Macronutrient composition of formulated diets for Atlantic halibut (Hippoglossus hippoglossus, L.) juveniles, II: protein/lipid levels at low carbohydrate

The aim of the present study was to show that Atlantic halibut on the weaning stage, fed low carbohydrate levels, tolerates a wide variation in lipid as long as protein requirement is fulfilled. The fish (0.4 g) were fed diets with lipid levels increasing from 50 to 300 g kg-1 concomitant with protein decreasing from 860 to 620 g kg−1 with 50 g kg−1 intervals. No carbohydrate was added and the diets contained less than 12 g kg−1 digestible starch. The experiment lasted for 95 days. An increase in growth rate with increasing dietary lipid was registered for the whole population, but for the largest half of the population, growth was not affected by the dietary treatments. No specific pathology was observed in liver or intestine on histological examination, but a consistent increase in hepatocellular vacuolization was observed with increasing dietary lipid levels. Increasing dietary lipid led to decrease in survival and the small fish were affected most severely by mortality. We suspect that high levels of soy lecithin supplemented as 25% of the lipid in the present experiment may have caused the dietary effect on mortality. Based on this assumption and combined with previous studies, our results indicate that weaning diets for Atlantic halibut should contain no more than 50 g kg−1 carbohydrate, a minimum of 580 g kg−1 protein and 50-300 g kg−1 lipid.