The effects of water hardness on the growth, metabolic indicators and stress resistance of largemouth bass Micropterus salmoides

Water hardness varies regionally and optimal levels for fish are often species specific. It is generally assumed that largemouth bass, Micropterus salmoides, require elevated water hardness, but this has not yet been investigated. In this two-part study, M. salmoides juveniles were subjected to 100, 200, 400 and 600 mg/l CaCO3 (by CaCl2 additions) and the survival, growth, ammonia-N excretion/oxygen consumption rates, biochemical composition and ion-regulatory enzymes were measured after one month. Afterwards, fish were subjected to a sudden salinity shock test (SSST) at 10 ppt. In experiment 2, M. salmoides juveniles were subjected to 50 or 400 mg/l CaCO3 for one month and measured for the above-mentioned parameters. In experiments 1 and 2, both feed intake and feed conversion ratio (FCR) significantly decreased at 400 mg/l CaCO3, compared to 50 or 100 mg/l CaCO3, however, hardness had no significant effect on survival or growth. There was no significant effect to gill Na+/K+-ATPase or H+-ATPase activities; however, survival to SSST was significantly less at 50 or 100 mg/l than the other treatments. At 400 mg/l CaCO3, ammonia-N excretion/oxygen consumption rates were significantly less compared at 50 mg/l CaCO3, indicating greater energy expenditure at lower water hardness. These results demonstrate that while high water hardness substantially improves stress resistance in M. salmoides juveniles, they do not require high water hardness for optimal growth.