OPTIMIZATION OF PRACTICAL DIETS FOR FLATFISH REARED IN EUROPE: A REVIEW AND PERSPECTIVES

Salmonids, gilthead sea bream (Sparus aurata L.) and European sea bass (Dicentrarchus labrax L.) play a lead role in the European fish production; however, other farmed fish species are required to differentiate and widen the market supply. Flatfish have long been of interest for aquaculture in Europe. In particular, turbot (Psetta maxima) is the most important cultured flatfish species in Europe, widely reared also in other countries such as East Asia, whereas the sole species common sole (Solea solea) and Segenegal sole (Solea senegalensis) represent an interesting alternative for the diversification of the European and Italian aquaculture, due to the high price and high market demand. To date, the production of these species in Europe and Italy are still minimal compared to other species and further improvement are needed to achieve a full sustainability of the production cycle. Above all, pursuing the ideal feed formulation and an adequate nutrient utilization are of particular relevance for the success of the aquaculture production of any given species. In this context, a critical review of our studies aimed at optimizing the diets for these flatfish species and conducted during the last ten years will be presented. Results indicated that soles species require innovative ingredients and have specific nutritional needs to improve feed intake and growth. In particular different dietary protein levels demonstrated a considerable influence on growth, feed utilization and nitrogen excretion with the highest growth and feed utilization achieved with a diet containing 57 % of crude protein (1). A feeding trial with different dietary energy levels evidenced that increasing dietary lipid level lead to a substantial decline in performance and affect gut health. Dietary lipid levels higher than 12 % depress growth and lipid utilization. High lipid diets lead to moderate to severe intestinal steatosis and ultrastructural evaluations display cellular suffering due to a lipid overload. Soybean meal (SBM) seems to be a good protein source in diets for Egyptian sole. A level of 30 % of SBM can be added in the diet without any reduction in growth rate and any effect on gut histology. Soy products may be promising protein sources for inclusion in sole diets (2). The substitution of fish meal by mussels meal improved performance and feed utilization. The mussels meal could mimic the characteristics of the sole’s natural prey, in terms of attractiveness and nutrient utilization. It is advisable to consider mussel meal, rather than fish meal as a reference diet to determine the growth potential of common sole. Turbot showed low tolerance to high plant protein inclusion, since vegetable ingredients negatively affected feed intake, growth and fish welfare. A substitution of a mixture of plant protein for up to 52 % of fish meal protein did not reduce feed intake, and at 39 % substitution, turbot maintained optimal growth rate and nutrient utilization. Worsened feed utilization in diets containing higher plant protein levels was not associated with a reduced digestibility of ingredients or alterations of gut histology (3). The administration of diet containing 35 % of fish meal ensured growth performance close to that containing 50 %, without significant effects on health and welfare of turbot juveniles. Diet containing 20 % of fish meal produced sub-optimal growth performance, metabolic stress and an immune response with consequences on health and welfare status. A 5 % fish meal diet caused a worsening of growth performance and fish welfare, probably due to an insufficient feeding and nutrients intake (4). The results of the different experiments will be provided and discussed in order to identify new perspectives for optimizing practical diets for turbot and sole in aquaculture.