Rainbow trout were studied at different rearing densities, fish sizes and feeding frequencies so that we could evaluate the effect of these parameters on fin condition, growth and feed utilisation. In one study, two sizes of rainbow trout (18-70 g or 48-125 g), fed to near satiation at 17.7°C, were examined at two rearing densities (11-41 kg m -3 or 21-92 kg m -3 ). This study showed that the anal fin was healthier (P < 0.05) at low densities. In the second study rainbow trout were again fed to near satiation and grown from 50 g to 125 g in 16.4°C water, and they were fed either once daily or three times daily at two densities (18-45 kg m -3 or 54-124 kg m -3 ). Rainbow trout growth and feed utilisation were slightly but significantly (P < 0.05) reduced at high densities, while dorsal fin condition, surprisingly, was better (P < 0.0001) at the high densities. Possible explanations to these findings are discussed. Condition of the left pectoral fin was improved at once daily feeding (P < 0.05) compared with three feedings per day, at which fights for feed possibly are more frequent.

The aim of the present study was to elucidate the nature of the relationship between enamel apatite and lesion fluid during demineralization. Powdered enamel in samples of 1.0 g was suspended in 3 ml of 10 30 50, or 70 mmol/l HCl under gentle agitation for up to 24 h at 20°C. After 20 min and 24 h pH and the concentrations of calcium and phosphate were determined and the degree of saturation with respect to various calcium phosphates calculated. The experiments were replicated 15 times using the same enamel samples. Twenty minutes after the start of dissolution, both pH and concentrations of calcium and phosphate had increased, and the solution became supersaturated with respect to hydroxyapatite and in some runs also with respect to brushite. During the subsequent 23 h and 40 min, pH continued to increase, despite the super-saturation with respect to apatite, whilst the concentrations of calcium and phosphate decreased due to formation of apatite and, occasionally, brushite mineral. The data indicated that release of carbonate from enamel and its conversion to H₂CO₃ caused the increase in pH and thus, played a major role in the dissolution-reprecipitation process.

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