DISCUSSION 



J. A. GuLLAND : Slobodkin's equation for experimental populations gives 

 rise to the following relationship : 



P. F 



F _ 



P 



= I 



Can this be apphed to exploited fish populations and the same answer 

 obtained? Some rewriting is necessary since the terminology is different: 

 thus F, which is the proportion of young fish, is equivalent to FJF + M, 

 and when re-written in my terms, we have: 



Pp_ 2M 



F+2M 



Represented graphically the relationship is comparable with the diagram 

 presented for west of Scotland hake populations (Gulland, Fig. i, p. 207). 

 For North Sea plaice it is known that exploitation has led to a tenfold dechne 

 therefore 



, 2M 



'I'^^FTm 



hence 



F=iSM 



It is also known from age composition data that the total mortality, F + M 

 = 0*7, hence 



M = 0-039 



The value of M usually taken, based on other evidence, is o-i, which is a 

 fair measure of agreement considering the rough nature of the approach. 



G. V. Nikol'skii: Do you consider that the stock of plaice may be 

 determined very early in life, by the amount of food available for the stages 

 immediately after metamorphosis? 



R. J. H. Beverton: For the North Sea the data are not sufficient. 

 Certainly once metamorphosis is completed the fish appear much more 

 hardy. It is the pelagic phases which seem, on circumstantial evidence, to be 

 the sensitive ones. 



T. B. Reynoldson: Beverton does not seem to have taken changes in 

 fishing efficiency into account, and in this his approach seemed to differ from 



