338 THOROLF LINDSTROM AND NILS-ARVID NILSSON 



if the life-span is short and sexual maturity soon attained — as in the case of 

 C. lavaretus in Lakes Uddjaur and Storavan — the demand on the resources 

 is greater than for a population with a longer life-span and retarded maturity 

 but with equal standing crop in weight and an essentially equal length 

 distribution. 



These points of view may also prove useful when discussing 'optima' 

 (Lindstrom, 1958). 'Optimal conditions' do not necessarily characterize those 

 habitats with the highest average density, as other habitats may induce a 

 faster rate of turnover and a greater demand on the resources and hence a 

 more successful competition with other species. 



EXPECTED CONSEQUENCES OF INTERSPECIFIC COMPETITION 

 FOR EXPLOITATION 



During the course of exploitation of a particular species the decreased density 

 of adults decreases intraspecific competition with better individual growth 

 and perhaps also decreased natural mortality rate for adults of that species as 

 a result. To the extent that the species itself cannot take care of the released 

 resources, the competition with other species also decreases, resulting in a 

 gradual breakdown of the limits of the niches with consequences for indi- 

 vidual growth and survival also in the interacting species. An important 

 question arises in this connection: is it possible to affect in any significant 

 degree the growth of a dwarfed plankton-eater by hard exploitation of that 

 species only, if it coexists with competing species that have occupied all food 

 niches that give fast growth? Hypothetically the answer is no, but we have 

 no material to illustrate this point. 



Exploitation of one species may also affect the survival in young stages of 

 all whitefish species in the lake, but the details are difficult to envisage. 



SUMMARY 

 The first-year biology of three Coregonus species has been studied. C. pidschian 

 and C. lavaretus have the same hatching places in some cases. The habitat 

 segregation between C. peled and C. lavaretus is broken down during late 

 summer and autumn; their feeding habits are similar and their growth 

 curves do not diverge until after the first year. These similarities are such 

 that it is difficult to envisage how the necessary isolation is established during 

 this year of high mortahty. 



In the adults there is an obvious segregation into different feeding niches. 

 It is suggested that this segregation is made possible by a behaviour mechan- 

 ism, resulting in a changeable food specialization and that the segregation is 

 primarily the result of species interaction. The interaction also has effects on 



