J. L. BROOKS 115 



the species in one and the same lake, but when the same species is 

 compared from different localities the variation is very considerable. 



The intra-lacustrine variation mentioned in the last sentence of 

 the quotation refers primarily to differences between fish of dif- 

 ferent year classes. 



Species Associations. Many lakes are known to have several 

 populations of Coregonus in them, and Svardson records two 

 lakes each of which has five species of the lavaretus group. The 

 populations that we now observe coexisting in some lakes are 

 known to have done so for long periods of time ( of the order of 

 a century or two) because fisheries records of this age mention 

 the local names for the different kinds of whitefish. It seems rea- 

 sonable to assume that many, if not most, of the Coregonus spe- 

 cies associations are of equal, and probably greater, stability. 

 However, the stability of such associations can be destroyed, as 

 has been indicated (Svardson, 1949) by an intriguing case, the 

 details of which are but sketchily known. In this instance a Lap- 

 land lake was known to have had two species of Coregonus which 

 maintained their integrity by having temporally separated spawn- 

 ing periods. When a third stock was introduced (apparently in 

 an attempt to improve the fishing), the result was the creation 

 of the large, panmictic, highly variable population that exists 

 there today. It is supposed that the introduced stock had a 

 spawning season that overlapped the hitherto separated seasons 

 of the other two. 



In his 1953 paper Svardson presents data concerning the 

 species of Coregonus found associated in three lakes of as many 

 different river systems in central Sweden. These rivers all flow 

 southeastward into the Baltic Sea from the mountains that run 

 down the middle of the Scandinavian peninsula. From south to 

 north the lakes are Lake Idsjon, Lake Storsjon, and the Hornavan- 

 Uddjaur series of lakes. Figure 12 indicates the species of Core- 

 gonus in each lake and some of the characteristics of each popula- 

 tion. Svardson considers the populations indicated in these 

 graphs to be species, but he did not assign them scientific names. 

 Their common names are therefore used. As indicated in the 

 previous section, the phenotypic characteristics of different year 



