Optimization of dipeptide–protein mixtures in experimental diet formulations for rainbow trout ( Oncorhynchus mykiss) alevins

Abstract

It has previously been shown that diets in which the amino acid portion is based on synthetic dipeptides result in positive growth of rainbow trout alevins. However, the protein-based diets used as a control performed significantly better. One objective of the present study was to identify a protein–dipeptide mixture ratio that results in comparable growth to a protein-based control, in alevins over the first 6 weeks of feeding. Seven diets were tested, including the casein–gelatin control, three diets with differing proportions of the casein–gelatin replaced with synthetic dipeptides (50P, 75P and 100P), one diet based on free amino acids and one reference commercial diet. In the seventh diet, arginine-containing dipeptides were withdrawn from the 100P dipeptide-based diet. To evaluate the diets, effects on growth, survival, feed intake and free amino acid (FAA) concentrations in muscle tissues were studied. The results on rainbow trout alevin performance using synthetic dipeptide (100P)-based diets resulted in a relative growth of 34% of the growth observed when alevins were fed the casein–gelatin control diet. The free amino acid (FAA)-based diet, however, resulted in a negative growth of the alevins. In contrast, a 1:1 ratio of synthetic dipeptides to casein–gelatin (50P diet) resulted in similar growth as the casein–gelatin control diet fed fish. Furthermore, the 50P diet resulted in a survival that was not significantly different from that found when using a commercial diet, which showed the highest survival. The concentrations of indispensable FAA (IAA) in muscle were with few exceptions similar in the control and 50P group, while the 100P dipeptide- or free amino acid-based diet fed fish showed lower levels of free IAA in muscle. Alevins fed dipeptide-based diets without arginine (100P w/o Arg) showed high mortality within 2 weeks and negative or minimal growth. This dietary group was restarted at 2 and 4 weeks, and similar results were obtained. In conclusion, the present study indicates that a 50% replacement of the protein portion with synthetic dipeptides is an acceptable diet formulation for rainbow trout alevins in terms of growth, survival and muscle levels of indispensable FAA. The effects of arginine-containing dipeptide withdrawal on fish growth and survival indicates that arginine is an indispensable amino acid in first-feeding alevins, despite their expression of the ornithine–urea cycle and related enzymes that may function in net de novo synthesis of arginine. The synthetic dipeptide diets used here were able to identify such limiting amino acids and may be further used to re-evaluate indispensable amino acid requirements in fish early life stages characterized with high growth.