Morphometric Changes in Gill Secondary Lamellae of Brook Trou (Salvelinus fontinalis) After Long-Term Exposure to Acid and Aluminum

Adult brook trout (Salvelinus fontinalis) were exposed for 147 d to three different combinations of acid and Al in soft water (2 mg Ca/L). Samples of gill tissue from each of the three exposure conditions (pH 6.64 + 0 μg Al/L, pH 4.91 + 4.3 μg Al/L, and pH 4.45 + 393 μg Al/L) were examined by light microscopy using high-resolution techniques and morphometric analysis. As compared with fish in control conditions (pH 6.64 + 0 μg Al/L), exposure to pH 4.91 without Al significantly increased volume density of lamellar chloride cells. Low pH and elevated Al (pH 4.45 + 393 μg Al/L) resulted in increased diffusion distance, white blood cell infiltration of the lymphatic space of the gill tissue (an indicator of tissue damage), and a dramatic increase in lamellar dense cells. These results are consistent with the physiological data that show ionoregulatory stress and decreased respiratory efficiency as typical responses of brook trout exposed to low pH and combinations of low pH and elevated Al.