GREAT LAKES COREGONIDS 



333 



PHYSIOLOGICAL BARRIERS BETWEEN THE SPECIES 



It does not appear to be profitable, in the present state of our knowledge of the 

 coregonids of other waters, to speculate further on the origin and precise relationships 

 of the forms in the Great Lakes; but two points are clear — first, that from whatever 

 source the species may have originated, certain factors are operating now to keep 

 them distinct; and second, as I have already indicated, tendencies that may result in 

 the formation of new species are manifesting themselves in at least some of the 

 species already formed. 



The factors that keep the species apart apparently are physiological differences 

 between the individuals of different species, differences that result (a) in the selection 

 of different habitats and (b) in breeding taking place at different seasons, at different 

 depths, and on different bottom. 



Segregation Through Different Habitat Selection 



The physical conditions in the lakes vary, and the adaptability of the species also 

 is different, so that it is not possible to generalize too strictly about the habitat 

 selection of any species in the basin. In some lakes species that regularly inhabit 

 shallow water elsewhere may be driven, by competition on the shoals or by absence 

 of shoals, to find a living in deeper water; and, being adaptable, they may thrive 

 there (Lake Ontario). In other cases species that regularly inhabit deep water have 

 been known to occur abundantly in shallow water only (Lake Nipigon) ; but, in gen- 

 eral, in any lake there are certain groups of species that are found in shallower 

 water than others. In general, artedi, clupeaformis, and quadrilaterale are shoal- 

 loving forms; alpense, zeniihicus, reighardi, and hoyi also like comparatively shallow 

 water; but johannx, nigripinnis, and Tciyi are found chiefly in the deeper waters. 



The bathymetric distribution of the species or groups of species is zonal. Each 

 occupies a rather broad zone defined by the depth of water at its margins. At the 

 center of the zone each has its greatest density of population, and this density dimin- 

 ishes toward the margin of the zone. Only a few stragglers are found beyond their 

 zones, except during the breeding migration. The zones overlap at their margins, 

 so that the different forms are intermingled there in relatively small numbers. 



There are no data to indicate why these zones have been selected by the various 

 species or groups of species. Nothing is known about their reactions to the various 

 physical and chemical factors of their environment. Possibly the selection is influ- 

 enced by the character of the bottom. Throughout the area inhabited by the shoal 

 group, the hydrographic map shows rock, gravel, and sand, and in the deeper parts of 

 the lakes clay and mud. While each species may range over all these types of bottom 

 within its zone, of course it is not only possible but probable that there are differences 

 in the character of the areas designated on the chart as mud, clay, etc., and that these 

 differences influence, indirectly, the distribution of the fish. Certainly all the forms 

 except artedi (which is a plankton feeder and therefore normally takes its food above 

 the bottom), so far as known, are confined to a bottom stratum of water of a thickness 

 of not more than 5 feet. In this stratum they find their food, which consists (in all 

 the forms) chiefly of various species of Crustacea and Mollusca. The character of the 

 food available probably is determined directly by the character of the bottom, and 

 therefore a knowledge of the food regularly taken by each species would be helpful in 

 defining this factor of the habitat. 



