SIMPLE MODEL FOR ASSESSING POTENTIAL LOSS 59 



Assumption 3 requires that the vuhierability of planktonic 

 organisms to entrainment does not change. This is not valid since, as 

 planktonic fish grow, swimming (avoidance) ability increases and 

 vulnerability decreases. This would result in an adult loss lower than 

 that predicted by the model. 



Assumption 4 requires that plankton which die naturally not be 

 entrained. Inclusion of organisms already dead in the estimated 

 number of organisms entrained would bias the model toward an 

 overestimation of loss. The magnitude of this increase of predicted- 

 vs. -actual impact depends on the proportion of dead organisms 

 entrained. 



The sensitivity analysis and these observations indicate that my 

 alternative version of the equivalent-adults model is biased toward 

 overestimating potential adult losses. Such bias should be generally 

 acceptable in terms of determining (in a "first-cut" approach) 

 whether or not entrainment at a particular facility is potentially 

 damaging to local fish populations. Use of this model will possibly 

 generate order-of-magnitude overestimates of loss in some cases (as 

 opposed to order-of-magnitude underestimates with the unrevised 

 model). For this reason, I suggest using this form of the equivalent- 

 adults model only in cases where information on planktonic 

 mortality rates or population dynamics does not exist. 



I do not consider this discussion to constitute a complete 

 treatment of the equivalent-adults model and its application in 

 impact-assessment situations. For a thorough discussion of other 

 aspects of the model, see Horst (1978). 



SUMMARY 



An examination of one of the assumptions inherent in applica- 

 tion of the equivalent-adults model of Horst (1975) indicated that 

 failure to satisfy the assumption could result in underestimates of 

 potential adult loss by a factor greater th£in ten. I suggest using an 

 alternative form of the model which requires no additional data. 

 Analysis of the assumptions of the alternative model shows that it 

 will overestimate impact, but such overestimates are considered 

 acceptable in using this model as a preliminary impact-assessment 

 tool. 



ACKNOWLEDGMENT 



I would like to thank Tom Horst and Carol Blakely for their 

 encouragement and comments. 



