300 POPULATION AND RECRUITMENT IN FISH 



organisms later consumed by the young fish, or otherwise improve the 

 habitat? 



E. D. Le Cren: There is no evidence on this. But Professor Nikol'skii 

 has pointed out that in the Pacific sahnon dead spawners provide food and 

 also fertilize the water. This may apply to a less extent for milt. The bulk of 

 the weight of these reproductive products, however, is made up of the egg 

 yolk, which is utilized directly by the larvae: in salmonids and perch most 

 of the eggs do hatch. 



L. B. Slobodkin: Work by Richman, on the populations of a small 

 pond in western Massachusetts, showed that there the amount of phyto- 

 plankton varied in direct relationship to the spawning of frogs. 



G. C. Varley: We have heard a great deal today about populations, but 

 very little about their effects on the organisms which serve them as food 

 supply. In several cases the populations which have been studied are co- 

 existing in the same areas, as Minke whale and fish do off the Norwegian 

 coast. Factors must exist which are common to these two: what is known 

 about these? 



A. Jonsgard: The Minke whale takes many fish, including young 

 herring. This is especially true in the autumn, when the southward-migrating 

 whales stop for a time in the herring grounds and feed on the young fish. I 

 have also mentioned the discovery of forty Lofoten cod in one whale 

 stomach ; translated into terms of herring this would be a vast amount of 

 fish. We kill about 4,000 Minke whale a year: a lot offish could be eaten by 

 them during their three- to four-week stay in the herring area. Other fish 

 are probably less affected. But we have no actual information on the inter- 

 action of these species. 



F. Raw: In connection with Le Cren's and Slobodkin's papers, I think 

 we should consider the efficiency of conversion of the food supply. If we 

 consider the actual production of yield, heavy stocking is relatively less 

 efficient than lower stocking. There is surely a carry-over since individuals 

 which die go through a series of cycles returning to the system in three or 

 four years. If energy loss from the system is considered, the number of stages 

 involved in it is important, and it is an asset if these are relatively few. 

 Slobodkin said that Daphnias conversion of food into biomass is 9-12 per 

 cent efficient. He said further that the range for aquatic organisms as a whole 

 was 5-15 per cent. Could he say more about the relation of these figures to 

 organisms and their food habits. Herbivores have a different and usually 

 lower conversion rate than carnivores, and this should be reflected in the 

 figure. 



E. D. Le Cren: There are an enormous number of complexities involved 



