16 



The procedure utilized is to estimate the flow of food through the 

 food chain, based upon the amount of carbon fixed annually, the 

 efficiency of material transfer up through the food chain, and choice 

 of the trophic level at which to calculate fish yields. 



Advantage : This form of survey is relatively inexpensive, carried 

 out through plankton tows, lab analysis of samples of water, and compila- 

 tion of records from many survey stations. 



Disadvantage : Estimates based upon such information are extremely 

 complex to perform and may vary from the correct figure by several orders 

 of magnitude. Regarding, particularly, forecasts from zoo-plankton 

 abundance, plankton nets are yery selective (depending on mesh size) 

 and no one net can yield a zoo-plankton sample in a manner that truly re- 

 flects the natural abundance of animals. 



2. Egg and Larval Surveys 



If the total number of eggs spawned in a season can be estimated 



and the mean fecundity determined, and if the percentage of females to 



males in the stock is known, the abundance of the mature stock can be 



calculated: 



w _ P_ M = mature abundance 



KF P = total eggs 



F = mean fecundity 



K = proportion of females in stock 



The estimation of the year-class for each stock (number of young fish 



entering the population) is critical, for the condition of the adult 



population ultimately depends on year-class populations. 



