Carotenoid deposition, flesh quality and immunological response of Nile tilapia fed increasing levels of IMTA-cultivated Ulva spp.

Increasing levels of a mixture of Ulva spp. (Ulva rigida and Ulva lactuca) produced in an integrated multi-trophic aquaculture (IMTA) system were evaluated in Nile tilapia. A control diet (CTRL) was compared with two experimental isonitrogenous (36 %) and isoenergetic (20 kJ g−1) diets containing 5 % (U5) and 10 % (U10) of Ulva spp. meal. Duplicate groups of 18 fish (255 g initial body weight) were reared at 25 °C and fed each diet for 68 days to evaluate carotenoid deposition, flesh organoleptic properties and immunological response. By the end of the trial, all groups of fish showed similar final body weight and specific growth rate. Whole body composition was also similar among treatments. The dietary incorporation of Ulva spp. meal increased total carotenoid content in the skin, with fish fed U5 displaying significantly higher levels (6.5 μg g−1) than the CRTL (1.4 μg g−1). No carotenoids could be found in tilapia muscle. Muscle colour determined using a spectrocolorimeter indicated that tilapia fillets from fish fed U10 showed the highest lightness (highest L* value) and yellowness (less negative b* value), but the lowest redness (lowest a* value). Furthermore, sensory attributes of flesh showed no significant effects of dietary treatments on visual, olfactory, texture and flavour parameters, with the exception of sour parameter that was lowest in U10-fed tilapia. The dietary inclusion of Ulva spp. meal had no beneficial effect on lysozyme or peroxidase activity, but the complement activity (ACH50), an important component of innate immune system in fish, increased concomitantly with the dietary inclusion level of Ulva spp. meal. The inclusion of Ulva spp. meal in diets for Nile tilapia seems to be possible up to 10 % without major effects on growth performance or flesh organoleptic properties but enhancing the innate immune response of the fish.