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A 13-week feeding trial was carried out with juvenile rainbow trout to test two diets: a control diet without astaxanthin (AX) supplementation (CTRL diet), and a diet supplemented with 100 mg/kg of synthetic AX (ASTA diet). During the last week of the feeding trial, fish were exposed to episodic hyperoxia challenge for 8 consecutive hours per day. Episodic hyperoxia induced physiological stress responses characterized by a significant increase in plasma cortisol and hepatic glycogen and a decrease in plasma glucose levels. The decrease of plasma glucose and the increase of hepatic glycogen content due to episodic hyperoxia were emphasized with the ASTA diet. Hyperoxia led to an increase in thiobarbituric acid-reactive substances in the muscle, diminished by dietary AX supplementation in both liver and muscle. Muscle and liver AX were increased and decreased respectively after 7-day episodic hyperoxia, leading to an increase in flesh redness. This augment of muscle AX could not be attributed to AX mobilization, since plasma AX was not affected by hyperoxia. Moreover, hyperoxia decreased most of antioxidant enzyme activities in liver, whereas dietary AX supplementation specifically increased glutathione reductase activity. A higher mRNA level of hepatic glutathione reductase, thioredoxin reductase, and glutamate-cysteine ligase in trout fed the ASTA diet suggests the role of AX in glutathione and thioredoxin recycling and in de novo glutathione synthesis. Indeed, dietary AX supplementation improved the ratio between reduced and oxidized glutathione (GSH/GSSG) in liver. In addition, the ASTA diet up-regulated glucokinase and glucose-6-phosphate dehydrogenase mRNA level in the liver, signaling that dietary AX supplementation may also stimulate the oxidative phase of the pentose phosphate pathway that produces NADPH, which provides reducing power that counteracts oxidative stress. The present results provide a broader understanding of the mechanisms by which dietary AX is involved in the reduction of oxidative status.

Red porgy, Pagrus pagrus, is a candidate for aquaculture diversification in the Mediterranean area. The main restriction for the development of this species, under cultured conditions, is the loss of their natural red–pink coloured skin. Several studies, using different dietary astaxanthin sources have given positive results on red porgy skin coloration. However, there is scarce information on the time required for cultured red porgy to obtain adequate skin colour when astaxanthin is included in the diet. This study aims to evaluate the effect of supplementing adult red porgy diet with shrimp shell meal, as a source of esterified astaxanthin, during different periods prior to harvest, on skin colour and skin carotenoid concentration. The experiment was carried out for 180 days, testing a control diet with no carotenoids and a shrimp shell meal diet (SM) with an inclusion of 16% shrimp shell meal in substitution of fish meal. Four treatment groups were established. Control treatment fish fed throughout the experiment on the control diet and SM60, SM120 and SM180 treatment fish fed first on the control diet and thereafter for 60, 120 and 180 days before harvesting on the SM diet. Growth was higher in fish from SM180 treatment group in comparison to control fish. Skin redness (a⁎) increased with increasing feeding period, and skin yellowness (b⁎) only until 120 days of feeding with SM diet. Whereas skin lightness (L⁎) does not seem to be influenced by dietary inclusion of shrimp shell meal for different periods of time. Skin carotenoid concentration also augmented with increasing feeding with SM diet. As a result, cultured red porgy fed for a period of 120 to 180 days, prior to harvest, on the SM diet may achieve similar skin coloration to wild red porgy, only differing in skin carotenoid concentration.

Whereas wild specimens exhibit a red pink silver colour, under captivity red porgy (Pagrus pagrus) skin turns dark grey, being directly associated with lower acceptance by the consumers and lower market value. This study was conducted to evaluate the effect of diet supplementation with two carotenoid sources, at two different concentrations, on growth and skin coloration. Fish of 44 g mean initial weight were fed the following diets: control (with no added carotenoids), CTX 40 and CTX100 (diets with 40 and 100 mg of synthetic canthaxanthin/kg); SM20 and SM40 (diets with 20 and 40 mg of astaxanthin from shrimp shell meal/kg). Three samplings were carried out in this study, at the beginning and after 75 and 105 days of feeding. At each sampling, individual weight and colour measurements were performed. Colour was evaluated in three different body zones: front lateral (zone I), front dorsal (zone II) and caudal fin (zone III). Growth was not influenced by the inclusion of carotenoids in red porgy diet. Only astaxanthin from shrimp shell meal, particularly at 40 mg/kg diet, was able to give red porgy skin an overall reddish coloration, suggesting a better utilization of astaxanthin from this source. Shrimp shell meal diets enhanced reddish hue and chroma values, nevertheless hue values at days 75 and 105 did not show a marked difference and chroma attained after 75 days of feeding time tended to be reduced at day 105, suggesting skin colour saturation after a certain feeding time. Carotenoid supplementation did not influence lightness of red porgy skin in the present experiment. From the three evaluated zones, zone I of fish fed SM40 diet presented lower hue, chroma and lightness variation; therefore, it could be proposed as a control zone for future studies. The results of this study suggests that the inclusion of astaxanthin, from shrimp shell meal, in red porgy commercial diets significantly improves skin coloration and markedly enhances the commercial value of this cultured species.