The supplemental effect of amino acids (AAs) to a soy protein concentrate (SPC) diet for red sea bream Pagrus major was evaluated for different‐sized fish. The fish were fed artificial diets based on 52% SPC for 60 days in juvenile (mean weight 11.7 g) and for 153 days in yearling (mean weight 178 g). To these diets L‐lysine hydrochloric acid (LS), DL‐methionine (MS) and both L‐lysine and DL‐methionine (LMS) were supplemented. A control diet without AA supplementation (NOS) and a reference diet based on brown fish meal (BFM) were also tested. In the juvenile fish, the growth performance of the MS group was significantly higher than the NOS group. The growth performance was further improved by the dietary supplementation of both Lys and Met (LMS group). Lenticular opacity was not observed in these two dietary groups. On the other hand, the growth performance of the NOS and LS groups was lowest, and the incidence of lenticular opacity of the LS group was higher rather than the NOS group. In the yearling fish, feed conversion of the LMS group was slightly higher than the NOS group, but the growth performance of the LMS group was similar to the NOS group. These parameters were similar or slightly lower in the LS or MS groups compared with the NOS group. Lenticular opacity was not observed in any dietary groups for yearling fish. In both juvenile and yearling fish, the performances of the BFM groups were superior to the other dietary groups. The results for juvenile fish indicate that the quality of the SPC diet is improved by the Met supplementation, and further improved by both Lys and Met supplementation. Furthermore, the results in the present study suggest that the AA requirement of red sea bream changes with growth and/or age.

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