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 different dietary lipid levels. Triplicate groups of 80 rainbow trout with a mean initial wet weight of 54 +/- 1 g were fed on pigmented fish meal-based diets containing 8.2% (A), 17.7% (B) or 26.7% (C) crude fat for 25 weeks. All diets contained synthetic astaxanthin concentrations ranging from 43.7 +/- 0.9 microgram/g to 49.7 +/- 0.8 microgram/g. The chromaticity of the dorsal, belly and tail regions of the fillets was assessed every 40 days by Minolta Chroma Meter and the astaxanthin concentrations determined by high performance liquid chromatography. At each sampling, the tail regions within all treatments had significantly higher concentrations of astaxanthin and a* values, than the dorsal and belly fillet regions, although among treatments, there were no significant differences in carotenoid content. At each sampling, the coefficient of variation of fillet pigmentation between the different fillet areas was not significantly different within and among the dietary treatments. The flesh astaxanthin concentrations ranged from 0.23 microgram/g in the fish at the start of the experiment to 13.1 microgram/g in the tail region of fish fed diet A with final flesh astaxanthin retention coefficients ranging from 11.6% in fish fed diet A to 14.9% in fish fed diet C. Fillet lipid content ranged from 2.3% in the tail musculature of the initial trial fish, to 10.3% in the belly muscle of fish fed diet C. Significant negative correlations were found between fillet lipid content and astaxanthin concentration in fish fed diets A and B. Colorimetry measurements demonstrated large variations in fillet colour irrespective of dietary treatment. Tristimulus chromometer a* values ranged from 4.83 to 25.96, b* values from 7.23 to 31.33, L* values from 34.72 to 63.39, while chroma and hue values ranged from 8.47 to 39.89 and 34.12 degree to 72.33 degree, 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 correlated 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 within and among treatments over the experimental period.