SIMPLE MODEL FOR ASSESSING POTENTIAL LOSS 55 



This equivalent- adults model can be expressed as 



Na - U' [(Ne X Sea) + (Nl x Sla)] (5) 



For purposes of illustration, Table 2 compares the results 

 generated by the equivalent-adults model of Horst (1975) without 

 providing for relaxation of the exploitation assumption; by the Horst 

 model with relaxation based on published planktonic mortality rates, 

 and by my version of the model for five of the species from Table 1. 



SENSITIVITY ANALYSIS OF THE ALTERNATIVE MODEL 



The equivalent-adults model I propose assumes (1) that all 

 mortality occurring during the first year of life takes place during the 

 planktonic life stage and (2) that Sea approximates Sq. The 

 sensitivity of the model to each of these assumptions was examined. 

 Sensitivity is expressed in terms of the ratio of the predicted impact 

 to the "actual" impact resulting when the model assumptions are not 

 satisfied by the natural situation. 



Figure 2 shows the relationship between the prediction of 

 equivalent-adult loss based on assumption 1 and the actual loss that 

 would result when planktonic mortality comprises only a specified 

 portion of estimated first-year mortality. This figure indicates that 

 the proposed model will always overestimate loss. The magnitude of 

 the overestimation is based on the relationship 



r = p-'^ (6) 



where p is the proportion of first-year mortality actually occurring 

 during the planktonic life stage(s) and r is the ratio of predicted to 

 actual loss. 



For example, if 25% of the total first-year mortality occurred 

 during the planktonic life stages, the proposed model would 

 overestimate loss by a factor of 2, or, if 1% of the total first-year 

 mortality occurred during the planktonic life stages, the proposed 

 model would overestimate loss by a factor of 10. This relationship is 

 independent of Sq . 



The effect on the model prediction resulting from the use of Sea 

 as an estimator of Sq is similar to that described. This relationship, 

 expressed in terms of the ratio of Sea to Sq , is 



(W 



So 

 where r is the ratio of predicted to actual loss. 



(7) 



