Growth performance of common sole (Solea solea)fed semi-moist diets supplemented with different feeding actractants-stimulants

A 111-day experiment was performed to compare the growth performance of common sole (Solea solea) fed a dry commercial diet or semi-moist feeds added or not with feeding attractants-stimulants. Three semi-moist preparations (75% dry matter) were obtained from a basal, complete dry diet including marine protein and oil sources (crude protein, 52%, total lipid, 13%) by adding just water (B), water plus betaine, 4.5% w/w (BB) or water plus a commercial feeding enhancer additive based on yeast extract and protein hydrolysates, 1.5% w/w (BC). A dry commercial diet (crude protein: 50%; total lipid: 20%) which was shown to ensure acceptable growth rates in previous experiment with sole, was used as a control treatment. The semi-moist preparations were obtained after cold pelleting the doughs through a dye, so as the resulting feed items were of the same size as those of the dry commercial diet (2 mm in diameter).The trial used 160 fish (mean initial body weight, 58±3 g) equally and randomly divided among 8 groups stocked in rectangular vessels (100x33x25 cm) kept in an indoor re-circulating marine water system ensuring nearly optimal water quality to sole (T: 20±1°C; Salinity: 35±1 g/L).The 8 groups were allotted to the 4 diets according to a random design (each diet fed to duplicate units). The diets were offered in a single daily meal equalised so as all units received the same amount of feed dry matter (i.e. 1% biomass). Response variables recorded at the end of the trial were subjected to monofactorial ANOVA and subsequent Duncan’s test for mean comparisons, considering fish groups as the experimental units. Overall mortality was low (6%) and not affected by the dietary treatment. Relative to fish groups fed with the control dry diet, those fed the semi-moist preparation-B resulted in higher final weight (94.3 vs. 76.4 g, P<0.05), specific growth rate (SGR, 0.43 vs. 0.24 % day-1, P<0.05), improved dry matter conversion rates (DMCR, 2.4 vs. 4.4, P<0.05) and productivity (gain in biomass, 15.5 vs. 7.6 g sqm-1day-1, P<0.05). The semi-moist feeds supplemented with feeding attractants-stimulants, resulted in a further improvement of the rearing performance. Diets BB and BC were equally effective over the unsupplemented preparation, in enhancing final weight (103.0 vs. 94.3 g, P<0.05), SGR (0.51 vs. 0.43 %day-1, P<0.05) and productivity (18.9 vs. 15.5 g sqm-1day-1, P<0.05). However, better growth with the supplemented feeds led just to a marginal improvement of DMCR (2.2 vs. 2.4, P>0.05), thus suggesting that increased weigh gain did not result from improved feed utilisation efficiency but, primarily, from higher feed intake induced by the feeding enhancers added to the diet. The results obtained here stress the importance of the sensory attributes of a complete feed (physical form, presence of chemio-attactants-stimulants compounds) in improving the farming performance of this slow growing fish species.