Analysis of the nutritional condition of fish larvae: studies with <i>Clarias gariepinus, Coregonus lavaretus</i> and <i>Scophthalmus maximus</i>

Among the environmental variables influencing the performance of fish larvae, the quantity and quality of food belong to the more important ones. Alterations of nutritional condition affect, apart from changing growth and survival, a number of structural and molecular parameters of the larvae. Particularly the absence of food has pronounced effects on histological features of, e.g. the liver and the intestine, on activities of digestive proteolytic enzymes, as well as on RNA/DNA ratios. The starvation-related changes are similar for the larvae of totally different fish species, e.g. Clarias gariepinus, a tropical freshwater species, the whitefish Coregonus lavaretus, from temperate freshwaters, or the turbot Scophthalmus maximus, a temperate seawater fish.Structural and molecular indicator parameters are employed by fishery biologists as sensitive diagnostic tools to estimate the percentage of starving fish larvae in the field. For the aquaculturist, on the other hand, the major problem is not the diagnosis of starvation in cultured larvae, but the detection of nutritional pathologies in relation to various dietary regimes. In addition, possible reasons for diet-induced differences of larval growth have to be evaluated. Studies with Clarias gariepinus and Coregonus lavaretus show that the methods which are valid to indicate food deprivation, are basically also suited to indicate malnutrition. However, further approaches are necessary in order to identify the factors responsible for larval malnutrition. A particular need exists for detailed knowledge of larval physiology, larval energetic and intermediary metabolism, larval nutritional requirements and adaptive plasticity of the larvae