118 FRESHWATER ANIMALS 



the coexisting species maintain their genetic integrity by con- 

 sidering the spawning behavior of the species of the Hornavan 

 lake system (Svardson, 1953). The pertinent information is 

 summarized in Table I. These differences in time of spawning 

 and types of spawning ground apparently suffice to permit the 

 populations to reproduce with relatively little intermixing. 



However, the reproductive isolation between the populations 

 is by no means complete. Ripe "asp" have been taken on the 

 spawning grounds of the great whitefish in December, even 

 though their normal breeding time is a month and a half earlier. 



Table I. Spawning Behavior of Coregonus Species of Hornavan Lake System 



Species Spawning 



(local name) period Spawning place 



"Asp" November In running water of rivers between lakes 



and in mouths of influent rivers 



Large whitefish 1 )ecember Shallow water above stony bottom; widely 



distributed in lakes 



Blue whitefish" December Deeper water over stony bottom; widely 



(two species) January distributed except in northern part of 



Lake Hornavan 



Plankton whitefish February Restricted in feeding and spawning to deep 



waters of Lake Hornavan; spawns on 

 bottom at relatively great depth (30 

 100 m.) 



" The spawning habits of the two species known by this name have not been 

 different iated. 



The local fishermen have long recognized a special kind of 

 whitefish that they call "albask," which Svardson thinks are 

 most likely hybrids between the "asp" and the great whitefish. 

 Hybridization between late spawners among the bine whitefish 

 and early plankton whitefish in areas where their spawning 

 grounds meet is to be expected, and Svardson finds some evi- 

 dence that would seem to substantiate this possibility. 



Intermediates and Hybridization. Individuals that appear in- 

 termediate between various of the species (like the "albask just 

 mentioned I occur with a low frequency. However, only a portion 

 of these can be established as hybrids; the only hybrids readily 



