During the 1 920 ' s the use of bioassays became more widespread. The 

 work of David Belding is noteworthy, as he had a good understanding of the 

 factors influencing the results of bioassays and the toxicity of wastes and 

 materials to aquatic life. In the 1924 paper by Belding, Merrill, and 

 Kilson, "Fisheries Investigations in Massachusetts," differences in the 

 sensitivity of different species to the same toxicant are pointed out. 

 They found that brook trout were seven times more sensitive than carp and 

 28 times more sensitive than goldfish to H 2 S. The authors stated, "There 

 is a marked difference in closely allied species such as the Salmonids." 

 They indicated that a species most sensitive to one material may be the 

 most resistant species in the group to another toxicant. They also pointed 

 out that fish vary seasonally in their resistance to toxicants; that the 

 quality of the receiving water affects toxicity; and that size or weight of 

 fish per volume of test solution, flow of water, 2 concentration, and 

 temperature are very important factors that influence results. Belding 

 developed these points further in the paper he presented to the American 

 Fisheries Society in 1927 entitled "Toxicity Experiments With Fish in 

 Reference to Trade Waste Pollution." In this report he discussed factors 

 that may be responsible for reported variations in toxicity of materials 

 and wastes such as: Individual variation in resistance among members of 

 the same species; differences in the sensitivity of a species to different 

 toxicants; differences in the sensitivity of different species to the same 

 toxicant; effects of age and size; differences in the dilution water, its 

 2 concentration, temperature, or dissolved materials; and differences in 

 type of test vessel used. Although he used only 24-hr tests, he recognized 

 that longer exposures at lower concentrations would produce kills. In his 

 bioassays he tested the toxicity of 20 materials to brook and rainbow 

 trout, Chinook salmon, carp, goldfish, and suckers. 



Reports are available from several other investigators who carried out 

 bioassays in the 1920's. In 1928 Nightingale and Loosanoff used early life 

 stages of the chinook salmon to test the toxicity of waste sulphite 

 liquor. Cole, Dilling, and Healey also conducted bioassays during this 

 period. In 1924 Thomas published a paper on the absorption of metal salts 

 by fishes. Wiebe conducted toxicity and pollution studies for a number of 

 years and reported on exposure of young fish to varying concentrations of 

 arsenic in 1930 and to sudden changes in pH in 1931; he also reported on 

 effects of dissolved phosphorus and organic nitrogen in the waters of the 

 Mississippi River in 1931. 



During the 1930's bioassays were increasingly used for the evaluation 

 of problems by state and federal investigators. Studies were made of the 

 toxicity of cyanides, phenols, gas house wastes, pulp and paper mill 

 wastes, oil and petroleum products, and metals. Extensive studies were 

 also made on 2 , C0 2 , temperature, and pH requirements. Many bioassay in- 

 vestigations were carried out by the states and the U.S. Bureau of 

 Fisheries, which later in the decade became the U.S. Fish and Wildlife Ser- 

 vice. Among the latter, the research of Ellis was outstanding. In 1931 

 Surber and Meehan reported on lethal concentrations of arsenic for certain 

 aquatic organisms. Galtsoff made valuable contributions to knowledge of 

 the effects of oil on marine organisms, especially its effects on 

 shellfish. In the late 1930's and early 1940's Tennessee Valley Authority 



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