MARINE MAMMALS 187 



seals will, of course, stay ashore without feeding for two to two and a half 

 months. 



I. A. McLaren: But so long a starvation seems unlikely in a female 

 feeding a pup. 



H. R. Hewer: When the seals haul out in the spring, are all age-groups 

 represented in the population ? 



I. A. McLaren: There is a progressive increase through the spring to a 

 threshold, when more or less the whole population is present on the ice and 

 all age groups are represented. 



H. R. PIewer : Of course it is common in Phocids for the younger age 

 groups not to return to the breeding grounds. 



L A. McLaren : Were this so in the ringed seal an age-group would be 

 evidently missing. 



E. D. Le Cren: Laws and Jonsgard both mentioned that a rise in growth 

 rate occurred when whale populations had been reduced by exploitation. 

 Does this mean that the whales exert a limiting effect on the plankton 

 populations on which they feed ? 



R. M. Laws: The evidence relating to growth rate is circumstantial but 

 I believe it to be correct. There is also a variation in size with longitude, 

 which may suggest that in the best feeding places there is competition 

 between older and younger whales. This size segregation is only seen on the 

 northern feeding grounds, and disappears when the whales move southwards. 

 There are, of course, other animals like crab-eater seals and pelagic birds, 

 which feed on krill and it is possible that their populations have increased 

 as a result of the exploitation of whales. 



A. Jonsgard : I agree that this could happen in Antarctic fm and blue 

 whales, which feed only on one type of food. But the minke whale is a 

 generahzed feeder with many kinds of diet. I know of one animal found 

 with a stomach full of Lofoten cod, a very large fish for a baleen whale to 

 eat. It is unlikely that food is a limiting factor for the minke whale unless 

 some special food is involved, of which a certain intake is nutritionally 

 essential. The complexities of these problems are well shown by Australian 

 work on the humpback whale, whose mean length at puberty was shown to 

 decrease over the period up to 1956. It was thought that this reflected a 

 decrease in growth rate : but it is now known to be due to gunners selecting 

 the largest available animals. 



