298 POPULATION AND RECRUITMENT IN FISH 



F. Raw: Fecundity should also be considered. An insect laying say one 

 hundred eggs need experience only the difference between a 96 per cent 

 mortahty and a 98 per cent for the population to be halved. In fish the 

 enormous fecundity, despite the stability of population, suggests a remarkable 

 buffering effect. 



E. D. Le Cren: Yet often there are fluctuations in the number of young 

 produced per year, due to climatic variations. But if the population includes, 

 say, ten distinct age-groups instead of one (as in most insects) these fluctua- 

 tions will balance out. One age-group can vary widely without the popula- 

 tion as a whole being much affected. 



J. B. Cragg: Referring back to McLaren's paper, he talked about an 

 equihbrium population in his seals. This seems in contrast to the grey seal 

 around Britain: has he any comment? 



I. A. McLaren : I can only say that I accepted the fact of an equihbrium 

 where the numbers seem stable. The reason for the difference may be that 

 whereas my ringed seals have had some 11,000 post-glacial years to reach 

 an equilibrium, the European grey seal has not yet recovered from very 

 heavy exploitation which has only recently terminated. 



J. G. Skellam: There is a problem about the use of the term 'equilibrium 

 population'. It has been used in two senses: 



(a) The total number of animals is more or less constant. 



[b) The age structure of the population is of more or less constant form. 

 It is possible to have a rising population which is yet maintaining a constant 

 proportion between its component year groups. Lotka (1925) demonstrated 

 this. If equilibrium be defmed in terms of age structure the presence of a 

 slow growth or decline in overall numbers is unimportant. 



M. H. Williamson: Concerning the relative stability of marine and 

 terrestrial populations, there are of course markedly unstable aquatic ones. 

 Plankton commonly vary tenfold in numbers and frequently a hundredfold: 

 thousandfold variations are known. 



J. B. Cragg: I think it is important to compare taxonomically related 

 organisms. 



G. C. Varley : There are insects with a stability of population com- 

 parable with that of fish. For example, ants and termites, in which the 

 reproductive life of a female extends over many years. 



R. J. H. Beverton: The fisheries work should not be taken as proof 

 that stabihty is the rule for marine organisms. There are exceptions, such as 

 the Norwegian herring which shows wide variations in abundance. 



G. Saetersdal: Records are available for several hundred years, but it 



