ABSTRACT
Acid 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 have
vanished. At present, rapidly expanding industrial activities in Asia have led to a continuous increase in emissions of
acidic pollutants, and rain at acidic levels of pH 4 has often precipitated throughout Japan. In order to investigate the
effects of acid rain on fish ecosystems and forecast the damages that will be sustained in the future, it is necessary to
clarify the biological responses of fish to an acidic environment. Therefore, we have investigated the effects of acid
exposure 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, resulting
in a decrease in plasma osmotic pressure. Dace Tribolodon hakonensis inhabiting Lake Osorezan in Japan, which is
highly acidic (pH 3.6), had profoundly differentiated gill chloride cells which showed high V-ATPase activity that stimulates
the 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 acidification
induces various physiological and ecological problems in fish. When juvenile carp were exposed to pH 4.5, plasma
cortisol levels peaked in response to acid stress, and immuno-globulin (IgM) levels subsequently decreased. This result
suggests 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 of
sex steroids and gonadotropin exhibited abnormally high levels and there was a possibility that acid stress disrupted the
endocrine control over reproduction. Additionally, the acidic condition of pH 5.8 completely inhibited the homing migratory
behavior 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 an
environment perceived to be deleterious to their offspring