220 DISCUSSION 



S. J. Holt: Referring to Slobodkin's comment on Watt's formula, I 

 agree with what he said but nevertheless believe that the use of an empirical 

 formula can be useful for interpolation. 



R.J. H. Beverton: Watt is attempting to set up an objective theory for 

 exploited population dynamics, but we cannot overlook the exploiter and 

 the economic factors governing his activities. The maximal yield obtainable 

 may not be the most economic one to take; the last lo per cent of the yield 

 may not be worth harvesting. The effort input must be taken into account 

 and the yield should be related to it. 



L. B. Slobodkin: There is no rigorous need to use biomass as the basic 

 parameter in Watt's theory of yield — one could equally use a value para- 

 meter and simply adjust the constants. 



R. J. H. Beverton : Certainly. My point was simply that it is misleading 

 to think only in terms of biomass as it may obscure the fact that the same 

 biomass production may be attained in several ways. 



M. Graham: The historical background to the equation described by 

 Gulland is an interesting one. It was produced not by zoologists becoming 

 devotees of mathematics, but because they were forced by policy to look for 

 a more or less rigid equation useful as a guide to factors of first and second 

 magnitude, and as a basis for advice to the Government which was engaging 

 in international negotiations. It was also of value in defining the kind of 

 data which needed to be obtained by such things as marking experiments, 

 to give critical information about important parameters. 



R. P. Silliman: There is a philosophical point raised by Beverton's 

 comment on the need to incorporate an economic term in Watt's system. 

 Watt's forecast is most relevant for the time when the human population 

 outstrips its food supply, and when the simple maximal yield will be the 

 important thing. 



J. G. Skellam : The philosophical problem stated in Watt's abstract just 

 referred to by Silliman is unfortunately considered from only one aspect, 

 maximum sustained productivity. Consideration of this aspect is, however, 

 neither necessary nor sufficient for the solution of the philosophical problem. 

 It is not necessary because the problem vanishes if the human population 

 were maintained by factors other than starvation at a sufficiently low density, 

 and it is not sufficient because the attainment of the maximum level of 

 productivity which the energy of sunlight and the efficiency of chlorophyll 

 can provide imphes the maintenance of a still greater human population at a 

 'bestial level of existence'. It is essential here to consider the ecosystem as a 

 whole, and it is up to man to decide how to control the growth of the human 

 population in the least objectionable way. 



