DISSOLVED CONTENT OF WATER. 895 



POISONOUS SUBSTANCES. 



The substances the variations of which have been referred to are substances 

 not harmful in themselves ; that is, the action is not a poisonous one. Substances 

 not commonly held in natural waters are usually unfavorable in their action 

 upon fishes, and in some cases there is a poisonous action tremendously greater 

 than with the same substance in higher animals. For instance, copper sulphate 

 is not extremely poisonous to mammals, but i part to 6}4 millions of water 

 has been observed to kill domesticated brook trout within 24 hours. The fatal 

 amount varies greatly even with the same species in different waters. At the 

 state hatchery at Bayfield, Wis., brook trout were not killed until the concen- 

 tration reached i to 400,000. Some waters precipitate the copper faster than 

 others, but there was probably also a greater resistance to copper sulphate 

 among the trout themselves. General statements of fatal concentrations of 

 such salts must apply only to the water in which the experiments were made 

 and to the fish adapted to that water. 



Silver nitrate is even more highly toxic than the salts of copper. In 

 experiments with Chinook salmon fingerlings three months old, one part of 

 silver nitrate to 20 million parts of water was fatal in a few hours and i to 22K 

 million was fatal within 48 hours. With i to 25 million the solution is so dilute 

 that about half the number of fish used were killed during 48 hours, the rest 

 surviving; i to 30 million and weaker solutions had no recognizable effect. 



Many of the metals are poisonous to fishes by lying in the same water with 

 the fishes. Copper is the most active of the common metals in this respect. 

 Twenty square inches in about 6 quarts of Potomac water killed 8 of 10 salmon 

 fry within 24 hours. Ten square inches in the same amount of Potomac water 

 killed 4 of 6 free swimming salmon fry within 2 days and 18 hours and all of 

 them within 3 days and 2 hours. The temperature was never higher than 68° 

 F., and the controls were good. 



Zinc, lead, and aluminum are toxic in the order named. Even tin seems 

 to have a very slight poisonous action. In vessels of any of these metals harm 

 to fishes is nearly always prevented by even a slight flow of water. It is when 

 the water is not changed that the injurious action is seen. Iron seems to have 

 no effect. Galvanized iron and lead, and asphaltum and enamel paints all have 

 more or less toxic action, but containers made of or painted with these sub- 

 stances become less harmful with use. Usually considerable time is required 

 before the toxic substance becomes in sufficiently concentrated solution in 

 standing water to have an effect, and this is the reason such containers are 

 often used successfully for transporting fishes. They have, however, been 

 repeatedly identified with certainty as the cause of loss of fish. 



