Effect of feed ingredients on digestive enzyme secretion in fish

In response to the limitation in the global supply of fish meal, the traditional protein source used in aquaculture feed, efforts have increasingly been focused on the use of alternative protein sources of plant origin. However, plant ingredients may cause growth retardation in aquaculture fish species. Feed nutrients must be digested for their utilization, and so pancreatic digestive enzymes have essential roles for the digestion. Also, cholecystokinin (Cck) is known to be a hormone that stimulates the secretion of digestive enzymes in vertebrates. To improve the utilization of plant based diets in fish, we investigated the effects of various feed ingredients on secretion of digestive enzymes in red seabream Pagrus major and yellowtail Seriola quinqueradiata, which are commercially important aquaculture species in Japan. We first investigated the effect of soybean meal on digestive enzyme secretion in red seabream. Activities of pancreatic digestive enzymes (trypsin, chymotrypsin, lipase, amylase) in the intestinal content of a soybean meal based diet (SBM) fed fish were lower than those of a fish meal based diet (FM) fed fish. Also, lower gene expression levels of the digestive enzymes in the hepatopancreas were observed in the SBM fed fish compared with the FM fed fish. These data indicate the FM diet stimulated the secretion/synthesis of pancreatic digestive enzymes to a greater degree than the SBM diet. Then, we tried to identify the stimulation factor in fish meal. Administration of a FM water-soluble fraction increased the gene expression of trypsin, lipase, cck and cck receptor (cck-1r) in yellowtail, suggesting the enzyme stimulation factor may exist in the water-soluble fraction of FM. Supplementation of the enzyme stimulation factor, although not identified yet, may improve the utilization of plant based diets in aquaculture fish species.