332 



THOROLF LINDSTR5M AND NILS-ARVID NILSSON 



June 



Pig, 5.— Growth curves for Coregonus peled (above) and C. lavaretns (below) during the first 

 ' summer and autumn. The curves are drawn firee-hand and based on some data on total 

 length observed from seine-caught young. 



The common diet of the C. peled and C. larvaretus young in the same 

 habitat also calls for an explanation. Lack (i947, I949) and Crombie (1947) 

 state that a common diet for closely related species in the same habitat can 

 be interpreted either as a sign of a temporary superabundance of food or as 

 an indication that parasites or predators are controlling the numbers of both 

 species. The latter interpretation is the more likely one in the present case 

 but there are certain specific characteristics. The abundance of food should 

 be important if the growth rate affects the length of the predation period. 

 Furthermore, the whitefish young themselves do not perhaps greatly affect 

 the general density, e.g. of zooplankton, but they may very v^ell temporarily 

 deplete rich zooplankton accumulations, and such accumulations are probably 

 important for the growth rate. Mortality is thus brought about through a 

 complex of factors, among which density of food and activity of predators 

 are important. Moreover, the measure of the similarity as regards choice of 

 food and habitat may be crude, particularly as the food-choice mechanism 

 is not yet well developed (see below). 



Some sets of circumstances during the first year should favour one white- 

 fish species, other situations should favour another species, but our knowledge 

 on this point is very incomplete. The biology of the whitefish species during 

 this year is, however, so similar that, in an imaginary situation where one 

 species was suddenly left as the only whitefish in the lakes, any one of them 

 would probably have a higher fraction surviving between hatching and the 

 end of the year. 



