The influence of mussel meal dietary inclusion on performance, fatty acid composition, gut and liver histology of common sole (Solea solea) juveniles

Common sole, Solea solea, farming in Europe, although potentially very promising, is still a little reality owing to the poor performances of fish when fed fish meal (FM) based diets. Mussel meal (MM), characterized by high palatability and good nutritional value, may represent an innovative ingredient in the formulation of a commercial feed for this species. The present study was carried out in order to determine the effect of graded level of MM dietary inclusion on growth performance, fatty acid composition, gut and liver histology of common sole juveniles. Four isoproteic (53%) and isolipidic (11%) pelletized diets were formulated to contain graded levels of MM (0%, 25%, 50% and 75% named respectively MM0, MM25, MM50 and MM75) to replace FM. Seventy sole juveniles (initial body weight 13 g) per tank were randomly distributed in twelve flat bottom 280 L square tanks (temperature 20 ± 1 °C and dissolved oxygen above 7 ppm). Animals were hand-fed twice a day to apparent satiation, over 91 days. Performance, carcass indices, gut and liver histology were investigated. One-way ANOVA and Tukey’s post hoc test was used to analyze data (P≤0.05). Diets containing MM gave significantly higher Specific Growth Rate (SGR) and lower Feed Conversion Rate (FCR) when compared to the control diet (MM0). The administration of MM50 and MM75 also resulted in a significantly higher voluntary feed intake (VFI). Carcass composition and carcass fatty acid composition were not influenced by dietary treatments. A significant decrease in VSI was observed in fish fed MM50 and MM75 in comparison with the control group. HSI of fish fed MM0 and MM25 was higher than that of fish fed MM75, even though in all the experimental groups the histological examination of liver parenchyma showed uniformly a moderate to intense accumulation of intracytoplasmic clear material (lipidic content) that was sometimes arranged in optically empty droplets. Histomorphologic evaluation of intestine did not show degenerative or inflammatory changes. According to those results, MM is an effective ingredient for enhancing growth and feed utilization in sole irrespectively to the inclusion levels used in this trial.