In natural waters having high dissolved oxygen, low carbon dioxide, and extreme hydrogen-ion ranges, fish of several species transferred from water of pH 8.4 to 8.6 to water of pH 4.4, and the reciprocal, have survived the transfer up to forty days, in most instances living as long as control fish kept under similar conditions but in their native waters. This indicates that it is not necessary to assume that fishes found in these extreme hydrogen-ion concentrations have gradually developed a resistance or are physiologically different from fishes of the same species from ordinary waters. Unionids, although not found in the more acid waters, have survived transfer to pH 4.4 to 4.6 up to forty-six days, the only observable effect being a slight corrosion of the outside of the shell, especially in the umbonal region. In a gradient tank supplied at one end by water from an acid lake, pH 6.4, and at the other end by water from a basic lake, pH 8.6, fish from the acid lake selected the acid end of the tank, fish from the basic lake selected the basic end of the tank, and fish from water intermediate in hydrogen-ion concentration selected the middle ofmore » the gradient. Since fishes can survive abrupt transference to a pH greater than could be produced by carbon dioxide, but have been killed by high carbon dioxide, even in the presence of high oxygen content, it is apparent that carbon dioxide has detrimental effects aside from its ionization in water as carbonic acid, and that, in order to determine the effects of hydrogen-ion concentration as such, there will have to be more observations and experiments upon pH in which the factor of high carbon dioxide is eliminated.« less

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