Two species of stonefly naiads were exposed to the chlorinated hydrocarbons DDT, aldrin, dieldrin, and endrin and to the organic phosphates parathion, malathion, guthion, Dylox, Di‐Syston, and Bayer‐29493. After the introduction of the pesticide, tests were run for 96 hours with observations made after each 24‐hour period. There was a definite increase in mortality for both Pteronarcys californica and Acroneuria pacifica with increasing exposure time. Higher concentrations of insecticides produced higher death rates more rapidly than lower concentrations. There were definite symptoms of poisoning noted during the 4‐day bioassays. The symptoms did not vary for the two species of stonefly naiads. Animals displaying tremors or convulsions usually died within 24 hours. Both species could recover from the effects of the poisons if they were removed from the solutions of toxicant before the tremors began. Removal from the test solution after the onset of tremors did not prevent death. With the same insecticide, the resistance of P. californica was greater than that of A. pacifica. Larger specimens of P. californica were more resistant than smaller specimens.
Two species of stonefly naiads, Acroneuria pacifica Banks and Pteronarcys californica Newport, were exposed to various organic insecticides for a period of thirty days in concentrations at or less than the 4‐day TLm concentrations for each species. In general, the concentrations at or near the 4‐day TLm concentrations were lethal to 50 per cent of the specimens within 4 to 10 days of continuous exposure. Less concentrated solutions produced toxic symptoms after 10 to 14 days. With some insecticides these symptoms appeared only after 21 days of continuous exposure. Normal molting of the specimens was inhibited in the aquaria at or near the 4‐day TLm concentrations. In lesser concentrations molting was not reduced until after 21 to 25 days of exposure, at which time marked reductions in molting rates were observed.
Since passage of the Clean Water Act in 1972, there has been a long, unresolved struggle to define a key phrase in Section 316(b) of the act: "adverse environmental impact" (AEI). Section 316(b) requires that the best technology available be used in cooling-water intake structures to minimize AEI due to entrainment and impingement of aquatic organisms. Various attempts were made to evaluate and define AEI, including focused national conferences on impact assessment. Unresolved arguments regarding AEI were reinvigorated following the 1995 Consent Decree requiring EPA to propose new rules to implement Section 316(b). This article reviews and compares eight proposed definitions of AEI. Six of the definitions define AEI as impact expressed at the population or higher level of biological organization. The two remaining definitions are unrelated to populations: a 1% cropping of the near-field organisms and "one fish equals AEI". The latter definition is based on the desire of some stakeholders to define AEI as the loss of any public trust resources. Equating loss of public trust resources with AEI hampers consensus on a definition because a societal-based policy concept (public trust resources) is commingled with science-based definitions based on population effects. We recommend that a population-based definition of AEI be incorporated into Section 316(b) guidance and observe that this will not preclude a state from exercising its law and policy to protect public trust resources.
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