Osmoregulatory Failure and Death of First-Year Largemouth Bass (Micropterus salmoides) Exposed to Low pH and Elevated Aluminum, at Low Temperature in Soft Water
Young-of-the-year largemouth bass (Micropterus salmoides) were exposed to pH levels from 8.0 to 4.5 in two water types, 1.5 and 13.4 mgCa/L. Exposures were conducted at 3.8 °C for 113 d, followed by 14 d of increasing temperature to 18 °C. Two treatments in the softer water, one each at pH 5.0 and 4.5, had Al added to attain 30 μg Al/L; all other treatments were at approximately 5 μg Al/L. The condition factor of fish in all treatment groups declined with exposure time at 3.8 °C. Fish in the 13.4 mg Ca/L water maintained osmotic homeostasis through pH 5.0. In the 1.5 mg Ca/L water, osmotic homeostasis was lost at pH 4.5 and at pH 5.0 when Al was added. Mortalities were most prevalent when exposed in the 1.5 mg Ca/L water with added Al. The probability of survival was directly correlated with blood osmolality; no correlation was found between survival probability and condition factor. A rise in blood osmolality occurred among fish from most exposure groups when the temperature was increased to 18 °C. When fish from these chronic treatments were challenged at pH 3.8, they had shorter survival times in the softer water and after longer preexposures.