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empirical data from stomach content analyses. 



The proportion of a prey actually "occurring" in a predator's diet in each 

 time step is then made a function of this suitability index and the prey's current 

 biomass. In the Andersen and Ursin (1977) model, prey switching by a predator is 

 implicit and is in direct proportion to the relative proportion of each prey biomass. 

 Laevastu and Larkins (1981) explicitly allow prey switching; the suitability of 

 any prey type at any time step is a function of the input suitability index and 

 the ratio of required to available biomass for each species. This latter approach 

 requires the input of a parameter to determine the proportion of each biomass 

 available for consumption in each time step; otherwise overgrazing would occur. 

 This parameter is a fixed proportion of the growth for all fish species. In the 

 model of Andersen and Ursin (1977), actual food consumption by any predator is 

 then determined from the available prey and a half saturation constant, or 

 coefficient of rate of search. Once the adjusted suitability index has been 

 computed in the Laevastu and Larkins (1981) model, no further adjustment to 

 feeding occurs; if more food is required from a prey biomass than is deemed 

 allowable, the predators requiring that food suffer starvation. Both models, 

 then, require the estimation of a parameter unverifiable given current data. The 

 method of Andersen and Ursin (1977) computes the half saturation constant for 

 each species by trial in the model. Laevastu and Larkins (I98I) designate allowable 

 consumption as a fixed proportion of growth for all species. In SKEBUB this 

 proportion is assumed identical for all fish species, reducing the required parameter 

 estimates to one. This global allowable consumption parameter is adjusted so that 

 the estimates of the percent of required food that is unobtained (starvation) appear 

 consistent with available data. The method of Laevastu and Larkins (I98I) as used 



