FISHES OF DOUGLAS LAKE, MICHIGAN. 221 



Douglas Lake was at one time continuous with the Great Lakes and with Burts, 

 Mullet, and Crooked Lakes to the south of it. The latter lakes continue to be broadly 

 connected with Lake Huron by means of Crooked and Cheboygan Rivers. Douglas 

 Lake, on the other hand, was long since separated from the other lakes. It has no 

 direct connection with the Great Lakes, but is connected with Burts Lake by the 

 Maple River. Thus its separation from the Great Lakes antedates that of Crooked, 

 Mullet, and Burts Lakes and is more complete. 



nSH HABITATS OF THE LAKE. 



The following four fish habitats of the lake are provisionally recognized. A possi- 

 ble, fifth habitat is suggested on page 246. 



BARREN SAND-SHOAL HABITAT. 



Wherever the terrace is without stones (pebbles may occur in the sand) it may be 

 referred to as a barren sand shoal. The sand is loose and shifting in the more exposed 

 of these shoals and practically always shows ripple marks. In less exposed places the 

 sand particles are loosely united by a deposit of marl, probably of algal origin. This 

 gives a certain firmness to the sand, as though it were mixed with clay and makes it 

 resistant to wave motion. Such protected sand shoals are often free from ripple marks. 

 The shore bordering all sand shoals is low, without an ice rampart of stones, and the 

 first land terrace or bluff is at some distance from it. The shoal may be narrow (only 

 a few yards or more) or wide (a hundred yards or more). The slope on its seaward edge 

 is steep, as steep as loose sand can lie, and the water over the seaward edge is commonly 

 about 4 feet deep. Near shore there may be a sparse growth of bulrushes but there is 

 no other vegetation. 



BARREN STONY-SHOAL HABITAT. 



Wherever the shore is bordered by a bluff or terrace which is being eroded the 

 shoreward margin of the shoal contains stones or small bowlders. Along such a shore 

 there is commonly formed by the action of the ice a rampart of stones, which borders 

 the shore like a low stone wall. The stony shoal is apt to be wide and the slope beyond 

 it less steep than that of the sand shoal. The water over its outer edge is often 6 or 7 

 feet deep. Its bottom may be of shifting sand or of sand agglutinated with marl. Its 

 shoreward border may support a growth of bulrushes or may be without them. Re- 

 garded as a fish habitat, its salient feature is the stones. Where the shore of the lake 

 shows a series of headlands with intervening valleys, it is being eroded along the head- 

 lands and built up between them. The headlands are bordered by stony shoals and the 

 intervening low shore by sand shoals. The two pass into one another without sharp 

 demarcation. 



THE VEGETATION HABITAT 



If we neglect the scant growth of bulrushes which may occur on the shoals, the 



vegetation of the lake is largely limited to the slope. In places the slope is continued 



into considerable areas of nearly level bottom covered by water less than 25 feet deep 



and overgrown with vegetation. Such an area, known as the "middle ground," extends 



19371°— vol 33—15 15 



