Abstract
The relative sensitivity of brook trout fry, salmon fry, and 1- and 2-year-old parrs to known quantities of DDT was studied in hatchery-trough experiments with larger numbers of specimens than were used by previous workers. In troughs, in which part-time water circulation was achieved and DDT used at the rate of 0.5 lb./acre (0.32 p.p.m.), high mortality rates (93–100%) were recorded for salmon fry, trout fry, and 1-year-old parrs. Moreover, it was shown that group susceptibility decreases in the following manner: salmon fry > 1-year-old parrs > trout fry. In another series of experiments, under identical conditions, it was shown that sensitivity in salmon decreases with age. Using salmon fry, in troughs deprived of water circulation, a study was made of the effects of various concentrations of DDT: 0.33, 0.18, 0.072, and 0.036 p.p.m. corresponding to 0.5, 0.25, 0.1, and 0.05 lb. of DDT/acre, respectively. The average lethal dose (L.D. 50) was found to be 0.072 p.p.m. A comparison between two series of experiments shows how mixing of the DDT with water makes the poison more deadly than when used as a DDT–oil solution on the surface of water.
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