The optimum dietary essential: nonessential amino acid ratio for rainbow trout (Oncorhynchus mykiss), which maximizes nitrogen retention and minimizes nitrogen excretion

Abstract

Although the ratio of essential to nonessential amino acids (EAA:NEAA ratio) in dietary protein has important effects on protein utilization by fish, there has thus far been little research aimed at quantifying these effects. In this study, we investigated the effect of dietary EAA:NEAA ratio on nitrogen (N) utilization and excretion of rainbow trout (Oncorhynchus mykiss). A 6-week dose-response experiment was conducted, with 6 levels of dietary EAA:NEAA ratio, ranging from 23:77 to 66:34. Protein content of all diets was 350 g kg⁻¹. Each experimental diet was fed to four replicate tanks of fish (35 fish per tank). Fish were fed a limited ration on the first two feedings of each day, and were fed to apparent satiation on the third feeding of each day. Response variables included feed intake, weight gain, lipid deposition, thermal-unit growth coefficient (TGC), feed efficiency ratio (FER), N retention, and N excretion (g kg⁻¹ gain). Mean feed intake, weight gain, TGC, FER and N retention all increased as EAA:NEAA ratio increased up to 57:43, and then decreased slightly as EAA:NEAA ratio increased further to 66:34. Maximum mean N retention of 46% and minimum mean N excretion of 29 g N kg⁻¹ gain was achieved by fish fed the diet containing an EAA:NEAA ratio of 57:43. Non-linear regression using a 2^nd order polynomial equation showed that 95% of the maximum N retention could be achieved with an EAA:NEAA ratio of 46:54, and 98% with an EAA:NEAA ratio ranging from 49:51 to 62:38.