The Toxicity of Potassium Cyanide to Trout

Lack of fundamental information on the factors determining the survival of fish in water containing poisonous substances has led to the use of a variety of experimental techniques and methods of interpreting results in studies on the toxicity to fish of constituents of waste waters discharged to streams; information on the survival times of fish in solutions where they survive for longer than one or two days is very scanty, and a programme of laboratory studies on the survival of rainbow trout in solutions of cyanide has therefore been started with the object of providing background knowledge for studies of this kind. In nearly all the toxicity tests reported in the literature, fish have been immersed in a fixed volume of poisoned water, but both Allee & Bowen (1932), using a suspension of colloidal silver, and Carpenter (1927), using solutions of lead nitrate, have shown that when fish are kept in the test solution the concentration of poison falls and survival time can be increased by increasing the number of fish in the solution. From their experiments these workers concluded that the reduction in concentration of poison was partly due to precipitation by mucus. It is probable that similar reactions reduce the concentration of many lethal agents and this, together with other changes produced in the water by the metabolism of the fish, would result in a continually changing environment during a test with a fixed volume of solution, so that the survival times could not be assumed to be representative of those of fish in a continuously polluted river. The simplest way to overcome this difficulty is to test the fish in a vessel through which a stream of poisoned water flows at such a rate that the activity of the fish cannot seriously alter its composition. Such an apparatus has been used by Erichsen Jones (1938), but it was too small for more than two minnows to be tested at one time. It is desirable to test more fish than this in one concentration if variations in resistance of fish to a poison are to be assessed. It was therefore decided to make tests in a vessel in which fifty yearling rainbow trout could be tested together in a stream of water in which concentration of poison, dissolved oxygen tension, and temperature were kept as constant as possible.