Use of Chlorella vulgaris as a carotenoid source for rainbow trout: effectof dietary lipid content on pigmentation, digestibility and retention in the muscle tissue

Apparent digestibility, deposition and retention of carotenoids in the musc le of rainbow trout, Oncorhynchus mykiss, were investigated comparing the f eeding of pigments from Chlorella vulgaris against commercially available p igments at two different total lipid contents (15% and 20% lipid). Algal bi omass (ALG) was included in rainbow trout diets and muscle pigmentation was compared to that obtained in trout fed diets containing a 5:3 mixture of c anthaxanthin and astaxanthin (MIX) (reflecting the relative concentrations of these carotenoids in the dry alga) or those fed a diet containing astaxa nthin only (AST). Apparent digestibilities of pigments and nutrients were d etermined by the indirect method, using Cr2O3 as indicator, and the colour intensity and pigment concentration were assessed in the muscle, using the Roche colour card for salmonids and W-vis spectrophotometry. After 6 weeks feeding, colour intensity was similar for the various pigment sources, achi eving levels 12 to 13, yet significantly higher in fish fed the diet with t he higher lipid content (approximate to 20%) p<0.05). Pigment concentration in the muscle was also higher in the fish fed the high-fat diet. Muscle pi gment concentrations were similar for fish fed diets ALG and MIX, and over 1.5 times higher than for diet AST (p < 0.05) after 6 weeks. Apparent diges tibility of dry matter, crude protein, lipid, total energy and specific car otenoid concentrations were also measured. Increased dietary fat content wa s shown to increase the deposition and the retention of carotenoids in musc le, and the difference increased with time (deposition increase of 10-20% a t week 3 and 30-40% at week 6 and retention increase of 10-15% at week 3 an d 30% at week 6). Pigment digestibility also apparently increases (10-20%) under those conditions despite the fact that no significant effects in term s of apparent digestibility increase were found for dry matter, protein, li pids or energy.