Includes bibliographical references (pages 11-12).

Introduction. Principal excretory products in rearing ponds. Effects of excretory products on the fish. Conclusions. Literature cited. In experiments at the Bureau's Salmon-Cultural Laboratory, to determine the excretory products of significance in rearing ponds and the possible effect of these products on fingerling chinook salmon (Oncorhynchus tshawytscha), the results demonstrated that ammonia and urea are the principal products. The relative amounts of these products vary with the concentration of fish and with their metabolic activity. Ammonia assumes dominance in an unfavorable environment. Water temperature, pH, and pond type affect the toxicity of ammonia solutions. Continuous exposure to levels of ammonia normally encountered in rearing ponds produced extensive hyperplasia of the gill epithelium in controlled experiments.