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ABSTRACTAcid rain induces the acidification of inland waters which results in damage to aquatic ecosystems that contain fish.In northern Europe and America, where damage by acid rain has been manifested, many populations of fishes havevanished. At present, rapidly expanding industrial activities in Asia have led to a continuous increase in emissions ofacidic pollutants, and rain at acidic levels of pH 4 has often precipitated throughout Japan. In order to investigate theeffects of acid rain on fish ecosystems and forecast the damages that will be sustained in the future, it is necessary toclarify the biological responses of fish to an acidic environment. Therefore, we have investigated the effects of acidexposure on physiological changes in fishes. Levels of pH 4 constitute acidic conditions lethal to most species of fish.When fish were exposed to low pH, loss of sodium (Na+) and chloride (Cl-) ions from the body fluid occurred, resultingin a decrease in plasma osmotic pressure. Dace Tribolodon hakonensis inhabiting Lake Osorezan in Japan, which ishighly acidic (pH 3.6), had profoundly differentiated gill chloride cells which showed high V-ATPase activity that stimulatesthe proton pump of the cell membrane to excrete H+ ions. Moreover, it was found that somatolactin, a pituitary hormone,is possibly involved in the acid-base regulation. Even if pH is not low enough to be lethal, the stress of acidificationinduces various physiological and ecological problems in fish. When juvenile carp were exposed to pH 4.5, plasmacortisol levels peaked in response to acid stress, and immuno-globulin (IgM) levels subsequently decreased. This resultsuggests that acid stress depresses the immune system of fish. When mature salmonid fishes are exposed to pH 4.5-5.0,inhibition of development and increases in malformation are observed in the embryos of their offspring. Plasma levels ofsex steroids and gonadotropin exhibited abnormally high levels and there was a possibility that acid stress disrupted theendocrine control over reproduction. Additionally, the acidic condition of pH 5.8 completely inhibited the homing migratorybehavior of land-locked sockeye salmon, and extremely slight acidification (near pH 6) inhibited their spawning behavior.These results suggest that salmonid fish are highly sensitive to slight changes in acidity and they attempt to avoid anenvironment perceived to be deleterious to their offspring
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