ALTERNATIVE PROTEIN DIETS FOR RAINBOW TROUT AND THE POTENTIAL TO REDUCE DIETARY CRUDE PROTEIN LEVELS
The utilization of alternative proteins in diets for rainbow trout has been increasingly valuable. As diets are increasingly refined to meet the nutritional demand of rainbow trout, savings may be employed by reducing crude dietary proteins when essential amino acids are supplemented to the feeds if growth efficiency can be maintained.
To determine if crude protein levels can be reduced in alternative protein feeds for rainbow trout, two fishmeal-free diet series utilizing either all plant proteins or a combination of animal and plant protein sources were formulated to contain 20% lipid. Both diet series were supplemented with increasing levels of lysine, methionine and threonine as digestible protein levels declined from 45 to 34% and 42 to 32%, in the plant based and animal based diets, respectively. Juvenile rainbow trout (65 g initial weight) were randomly assigned to triplicate tanks/diet and fed to apparent satiation for 12 weeks in a 15 °C recirculating system.
Significant effects of protein sources and protein levels on final fish weight with significant interactive effects were detected. Final mean fish weight ranged from 334 to 362g for fish fed the plant protein based series of diets and ranged from 332 to 395 g for fish fed the animal protein based series of diets. Regression analysis indicated final fish weight increased with dietary protein level in a linear fashion for animal protein based diets while for fish fed the plant protein series a Gaussian fit predicted a maximum protein concentration of 41% digestible yielded the largest final fish weight. Higher feed efficiency was observed for fish fed the plant protein series of diets as compared to the animal protein series diets. Increasing dietary protein improved feed efficiency with no interactive effects between protein source and protein level; peak feed efficiency was predicted at 41.9% digestible protein. Feed intake was higher in fish consuming the animal based diets as compared to fish fed the plant based diets. Increasing dietary protein level also decreased feed intake but no interactions between protein source and level were observed. Higher protein retention was observed for fish fed the plant protein based diets compared to the fish fed animal protein based diets. Increasing dietary protein in the plant based diets reduced protein retention from 50% to a low of 41% whereas it reduced protein retention of the fish fed the animal based diets from 46% to 41%. Estimated maximum retention was obtained at 36% digestible protein utilizing either plant or animal proteins.
These results demonstrate that alternative protein diets from either plant or animal sources can maintain growth and feed efficiency of rainbow trout. Further, amino acid supplementation was shown to be successful at reducing crude protein levels without reducing performance of rainbow trout fed either series.