THE EFFECT OF DIETARY-LIPID LEVEL ON VARIATION OF FLESH PIGMENTATION IN RAINBOW-TROUT (ONCORHYNCHUS-MYKISS)

Variation in flesh pigmentation was assessed in rainbow trout fed diff erent dietary lipid levels. Triplicate groups of 80 rainbow trout with a mean initial wet weight of 54 +/- 1 g were fed on pigmented fish me al-based diets containing 8.2% (A), 17.7% (B) or 26.7% (C) crude fat f or 25 weeks. All diets contained synthetic astaxanthin concentrations ranging from 43.7 + 0.9 mu g/g to 49.7 +/- 0.8 mu g/g. The chromaticit y of the dorsal, belly and tail regions of the fillets was assessed ev ery 40 days by Minolta Chroma Meter and the astaxanthin concentrations determined by high performance liquid chromatography. At each samplin g, the tail regions within all treatments had significantly higher con centrations of astaxanthin and a values, than the dorsal and belly fil let regions, although among treatments, there were no significant diff erences in carotenoid content. At each sampling, the coefficient of va riation of fillet pigmentation between the different fillet areas was not significantly different within and among the dietary treatments. T he flesh astaxanthin concentrations ranged from 0.23 mu g/g in the fis h at the start of the experiment to 13.1 mu g/g in the tail region of fish fed diet A with final flesh astaxanthin retention coefficients ra nging from 11.6% in fish fed diet A to 14.9% in fish fed diet C. Fille t lipid content ranged from 2.3% in the tail musculature of the initia l trial fish, to 10.3% in the belly muscle of fish fed diet C. Signifi cant negative correlations were found between fillet lipid content and astaxanthin concentration in fish fed diets A and B. Colorimetry meas urements demonstrated large variations in fillet colour irrespective o f dietary treatment. Tristimulus chromometer a values ranged from 4.8 3 to 25.96, b values from 7.23 to 31.33, L* values from 34.72 to 63.3 9, while chroma and hue values ranged from 8.47 to 39.89 and 34.12 deg rees to 72.33 degrees, respectively. In all treatments, L values were significantly correlated with flesh lipid content, however, only the a, b and chroma values of fish fed diet A were significantly correlat ed with flesh lipid content. Although the flesh deposition efficiency of astaxanthin increased with increasing dietary lipid level, this did not reduce the amount of variation in fillet pigmentation observed wi thin and among treatments over the experimental period. (C) 1998 Elsev ier Science B.V.