ON THE COMPETITION BETWEEN WHITEFISH SPECIES 329 



view on the systematic position of the whitefish species : they are so closely 

 related that introgression occurs very frequently (and has gone very far in 

 environments with few niches, reaching replacement or complete fusion of 

 populations). 



Darwin pointed out that the intensity of competition must be directly 

 related to the degree of ecological similarity between the species involved. 

 The mechanisms of competition are still very incompletely known, and the 

 present material on the biology of young and adults of closely related and 

 ecologically similar whitefish species allows of only a fragmentary interpreta- 

 tion of the competitive mechanisms working. 



COMPETITION DURING THE FIRST YEAR 



It is often stated that fish are subject to a heavy mortality in their fry and 

 young stages. As the factors governing this mortality vary between years, 

 the strength of the different year classes varies correspondingly. Most likely 

 the instantaneous mortality coefficient (Beverton & Holt, 1957; Ricker, 

 1958) typical for the first-year biology of a particular population, varies not 

 only between years but also with the season and the density of the popula- 

 tion. If the whitefish species populations adversely affect each other's survival 

 during these early life stages this is an example of competition. A survey of 

 the first-year biology will give some suggestions. 



The first-year biology has been studied in Lakes Uddjaur and Storavan 

 (Fig. 2). There exist three species: Coregonus peled (Gmelin), Coregonus 

 lavaretus (Linnaeus) and Coregonus pidschian (Gmelin). The spawning . places 

 o£ Coregonus peled — in the streams — are well separated from those of other 

 whitefish populations. C. pidschian and C. lavaretus have in some cases the 

 same spawning places and the same spawning time. 



There is reason to beHeve that for any of the three species the fraction 

 surviving until hatching could increase in an imagined situation where the 

 species was suddenly left as the only spawning whitefish in the lake. It does 

 not necessarily follow that the relative numbers of the three whitefish species 

 at hatching in the situation which actually exists have any significance for 

 the relative numbers of the adults : this is still an open question. 



The hatching times of the whitefish species differ. The different sizes of 

 the eggs of the species result in different sizes at hatching, the fry of C. 

 lavaretus being smaller than the others. Strong evidence of the concurrence 

 of large size and good survival during the first year has been gathered for 

 territorial fish (Brown, 1946, 1957; Kalleberg, 1958; cf Miller, 1959). The 

 same probably also applies to schooling fish (Svardson, 1949), but the 

 relations between species must be determined in a more complicated way. 



