Effects of cage netting colour and density on the skin pigmentation and stress response of Australian snapper Pagrus auratus (Bloch & Schneider, 1801)

The unnaturally dark pigmentation of cultured Australian snapper Pagrus auratus can be improved through dietary astaxanthin supplementation and by holding fish in tanks with a white background. The practical application of these laboratory-based findings was examined with two experiments to establish if the advantages of transferring fish to light coloured tanks before harvest could be achieved on-farm using white cages and to determine the effects of fish density on skin colour. For the first experiment, snapper (meanTL = 29.7 cm) were transferred from a commercial snapper sea cage to black or white netted cages and fed diets supplemented with unesterified astaxanthin (supplied as Lucantin (R) Pink, BASF) at 0 or 39mg kg(-1) for 42 days. Skin colour was measured using the CIE L* (black-white), a* (green-red), b* (blue-yellow) colour scale. Snapper held in white netting cages became significantly lighter (higher L*) than snapper held in black cages; however, values were not as high as previous laboratory-based studies in which snapper were held in white plastic-lined cages. Snapper fed astaxanthin displayed significantly greater a* and b* values, and total carotenoid concentrations after 42 days. In addition, total carotenoids were higher in fish from black than white cages. The second experiment was designed to investigate whether density reduced the improvements in skin colour achieved by holding fish in white coloured cages and whether cage colour affected stress. Snapper (mean weight = 435 g) were acclimated to black cages and fed 39mg kg(-1) astaxanthin for 44 days before transferring to black or white plastic-lined cages at 14 (low), 29 (mid) or 45 (high) kg m(-3) for 7 days after which time skin colour, plasma cortisol and plasma glucose concentrations were measured. Skin lightness (L*)was greater in snapper transferred to white plastic-lined cages with the lightest coloured fish obtained from the lowest density after 7 days. Density had no effect on plasma cortisol or glucose levels after 7 days, although plasma cortisol was elevated in snapper from black cages. For improved skin colouration we recommend feeding unesterified astaxanthin at 39 mg kg(-1) for approximately 6 weeks and transferring snapper to white plastic-lined cages or similar at low densities for short periods before harvest rather than producing fish in white netting sea cages subject to biofouling.