366 



GENERAL DISCUSSION 



M. Graham: There are some questions which can at present only be 

 answered empirically. A complex population can only be dealt with in a 

 simple way because the treatment works. The yield of North Sea fisheries 

 has fitted the theoretical predictions; despite great changes in gear and 

 methods, the cessation of fishing during six or seven war-time years did 

 have much the effect predicted. This suggests that however complex the 

 parameters, they can yet be handled in a simple way. 



It is quite evident, for example from the data that Beverton presented, 

 that some populations can fluctuate about stable, and others about rising 

 means. Sole, for example, follow the latter, and plaice the former pattern. 



S. J. Holt: We have tended to think of the relationship between 

 recruitment and the preceding stock in terms of a basic assumption that 

 recruitment in succeeding generations is not affected by stock size. Le 

 Cren has now stated that whatever the amount of eggs produced, the 

 number of recruits remains the same. It has also been suggested that recruit 

 numbers and stock size are independent. But this is not the same thing from 

 the point of view of regulatory mechanisms. In the first case we have fluc- 

 tuating egg numbers but constant recruitment; in the second the stock size 

 or egg number may vary little but the recruits considerably. Yet fisheries 

 biologists perhaps wrongly take these two patterns as comparable. 



t 



Eggs 



t 



Recruits 



t 



Eggs 



t 



Recruits 



Time Time 



Fig. 3. — (S. J. Holt): Mortality and recruitment in fish — see text. 



The difference is that the process controlling the fluctuation operates at 

 different stages of the life cycle. In Le Cren's example it occurs before 

 recruitment; in other instances after it. In the second case the damping 

 through the presence of a succession of age-groups in the population is the 

 sole influence. 



Thus on Le Cren's pattern, whatever the number of eggs laid, the same 

 number of recruits result, as in Fig. 3^7. While on the other hand in Fig. 3/j, 



