SIMPLE MODEL FOR ASSESSING POTENTIAL LOSS 51 



4. The life span of an individual is equivalent to the mean 

 generation time (MGT) for females in the population. 



5. Exploitation of planktonic eggs occurs instantaneously at 

 spawning, and exploitation of larvae occurs instantaneously at 

 hatching (becoming planktonic). 



Horst (1978) discussed methods of dealing wdth departures from 

 several of these assumptions. In many instances the method involves 

 a simple modification of the calculated survival rate from egg to 

 adult. Horst also discussed modifying the modeling procedure to 

 approximate MGT. 



In this paper I discuss aspects of the equivalent-adults model 

 associated with the mortality of eggs and larvae, A conflict exists 

 between assumption 5 and the "real-world" case; i.e., continuous 

 exploitation of planktonic Hfe stages by the power plant. I ex- 

 amine the effect of commonly observed egg and larval mortality 

 rates on the results of the model and suggest an alternative method 

 of computing the loss of equivalent adults which requires no 

 additional data. 



EFFECT OF EGG AND LARVAL MORTALITY OIM THE RESULTS 

 OF THE EQUIVALENT-ADULTS MODEL 



I have found that very seldom are data available to allow a 

 detailed estimate of rates of egg and larval mortality. Two cases with 

 which I am familiar are the analysis of egg and larval mortality of the 

 Atlantic mackerel, Scomber scombrus (Sette, 1943), and the analysis 

 of larval mortality of winter flounder, Pseudopleuronectes ameri- 

 canus (Pearcy, 1962). In both these cases mortality conforms to the 

 exponential model. My discussion here assumes that all mortality of 

 eggs and larvae follows this model. This allows the survivorship to the 

 end point of a life stage (e.g., hatching success) to be converted to an 

 instantaneous mortality rate: 



M = ^J^ (2) 



where M is the instantaneous natural mortality rate, S is the 

 survivorship to the end of the life stage, and t is the duration of the 

 life stage in days. 



Table 1 gives the results of a survey of daily natural mortality 

 rates for the entrainable (planktonic) life stages of several species. 

 The information in the table indicates that natural mortality rates for 



