220 



BULLETIN OF THE BtJREAU OF FISHERIES. 



measurements have been made. The bottom temperature at a depth of 70 feet on 

 July 10, 1912, was 47° F. (8.3° C). At a depth of 82 feet in South Fishtail Bay a 

 temperature of 6° C. has been recorded in July. There is a well-defined thermocline 

 at a depth of 40 to 45 feet. Its unusual distance from the surface is due to the heavy 

 winds which cause the surface waters to be intermingled. Above the thermocline 

 the temperature rises until in August it reaches 20° C. at the surface. 



Fig. i.^Map of Douglas Lake. Cheboygan County, Mich. The numbered circles show where collections were made. 

 I, Maple River; 2, two hundred yards east of Grapevine Point; 3, at the biological station; 4. sand bar; 5, protected 

 cove on North Fishtail Bay; 6. deep channel east of Grapevine Point; 7, northeast of Faii^ Island; S, stony shoal on 

 Grapevine Point; 9. protected bay at Bryant's lauding: 10, east shore of South Fishtail Bay; 11. west side of entrance 

 to North Fishtail Bay; 12, west side of South Fishtail Bay o3 Grapevine Point. 



Above the thermocline in August there is abundance of dissolved oxygen, about 

 5.5 cc. per liter at the surface and 4.5 cc. at a depth of 33 feet. Below the thermocline 

 the amount of dissolved oxygen is diminished. It varies in August from about 0.6 cc. 

 per liter at a depth of 50 feet to nothing at a depth of 63 feet or more (Tucker, 191 3). 

 Below a depth of 45 feet the lake does not afford, in summer, enough oxygen to make 

 it a suitable habitat for fish. 



Little is known of the distribution of the vegetation in the lake. It is briefly 

 discussed unOer fish habitats, but should be made the subject of special study. 



