54 



SAUNDERS 



entrained had all entrainment taken place immediately after spawn- 

 ing or hatching. 



At natural mortality rates of 5%/day or greater and for an 

 entrainable life-stage duration of 30 days, the equivalent-adults 

 model underestimates potential adult loss by a factor greater than 2 

 (Fig. 1). In assessing larval entrainment of winter flounder, which has 

 a larval mortality rate of 21%/day during the first 25 days after 

 hatching (Pearcy, 1962), the model would underestimate potential 

 adult loss by a factor of 18. 



These facts suggest that the equivalent-adults model is very 

 sensitive to the assumption that exploitation occurs instantaneously 

 when eggs are spawned or larvae hatch. Because the model could 

 grossly underestimate adult loss, it may be of questionable value 

 from a regulatory viewpoint since, in a proceeding intended to 

 protect the environment from adverse effects, acceptable impact 

 predictions usually must be able to assert that impact in the 

 real-world case will not be greater than that predicted. Therefore, to 

 assure this "conservatism," we must provide for relaxation of the 

 assumption of immediate exploitation. 



REVISION OF THE EQUIVALENT-ADULTS MODEL 



When sufficient data are available to calculate mortality rates for 

 planktonic life stages, the equivalent-adults model can be modified 

 by incorporating U (Eq. 3) into the loss calculation. This can be 

 accomplished by using data available in the literature or by 

 estimating planktonic mortality rates directly from field-survey data, 

 as discussed by Polgar (1977) and Hackney (1977). When restrictions 

 of time or resources prevent direct estimation of U, however, 1 

 propose that an alternative form of the equivalent-adults model be 

 used. 



My model requires an estimate of survival through the first year 

 (So). To relax the assumption concerning time of entrainment 

 without introducing a number of new factors into the model, 1 have 

 taken a pragmatic approach to estimating Sq • Since first-year 

 mortality is high in comparison with mortality in older fish, I use the 

 survival from egg to adult (Sea)' based on fecundity and life span 

 (Eq. 1), as an estimator of Sq . If we then assume that all first-year 

 mortality occurs during the planktonic life stage, a correction factor 

 (U') can be calculated for the model: 



U' = g-lnSEA/2 (4) 



where Sea is the egg-to-adult survival as defined by Eq. la. 



