Trout maintain growth with amino-acid balanced diets, reduced protein

A team of researchers from the University of Idaho explored the use of balancing essential amino acids, reduced fishmeal (FM) and low crude protein (CP) diets on the growth performance, production and stress response of rainbow trout. The study was published in the journal Aquaculture​​​.

“We hypothesized that both dietary CP and FM content could be successfully reduced to maintain growth performance and feed efficiency as long as EAA requirements are met, but that reduced dietary CP and FM content would exacerbate the response to chronic stress,” ​the researchers said. “Therefore, the present study evaluated the factorial effects of lowering dietary FM and CP levels, while maintaining dietary EAA concentrations through synthetic amino acid supplementation, on rainbow trout growth performance, nutrient utilization, and response to chronic handling stress.”​

They found that reducing crude protein levels to 42% maintained trout growth. Providing 20% fishmeal (FM) in diets increased whole-body dry matter, crude protein (CP) and total essential amino acids (EAA) compared to results for fish receiving diets with 5% fishmeal.

However, increasing dietary CP levels lowered dry matter, crude fat and gross energy while increasing EAA amounts, they said. Lowering both fishmeal and CP amounts did not alter stress indices based on plasma cortisol, glucose and lysozyme activity following 6 weeks of handling stress.

In an assessment of gene expression, general control nonderepressible 2 (gcn2) was lower as more crude protein was included in the diet, but other genes evaluated were not influenced by diet, they said. “Gene expression results suggest amino acid limitations on muscle protein metabolism as a result of feeding diets below 42% CP, even when supplemented with synthetic EAA to meeting published dietary requirements,” ​they added.

“We observed that diets balanced for amino acids and other critical nutrients show an opportunity to reduce dietary CP level in feeds for rainbow trout from 48% to 42% without reducing growth performance, feed efficiency, body composition, metabolic amino acid sufficiency or stress tolerance​. ​

“Reducing the dietary FM level to 5% also significantly reduced rainbow trout performance, possibly a result of reduced feeding stimulants and/or non-nutritive growth promoters associated with FM; however, neither low dietary CP nor FM impacted the physiological stress response following six weeks of chronic stress.”​

The research project was supported by Evonik Nutrition & Care GmbH.

Reducing fishmeal use​

Feed producers in the aquaculture industry have sought to lower fishmeal use in feeds and focused on alternative proteins – including plant-based proteins – to mitigate price increases in fishmeal, the researchers said.

However, there are differences in the amino acid profiles of plant proteins compared to those in fishmeal (FM), which has established some limitations to the use of alternative proteins, they said. Supplemental amino acids are increasingly used to meet physiological needs.

Feed costs are influenced by protein inclusion level and protein type, they said. Commercial feeds for rainbow trout can include up to 48% crude protein but supplemental amino acids provide a way to reduce reliance on intact protein sources and lower dietary crude protein.

“A more precise understandings of nutritional requirements of the target species and raw material digestibility play an important role in successfully reducing expensive dietary FM and CP levels and overall feed cost,”​ they said.

Does fish diet alter stress response?​

Aquafeed producers have established a set of nutritional recommendations to support fish needs during “dynamic production conditions,”​ the researchers said.

“Nutrient requirements of fish under commercial production conditions can be affected by various biotic (e.g., fish density, sex, health conditions) and abiotic (e.g., water quality, feed quality) factors,”​ they said. “Stress associated with fish culture practices, such as stocking density, grading, netting, and hauling of fish, may also affect nutrient requirements at fixed feeding rates.”​

However, little is known about how fish on different diets manage stress – especially relating to periods of chronic stress, they said.

“There appears to be no effect of varying protein and lipid concentration in rainbow trout diets on baseline cortisol concentrations; yet, plant-based diets have been shown to result in a stronger acute, cortisol stress response,” ​they said. “The potential for interactions between diet and stress has significant implications for fish wellbeing.”​

Feeding trial details​

The feeding trial provided eight experimental but isocaloric diets to rainbow trout for a period of 15 weeks, the researchers said. After nine weeks, half of the fish on all diets faced exposure to twice-weekly handling stress.

The diets included two levels of fishmeal at 20% or 5% and four levels of crude protein (CP) at 48%, 45%, 42% or 39% – feeds were supplemented with lysine, methionine, threonine, tryptophan, arginine, histidine, isoleucine and valine to sustain essential amino acid (EAA) levels found in a diet with 20%FM and 48%CP.

Fish were weighed at the start of the feeding trial and after nine weeks to establish weight gain (WG), specific growth rate (SGR), the feed conversion ratio (FCR) and survival rate, they said. At the end of the 15-week period, a selection of fish on each diet with harvested for whole-body analysis and additional fish were sampled for muscle gene expression and plasma analysis.

Fish were collected one hour after the stressor to assess plasma stress indices and blood was collected to check physiological stress response, they said. Feeds and fish samples were checked for energy content and proximate composition.

Results​

Overall, mortality rates were similar for fish across all diets, the researchers said.

During the first nine weeks, lowering fishmeal from 20% to 5% reduced final body weight and growth rate, limited feed intake, PER and PRE and increased FCR, they said. Trimming CP from 48% to 42% did not alter weight gain but cutting CP to 39% reduced weight gain.

However, lowering CP amounts did not alter FCR and protein retention improved as dietary CP fell, they said. “The interaction of the two main factors (FM and CP) significantly impacted feed intake, but had no significant effects on growth performance or feed utilization,”​ they added.

“Our study demonstrated that 5% dietary FM is insufficient for maximal growth performance, while diets balanced for EAAs show an opportunity to reduce CP level from 48% to 42% without any reduction in growth performance, body composition, metabolic amino acid sufficiency or tolerance to chronic stress,”​ the researchers said.

After 15 weeks, the reduction in FM level lowered whole-body dry matter, crude protein and raised whole-body ash for fish on the 5% FM diets, they said. Reducing dietary CP increase whole-body dry matter, gross energy and crude fat.

“There were no interactions between dietary FM, CP, and stress on whole-body proximate composition,” ​they said. “Stress (non-stress group vs. stress group) significantly increased whole-body dry matter (32.5 vs. 33.5%), crude protein (15.9 vs. 16.2%), crude fat (13.7 vs. 14.6%) and gross energy (9.20 vs. 9.66 MJ/kg).”​

Stress was found to lower amounts of whole-body EAAs and for non-essential amino acids, they said. Amounts of total and essential amino acids fell as dietary CP was reduced.

“The interactions between dietary FM, CP, and stress were not significant for whole-body total EAA composition,”​ they added.

Stress also increased plasma levels of cortisol, glucose and lysozyme activity, the researchers said. But neither dietary FM and CP nor the interaction of FM and CP altered plasma biochemical results, the researchers said.

In terms of gene expression, gcn2 increased as levels of CP were lowered and atf4 expression rose in stressed fish, they said.

Source: Aquaculture​

DOI: doi.org/10.1016/j.aquaculture.2019.734435​​

Title: Effects of lowering dietary fishmeal and crude protein levels on growth performance, body composition, muscle metabolic gene expression, and chronic stress response of rainbow trout (Oncorhynchus mykiss)​

Authors: S. Lee, K. Masagounder, R.W. Hardy, B.C. Small