Aquatic weed Ceratophyllum sp. as a dietary protein source: its effects on growth and fillet amino acid profile of rabbitfish, Siganus guttatus

A 16-week feeding trial was conducted to determine the potential use of the aquatic weed, Ceratophyllum sp., meal to replace soybean cake meal (SCM) as a protein source in diets for grow-out of rabbitfish, Siganus guttatus. Four iso-nitrogenous diets were formulated to contain different levels of Ceratophyllum weed meal (CWM) at 0 (CWM0), 20 (CWM20), 30 (CWM30) and 40 (CWM40) g 100g-1 diet substituting SCM from 38 to 0 g 100g-1 diet, respectively. Fish meal was included in the diets at the same proportion in order to meet the protein requirement level for grow-out of rabbitfish of around 28%. Rabbitfish with initial body weight of 109±1.3 g were stocked into 15 cages each 1x1x2.5 m3. Each diet was fed twice a day at a rate of 2.5-3% of biomass. Fish were measured every month to record growth and adjust the feed ration. After 16 weeks, the weight gain and specific growth rate (SGR) of fish fed the test diets containing 0-30% CWM were similar, whereas the fish fed the CWM40 diet demonstrated lower weight gain and specific growth rate (ANOVA, p > 0.05). Feed conversion ratio (FCR) was relatively high for all groups and the FCR of the control diet (without CWM) was similar to diets containing CWM up to 30%. Fillet protein content of fish fed CWM20 and CWM30 diets were similar and both were significantly different (p < 0.05) from other two treatments (CWM0 and CWM40). Fish fed the CWM20 diet had the highest total amino acid in fillet (74.44±0.28%) which was significantly different (p < 0.05) from fish fed CWM0 (68.53±1.17%) and CWM40 (65.69±0.09%). Fish fed 20% CWM had significantly higher total amino acid content in fillet compared to the wild rabbitfish (70.09%). Based on the biological performance and the total amino acid content in fillet, rabbitfish could utilize CWM as dietary protein source up to 30% for grow-out in floating net cages.