Fig. 2
Ingestion rate of the juvenile O. marmoratus on the TS pellets with inosine (INO) or inosine 5′-monophosphate disodium (IMP•Na2) at different concentration levels
Betaine is commonly known to possess synergistic effects with other chemical substances, especially amino acids; hence it can be a feed enhancer to some fish species (Kasumyan and Døving 2003). In this study, betaine was neither a feeding stimulant nor a feed enhancer to the juvenile O. marmoratus. Figure 3 shows the ingestion rates of the fish to the agar gel pellets with betaine, amino acids mixture (AAM) or betaine + AAM at different rates. Inclusion of betaine to the AAM did not improve the ingestion rates of the TS pellets despite changes in the ration of betaine and AAM. Such results were in contrast with those reported on brown-marbled grouper Epinephelus fuscoguttatus, where the ingestion rate of the agar gel pellet significantly (P < 0.05) increased after the inclusion of betaine to the AAM, even in a small portion (Lim et al. 2016). Indeed, betaine was also rejected by S. salar as reported by Mearns et al. (1987) through behavioural assays.