by Karthik Masagounder, Evonik Operations GmbH

 

Seunghan Lee and Brian Small, University of Idaho

Rainbow trout (Oncorhynchus mykiss) is very popular with consumers – particularly as it is rated in the top five healthiest fish to eat – so it is an important species for aquaculture producers. Increasing raw material and feed cost is always a worry for trout feed and fish producers. Feed cost is contributed largely by the dietary protein level and sources.

Crude protein (CP) levels have traditionally ranged from 42 percent to 48 percent, depending on fish size, in commercial grower feeds for rainbow trout (Hardy, 2002). Over the past decade, feed producers have significantly reduced fishmeal (FM) levels in feeds, replacing it with alternative protein sources, especially those of plant origin. This helps producers cope with volatile FM prices among other benefits.

It is not a straight swap though. The amino acid profiles of plant protein meals differ from those of FM protein (NRC, 2011), and this can limit their levels in fish feeds. Supplemental amino acids can help in meeting physiological requirements for limiting amino acids and have been increasingly used in fish feed formulations based on plant protein feedstuffs.

Minimising dependency on intact protein sources

Fish require a diet that is well-balanced in amino acids, rather than CP, per se. Supplemental amino acids allow producers to minimise dependency on intact protein sources to meet target level of individual amino acids in feeds and reduce dietary CP levels.

A more precise understanding of nutritional requirements of the trout and raw material digestibility plays an important role in successfully reducing expensive dietary FM and CP levels and overall feed costs.

While many nutritionists use National Research Council (2011) data as the baseline for the nutrient requirements of fish and shrimp when formulating feeds, the aquafeed industry has developed to some extent higher parallel recommendations to buffer the requirements of fish under dynamic production conditions.

The nutrient requirements of fish are not constant under commercial production conditions and can be impacted by several biotic (For example: age, growth potency, sex and health conditions) and abiotic (Water quality) factors. Requirements for certain amino acids and energy can also be affected by stress associated with fish culture practices, such as stocking density, grading, netting, and hauling of fish.

Therefore, requirements of certain nutrients generated under ideal growing condition might become limiting under health challenging conditions. How fish following different dietary histories handle stress is unclear, especially during periods of chronic stress. The potential connection between diet and stress has significant implications for the health and welfare of fish.

We hypothesised that both dietary CP and FM content could be reduced while still maintaining growth performance and feed efficiency, if essential amino acid (EAA) requirements were met. However, reduced dietary CP and FM content may affect fish performance under chronic stress conditions, as the requirements of certain functional amino acids may become limiting.

We therefore conducted a study to evaluate the effects of lowering dietary FM and CP levels, while maintaining EAA levels, on growth performance, body composition, muscle metabolic gene expression, and chronic stress response of rainbow trout, with and without handling stress.

Assessing growth performance under laboratory rearing conditions

Eight experimental diets (isocaloric) with a 4×2 factorial design were formulated to contain two levels of FM (20%, 5%) and four levels of CP (48%, 45%, 42%, 39%). Diets were supplemented with increasing levels of limiting amino acids to maintain dietary EAA levels. Reduction of fish meal (by 15%) in the diet was compensated by increasing soy protein concentrate (by 8-10%), corn protein concentrate (by ~3%) and poultry byproduct meal (by ~2%).

Protein level within each FM level was reduced by reducing the inclusion levels of protein sources (mainly poultry byproduct meal, DDGS, corn protein concentrate) and increasing wheat flour and fish oil levels. Amino acids were balanced by increasing the levels of supplemental sources.

All the diets were formulated to have a constant digestible energy at 17.8 MJ/kg. Trout (~35 g, initial body weight) were fed to apparent satiation twice daily for nine weeks to assess growth performance under laboratory rearing conditions, and then for an additional six weeks with and without exposure to handling stress (30 sec of chasing followed by 30 sec of netted air exposure) twice per week.

The nine-week growth trial demonstrated that reducing dietary FM levels from 20 percent to 5 percent significantly reduced fish growth and increased feed conversion ratio (P<0.05). Reducing dietary CP levels from 48 percent to 42 percent did not affect trout growth, feed intake or FCR, while showing significant improvements in protein efficiency ratio and protein retention efficiency.

However, further reduction in CP to 39 percent caused a drop in body weight, although it also resulted in further improvement in protein retention. A dietary FM level of 20 percent significantly increased whole-body dry matter, CP and total EAAs (P<0.05) compared to 5 percent FM inclusion while increasing dietary CP level significantly decreased dry matter, crude fat, and gross energy but increased total EAAs (P<0.05).

Although the digestible energy level (17.8 MJ/kg) was kept constant across diets, reducing dietary protein level increased whole-body fat content fish (13% fat for 48% CP diet versus 15% fat for 39% CP diet, on a wet basis). It is known that energy utilization is lower for protein compared to lipid in trout (Schrama et al. 2018), which could have played a role in the increased fat deposition in the reduced protein diet.

Handling stress significantly increased plasma stress indices including cortisol, glucose and lysozyme activity. However, reducing FM and CP levels had no effect on the measured stress indices (P>0.05). The mRNA expression of genes in the gcn2/eif2α/atf4 pathway in the muscle tissue (normalised against arp), known to be triggered in response to protein or amino acid starvation, were evaluated.

General control nonderepressible 2 (gcn2) decreased with increasing dietary CP level above 42 percent (P<0.05), but there were no dietary effects (FM or CP levels) on eif2α (eukaryotic initiation factor 2α) or atf4 (activating transcription factor 4) expression. In total, gene expression results suggest amino acid limitations on muscle protein metabolism as a result of feeding diets below 42 percent CP, even when supplemented with synthetic EAA to meeting the known dietary requirements. Reduced fish growth with increased expression of gcn2 at CP below 42 percent suggest the possibilities of higher requirements for certain amino acids.

Improving the sustainability of trout production

This study demonstrated that 5 percent dietary FM is insufficient for maximal growth performance, possibly because of reduced feeding stimulants, and unidentified growth promoters associated with FM. Also, differences in the nutrient digestibility among diets cannot be ruled out.

The diets balanced for EAAs show an opportunity to reduce CP level from 48 percent to 42 percent in juvenile trout feed without any reduction in growth performance, body composition, metabolic amino acid sufficiency or tolerance to chronic stress, while improving dietary protein utilisation. Results of this study can be used to help to improve the sustainability of trout production.

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