50 SAUNDERS 



from the entrained plankters. Horst (1975) suggested using an 

 equivalent-adults model to translate the number of eggs and larvae 

 entrained to an equivalent number of adults lost. 

 The reduced form of the model is 



Na = (Ne X Sea) + (Nlx Sla) (1) 



where Na = number of adults potentially lost as a result of entrain- 

 ment 

 Ne = number of newly spawned eggs entrained 

 Sea ~ survival from egg to adult 



Nl = number of newly hatched larvae entrained 

 Sla ^ survival from larva to adult 



and 



SEA = (Pf X F5^x L)-i (la) 



where Pf is the proportion of the population which is female, Fx is 

 the average fecundity per female per year, and L is the reproductive 

 life expectancy of an average organism. 



The term Sla can be defined as 



Sla = Sea x Sel (lb) 



where Sel is the survival from egg to larva. 



The equivalent-adults model provides an approach to the 

 assessment of entrainment impact on a species population in 

 instances where little information is available on the life history of 

 the species. The only life-history information required to use the 

 model is the fecundity of a female, the reproductive life span, and 

 the sex ratio. A number of other life-history parameters can be 

 incorporated into the model's calculations, however. Thus the 

 equivalent-adults model is a valuable expedient that can be used in 

 the impact-assessment process when data are insufficient to allow a 

 more sophisticated approach, such as the Leslie (1945) model (Horst, 

 1978). 



A number of assumptions implicit in its formulation must be 

 recognized and addressed when we use the equivalent-adults model: 



1. The adult population is at equilibrium, with no change in its 

 size occurring over time. 



2. The population maintains a stable age distribution. 



3. The ratio of males to females is constant through time. 



