Effects of phytase addition on P utilization in Japanese flounder Effect of Phytase on Growth and Phosphorus Utilization in Japanese Flounder ( Paralichthys olivaceus )

An experiment was conducted to investigate the effect of phytase supplementation on weight gain, phosphorus and protein digestibility and retention in Japanese flounder (Paralichthys olivaceus) fed a soybean meal (SBM)-containing diet. Six levels of phytase-supplemented diets containing 0, 150, 300, 450, 900 and 1500 FTU (phytase unit)/100g diet were assigned to triplicate tanks and fed to Japanese flounder (20 fish/ tank, initial average weight 151.4 g) for 40 days. The increase of soluble phosphorus and decrease of phytic acid remained relatively constant for all levels receiving the 300 FTU diet and greater. Significantly (P<0.05) greater weight gain and higher feed conversion ratios (FCR) were observed in fish fed diets supplemented at 300 FTU or greater compared to the control (0 FTU) diet. Significantly (P<0.05) improved apparent protein and phosphorus digestibility, as well as serum calcium concentration were found in fish fed the 300 FTU diet. All International Journal of Recirculating Aquaculture 10 (2009) 25-42. All Rights Reserved, © Copyright 2009 by Virginia Tech, Blacksburg, VA USA 26 International Journal of Recirculating Aquaculture, Volume 10, June 2009 Effects of phytase addition on P utilization in Japanese flounder diets supplemented at 300 FTU or greater also showed a significantly (P<0.05) improved concentration of bone calcium and zinc. The 150 FTU inclusion level showed only better protein and phosphorus retention, bone phosphorus and magnesium than the control (0 FTU) diet. Therefore, this study indicated that supplementation of phytase is effective and that the 300 FTU/100g SBM-containing diet resulted in the maximum release of soluble phosphorus, and as a consequence improved weight gain, FCR, bone minerals, phosphorus and protein digestibility and retention in Japanese flounder. INTRODUCTION Soybean meal (SBM) is an important dietary protein source for cultured Japanese flounder (Paralichthys olivaceus). After oil extraction, SBM is used as a protein source in animal feeds due to its well-balanced amino acid profile and relatively high crude protein level (Cheng and Hardy 2003). Soybean meal is widely used in fish nutrition research, as well as for commercial fish feed purposes (Robinson et al. 1985, Hughes 1988, Riche and Brown 1996). However, it is also reported that SBM is rich in phytic acid, which reduces the availability of nutrients such as protein and minerals. Phytic acid has been shown to have a strong anti-nutritive effect due to its tendency to form insoluble complexes with diand trivalent minerals, rendering these minerals unavailable to fish (Kerovuo 2000). Approximately two-thirds of the total phosphorus (P) in various plant ingredients is present as phytic acid (Ketola 1994). It is known that fish cannot utilize phosphorus bound in the phytic acid complex molecule because fish lack the phytase enzyme needed to hydrolyze bound phytic acid phosphorus to an available form of phosphorus. On the contrary, commercial phytase can hydrolyze the phytate complex bond producing a simpler form of P, thereby increasing the bioavailability of mineral elements (Persson et al. 1998). This holds promise for reduced phosphorus in effluents and reduced phosphorus pollution. Use of commercial phytase has been reported to improve phytate P bioavailability in poultry, swine, and fish, which should decrease the amount of phosphorus excreted (Nelson et al. 1971, Simons et al. 1990, Crowell et al. 1993, Rodehutscord et al. 1995, Schafer et al. 1995, Jackson et al. 1996, Lanari et al. 1998, Veilma et al. 1998, Veilma et al. 2000). The increased nutrient utilization due to the addition of phytase in the diet has yet to be widely studied in fish. Therefore, the present study was conducted to examine the effects