Use of Synthetic Diets and Biochemical Criteria in the Assessment of Nutrient Requirements of Fish

Dietary allowances of nutrients have been formulated mainly from growth–response curves. The use of other criteria, especially those which exploit the biochemical role of nutrients, is discussed by reference to three different nutrients, namely thiamin, magnesium, and essential fatty acids.It is shown that erythrocyte transketolase activity provides a nutritional index of thiamin status in turbot (Scophthalmus maximus). The enzyme is saturated with coenzyme (thiamin pyrophosphate) at a dietary thiamin level of 2.6 mg/kg.Assessment of mineral requirements is shown to present special problems because complex interrelationships exist between some dietary minerals. It is shown that dietary magnesium deficiency (4 mg magnesium/100 g diet) leads to renal calcification in rainbow trout (Salmo gairdneri) at dietary calcium levels of 2.7 g/100 g (Ca: P ratio 1: 1). Elevation of dietary magnesium to 100 mg/100 g under these conditions gave freedom from pathology and enhanced growth.Ratios of certain fatty acids in the tissue phospholipids of rainbow trout are known to provide a useful index of essential fatty acid status. These ratios cannot be applied to turbot as this species does not chain elongate and desaturate 18-carbon acids at appreciable rates. Long chain polyunsaturated fatty acids of the ω3 series must be supplied preformed in the diet of turbot.