The flathead grey mullet is a catadromous species where the larvae are strict carnivores . On the other hand, juvenile fish become omnivorous during their migration to a more hypersaline, estuarine environment. This requires that mullet adjust their biosynthetic and digestive capacity, which affects their dietary requirements. Our group tested different levels of dietary DHA (0.7, 0.9 and 1.2% DW diet) at two salinities (15 and 40 ‰) on juvenile mullet DHA synthesis . Fish exposed to 15 ‰, increased DHA synthesis through a series of desaturation and elongation steps that were regulated by transcription factors (PPARα and SREBP-1) resulting in similar growth among treatments . However , at 40‰ mullets displayed a significant (P < 0.05) dietary DHA dose-dependent response on weight gain (Fig. 1), while the expression of the transcription factors was ca. 50% lower. These results suggested that dietary levels of DHA can be reduced during grow-out of farmed juvenile mullet, provided the salinity is lowered to 15‰.
NCM studies testing 3 rotifer taurine levels (1.1, 4.4, 6.4 mg g-1 ) on 3-12 dph larvae, found a significant (P < 0.05) taurine dose-dependent growth response in 12 , 19 and 44 dph (Fig. 2) demonstrating the far-reaching effect of rotifer taurine on post-larvae and juveniles. On the other hand, when older juveniles were fed different dietary taurine levels (0, 0.5, 1.0 and 2.0% DW diet), only the 0.5% treatment elicited a better (P < 0.05) growth performance, while there was a marked (P < 0.05) up- regulation of liver cysteine sulfinic acid decarboxylase (CSD) . This is the main rate limiting enzyme in taurine biosynthesis and suggests that mullet juveniles have the capacity for endogenous taurine production as they move towards taurine- poor estuarine waters.
The most effective weaning diet also reflects changes in digestive capacity as the mullet larvae develop into juveniles. Therefore, three weaning dietary treatments (herbivorous, carnivorous, and omnivorous) were tested in 24 to 53 dph juveniles. The omnivorous weaning diet resulted in the best fish growth (P < 0.05) , which may be tied to high gut production of both α-amylase and proteolytic enzymes. Moreover, t he herbivorous diet delayed fish gut maturation and mucosal absorption reducing growth, while hydrolysates in the carnivorous and omnivorous diets may have increased these parameters . Taken altogether, this study recommends that mullet weaning and grow-out feeds should be designed for omnivorous feeding.