RESPONSES OF JUVENILE LARGEMOUTH BASS TO DIFFERENT PH AND ALUMINUM LEVELS AT OVERWINTERING TEMPERATURES - EFFECTS ON GILL MORPHOLOGY, ELECTROLYTE BALANCE, SCALE CALCIUM, LIVER-GLYCOGEN, AND DEPOT FAT

Young-of-the-year largemouth bass were exposed to simulated overwintering conditions, 4-degrees-C for 113 days, in soft ([CaCO3] 48 mg/L, [Ca] 13.4 mg/L) and very soft ([CaCO3] 4.6 mg/L, [Ca] 1.5 mg/L) water. In some treatments pH was reduced to 5.0 and 4.5, with no added Al; in others, a monomeric Al level of 30 mug/L was maintained. In the neutral pH treatments the bass had greatly thickened gill respiratory epithelium and large numbers of chloride cells (compared with bass kept at 20-degrees-C in a prior experiment), and blood osmolalities declined and stabilized at about 260 mosmol/kg. In the acid treatments die bass had thickened respiratory lamellae and chloride cell numbers similar to those found in the neutral pH treatments. In very soft water, mean blood osmolalities declined to 242 and 219 mosmol/kg at pH 5.0 and 4.5, respectively, and did not stabilize. In the acid + Al treatments, respiratory lamellae were frequently obliterated by hyperplasia of the interlamellar epithelium, suggesting markedly compromised respiratory ability. In addition, chloride cell numbers were usually depressed, and those cells that were present were often vacuolated or covered by layers of pavement epithelium; blood osmolalities declined to the lethal and near-lethal level of about 200 mosmol/kg, and heavy mortalities occurred. After 84 days in cold soft water (pH 7.8), scale Ca content declined; this effect was more severe in very soft water (pH 7.0). Acidification did not cause further decline in scale Ca content in the very soft water. At pH 4.5 + Al, scale Ca levels were higher than at pH 4.5 without added Al and higher than at pH 7.0, suggesting that Al may interfere with normal utilization of scale Ca reserves. Substantial lipid reserves persisted throughout the experiment. Mortalities were correlated with pH-associated gill damage and electrolyte losses rather than with duration of exposure to low temperature per se. Modest levels of Al exacerbated gill damage and electrolyte losses. We suggest that overwintering, with only moderate elevation of acid and Al levels, may represent an important critical period for survival to young-of-the-year fish in northern climates, principally by adversely affecting electrolyte balance. The gills of juvenile largemouth bass undergo marked structural changes with prolonged exposure to cold water, possibly to retard electrolyte losses.