Giant grouper growth was comparable to the growth rates documented for other grouper species of similar initial body weight when fed diets containing alternative ingredients of animal origin.
Replacing up to 40% of total marine protein ingredient content with the alternative ingredient blends we tested did not profoundly reduce fish growth performance or feed utilization in E. lanceolatus.
This result is within the range of previously reported results of successful FM replacement by SBM and SPC in diets for carnivorous marine fish, which include 20% in diets for Japanese yellowtail 40–50% for cobia and 50% in Atlantic cod.
All experimental feeds were well-accepted by giant grouper, however, the higher FCR in FM20 fed fish suggests a less efficient feed utilization compared to the other treatments.
Nonetheless, FCR for all treatments was similar to values gathered in feeding studies with other grouper species.
The reduced growth and higher FCR observed with 80% FM replacement could be related to deficiencies in essential amino acids or a reduction in diet digestibility with higher soy protein inclusion in the diet.
Dietary methionine levels decreased with increasing FM replacement and higher soy protein inclusion. Methionine levels were lowest in the FM20 diet, and all feeds except for the FM100 were below the required 1.3% dietary methionine level suggested for juvenile orange-spotted grouper.
Though the specific amino acid requirements for giant grouper remain relatively unknown, it is likely that their dietary needs are similar to other groupers, withmethionine as a potential limiting nutrient in the experimental diets.
Supplementation of methionine in diets with high inclusion of plant ingredients could contribute to the improvement of growth rates in the species. In addition, ingredients such as SBM are typically less digestible than FM formanymarine fish such as cobia summer flounder and yellowtail kingfish.
This could further explain the slightly lower fish growth observed with the FM80 and FM60 diets.
Although SPC has a higher digestibility value than SBM the FM20 diet which contained the highest level of SPC of all diets also had the highest inclusion level of S. limacinum meal, which may have contributed to a decreased digestibility of this diet.
In this regard, the digestibility of algal meals used in fish diets needs to be studied.
SPC did not affect protein digestibility in diets formulated for rainbow trout or yellowtail kingfish though more than 50% FM replacement by SPC reduced fish growth in the latter Thus, a possible effect of high SPC dietary levels on reduced growth
performance in giant grouper cannot be completely excluded in the present study.