Effects of acidic water on young-of-the-year smallmouth bass (Micropterus dolomieui)

Smallmouth bass (Micropterus dolomieui) populations are declining in areas vulnerable to anthropogenic acidification. To determine if populations may be lost directly from acid effects (i.e., in isolation from synergists), we performed a laboratory study on the effects of acid exposure on growth, survival and histology of young smallmouth bass (3 to 36 d post swimup). We exposed them to declining, then fluctuating acid levels averaging pH 7.4 (control), 5.7 and 5.0. Substantial decreases in survival were observed, as well as damage to tissues. Survival declined with increasing acidity to 43% of that in controls at pH 5.7 and to 4% at pH 5.0. Histological analysis revealed severe changes in the most acidic treatment, with damage to blood, gill, and epithelial tissue. In contrast, growth was less useful as an indicator of response to acidity. Growth in both length and dry weight was greater at the lower pH levels than in controls. This was probably a reflection of lower survival at the lower pH levels, reduced density, and hence less competition for food. Our results suggest that an increase in environmental acidity is sufficient to cause losses in this species, even in the absence of synergists such as heavy metals.