EXPERIMENTAL AND THEORETICAL APPROACHES 261 



that of other speakers who related catch to effort. Could efficiency changes 

 be related to changes in his abundance curves ? 



R. J. H. Beverton: I was using catches, and not more refined measures 

 of population, which are not available for the sixty-year period under 

 consideration. There does not seem to have been any great overall change 

 in fishing pressure in this period, and I believe that the trends indicated are 

 genuine ones. The catch per unit of effort could be determined for the past 

 thirty years, but this treatment would emphasize the post-war bulge. 



G. V. Nikol'skii: I do not agree that Slobodkin is right when he says 

 that the populations he has been studying are of no economic importance. 

 Daphnia is a most important food organism for fisheries. 



I think that for many predator-prey relationships, however, the mathe- 

 matical models must be different, to take into account the existence of real 

 predators in contrast to the method of abstraction of animals used by 

 Slobodkin. 



L. B. Slobodkin: I am using predation in a simple sense. The only 

 biological feature of my predator is that it takes Daphnia. 



W. Edmondson: I am not sure that Slobodkin is a very abnormal 

 predator. It may be that he takes only big or small animals, but many 

 natural predators, like Chaoborus, are highly selective 



L. B. Slobodkin: But the degree of predation in my case is not related 

 to previous predation. My own population dynamics are in no way depend- 

 ent upon those o£ Daphnial 



M. E. Solomon: One of Beverton s graphs showed a rise in the plaice 

 population during the war. Later, I think he stated that competition. for 

 food among the newly hatched animals was the key factor. Surely these 

 statements are contradictory? With less fishing there should be higher egg 

 production and so no population rise, or even a decline ! 



R. J. H. Beverton: The war-time rise in population was due to the fall 

 in adult mortahty from 50-60 per cent per annum under fishing conditions, 

 to about 10 per cent per annum. This caused a rise in the total population, 

 but the size of the individual year classes was unaffected: the bulge was thus 

 largely due to improved adult survival. 



M. E. Solomon: And what happened after the war? 



R. J. H. Beverton: The sizes of year classes hatched during the war 

 were about average : despite very abundant egg production the same number 

 of recruits came through. Thus there is definite evidence that regulation 

 occurs after hatching. 



A Milne: Is predation, especially on young fish, important? 



R. J. H. Beverton: Predation on adult plaice is probably shght; at 

 least there are no indications that they are eaten to any significant extent 



