156 Roosevelt Wild Life Bulletin 



furnish cool pools and perhaps an entire lower stratum of cool 

 water ; and with the increased volume of water would come an 

 increased food supply. 



Mr. H. D. Cornwall, in a letter of September 7 last, informs 

 me of a conversation he had had with a woodsman who told him 

 of " having seen trout and bullheads dead and others in a very 

 much weakened condition swimming on the surface of water in 

 ponds caused by beaver dams on small streams where in low water 

 the condition is such that there is not sufficient new water coming 

 into the pond to freshen it." A similar experience was related 

 to me by Ranger David Conkey in connection with beaver ponds 

 which had gone down as a result of disrepair of the dams. While 

 I personally saw a number of ponds that had been lowered to such 

 an extent that a large part of the bottom was exposed, I did not 

 happen to see any evidence of dead or dying fish, although it is 

 possible that sufficient search might have revealed them. A 

 decided stench was noticed in one or two such places but so far as 

 I could determine it seemed to arise from decaying vegetable 

 matter. 



There would seem to me to be little doubt that conditions in 

 some beaver ponds may readily become fatal to certain fishes, the 

 trout probably being more sensitive than other species in the Adi- 

 rondack streams. In periods of drought during the summer months, 

 with partial or complete drying up of the springs, it is possible that 

 in the beaver-dammed streams there may be an excessive accumu- 

 lation especially of carbon dioxide and nitrogen, these gases arising 

 in the silt and humus on the bottom of the ponds ; for according to 

 Shelford ('13, p. 60), "Nitrogen and carbon dioxide are produced 

 especially near the bottom and if the water did not circulate they 

 would be too abundant in some places and deficient in others for 

 animals to live." Again (pp. 59-60), "Several workers have 

 shown that carbon dioxide is very toxic to fishes. . . . Fishes for 

 example turn away when they encounter as small an increase as 

 5 c.c. per liter of carbon dioxide. Since a large amount of dissolved 

 carbon dioxide is commonly accompanied by a low oxygen content 

 as well as other important factors, the carbon dioxide content 

 of water (strongly alkaline waters excepted) is probably the best 

 single index of the suitability of the water for fishes." Further 

 (P- I 33)- "Analyses of the bottom water from ponds with humus- 

 covered, bottoms showed that it contained no oxygen. The open 



