reach a 



. • '/ 

 stable position.— 



SIMULATION OF FOOD SUITABILITY AND 

 AVAILABILITY DEPENDENT FEEDING 



Predator-prey relationships provide a readily comprehensible mechanism for 

 species interactions and are used for this purpose in many ecosystem models 

 (e.g., Andersen and Ursin 1977, Pope 1979, Helgason and Gislason 1979, Laevastu 

 and Larkins 1981). Ursin (1981) has discussed the constraints of the models of 

 Pope(l979) and Helgason and Gislason (1979), in particular their assumption 

 that growth and food consumption rates are independent of food concentration. 

 The remaining two models allow underconsumption by a predator species when prey 

 sources are limiting. In both instances the prey consumption by a predator is 

 made dependent on an index of suitability and the prey's biomass. 



Andersen and Ursin (1977) compute the suitability, or vulnerability, index 

 as a composite of suitability with respect to size, the fractional overlap of 

 predator and prey in time and space, and the chance of encounter due to the 

 behavior of predator and prey. This index is frequently compared with available 

 stomach contents data to check the realism of the arrays. Laevastu and Larkins 

 (1981) have taken a more direct approach and base their suitability index on 



1/ A "stable position" is defined as one where further iterations do not affect 

 ~ year and biomasses. in practice this stabilization occurs when the percent 



annual change in individual biomass varies from to about 5%. Global adjustment 

 to the allowable percent consumption parameter is used to adjust the equilibrium 

 position so that the mean of the percent annual changes in individual biomasses 

 (which can be + or -) is close to zero. 



