Effects of chronic exposure to acidified water on chemoreception of feeding stimuli in fathead minnows (Pimephales promelas): Mechanisms and ecological implications

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Abstract
A computer‐automated behavioral assay was used to assess the effects of reduced ambient pH on the response of fathead minnows to chemical feeding stimuli. The testing system quantified eight behavioral parameters based on the activity level of individual fish before and after exposure to a single‐pulse dose of a feeding stimulus—a tissue culture medium (1 ml; Eagle minimum essential medium with Earl's salts and L‐glutamine) — injected into the water circulation. Twentyfive adult minnows were tested after exposure to one of five pH levels: 8.0 (control), 7.0, 6.5, 6.0 and 5.5. Complete elimination of the feeding response occurred at pH = 6 and lower; responses at higher pH levels were not statistically different from those in the controls. Differences in water hardness ( 160 mg/L CaCO3) and duration of exposure (72 h versus 30 d) had no influence on the pattern or degree of the pH effects. Scanning electron microscopy revealed no pathological or gross morphological correlates of the behavioral changes. To test for possible recovery, minnows that did not respond at pH = 6 were placed in water of control pH (8.0) for 24 h and then tested again; a statistically significant response to the feeding stimulus was restored. These results indicate that acute and chronic sublethal levels of acidification caused a reversible impairment of chemoreception and that the impairment can occur at a relatively high pH. The toxic effect probably involves mechanical and chemical inhibition of receptor cells in the olfactory and gustatory epithelia rather than actual destruction of chemosensory tissue. In nature, reduction of pH to approximately 6.0 could impair feeding behavior and reduce food intake, fecundity and long‐term survival of fathead minnow populations. Our results correspond well with environmental studies that show that fathead minnows are eliminated from natural waters when pH levels reach 5.8 to 6.0.