What is the optimal level of tryptophan for feeding rainbow trout?

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By Milthon Lujan

Trout. Source: INRAE
Trout. Source: INRAE

Feeds are fundamental to the growth of the rainbow trout aquaculture industry, thus providing nutritionally balanced food that meets the nutritional needs of the fish is essential.

A team of scientists from the University of Kashmir conducted a feeding trial to assess the dietary requirements of tryptophan for rainbow trout, Oncorhynchus mykiss.

Essential amino acids: Basic components for fish health

Amino acids, like proteins, play a vital role in fish health and growth. Among them, tryptophan holds a unique position due to its diverse functions.

Tryptophan plays a crucial role. It not only contributes to protein synthesis but also acts as a precursor to several vital molecules in fish. These molecules impact the immune system, antioxidant status, stress response, and behavior.

Various functions of tryptophan

  • Precursor to vital molecules: Tryptophan is a basic component of serotonin (a neurotransmitter), melatonin (a hormone), and kynurenine (an amino acid). These molecules influence the nervous, endocrine, and immune systems.
  • Optimizes growth and health: Tryptophan deficiency can lead to impaired growth and scoliosis (curved spine) in fish, highlighting its importance for healthy development.

Research objectives

The study aims to determine the precise dietary requirement of tryptophan in rainbow trout fry by analyzing:

  • Growth performance: Weight gain, feed efficiency, and specific growth rate will be measured.
  • Hematobiochemical parameters: Blood cell count and biochemical markers will be analyzed to assess overall health.
  • Antioxidant status: Antioxidant levels, which combat harmful free radicals, will be evaluated.
  • Gene expression: Expression of crucial genes for growth will be examined.
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The experiment

To determine the perfect level of tryptophan for optimal growth and well-being of rainbow trout, scientists conducted a meticulous 56-day feeding trial. Here’s the breakdown:

  • Subjects: 360 rainbow trout fry were carefully distributed among six tanks, with three replicates per treatment group.
  • Diets: Six specially formulated diets were prepared, all with the same energy and protein content but with varying levels of tryptophan (ranging from 0.40 to 6.40 g/kg).
  • Feeding: Fish were carefully hand-fed twice daily, adjusting feed amounts according to body weight.

Key results

The results speak volumes about the power of this essential amino acid:

  • Maximum growth rates: Fish fed the diet with 4.0 g/kg tryptophan showed the best growth performance.
  • Ideal body composition: This group also exhibited the optimal balance of protein (high), fat (intermediate), and moisture (low).
  • Improved health: Key blood parameters (hemoglobin, hematocrit, red blood cells) and protein levels increased significantly in the 4.0 g/kg tryptophan group.
  • Liver health check: Indicators of liver function (ALT and AST enzymes) showed positive trends up to the optimal level of tryptophan.
  • Intestinal and antioxidant support: Clear improvement was observed in intestinal enzymes and antioxidants (except MDA) up to the 4.0 g/kg level.
  • TOR signaling connection: Muscle genes linked to growth (TOR and 4E-BP1) experienced a significant increase in expression with increasing tryptophan up to the optimal level.

The optimal point: 4.0 g/kg

In-depth analysis points to 4.0 g/kg of tryptophan in dry diet (8.89 g/kg dietary protein) as the ideal amount, leading to:

  • Maximum growth potential
  • Solid blood health
  • Improved liver function
  • Balanced body composition
  • Enhanced immune defense and antioxidant capacity.
  • Optimal expression of growth-promoting genes.
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The benefits of understanding tryptophan requirements

By determining the optimal level of tryptophan in the diet for rainbow trout, this research can:

  • Optimize feed formulations: This can lead to cost-effective and environmentally sustainable practices in aquaculture.
  • Enhance fish health and growth: Adequate tryptophan intake can promote optimal fish growth and well-being.
  • Contribute to sustainable food security: This research paves the way for efficient and responsible aquaculture practices, contributing to a sustainable food source for a growing population.


This study delves into the multifaceted role of tryptophan in fish, using rainbow trout as a model. By understanding its optimal dietary requirements, we can contribute to sustainable and responsible aquaculture practices, ensuring a healthy future for both fish and humans.

While this study offers valuable insights, further exploration could include the interaction between tryptophan and other essential amino acids for truly optimized fish nutrition.

The study has been funded by the Department of Biotechnology (DBT) of the Government of India.

Imtiaz Ahmed
Fish Nutrition Research Laboratory, Department of Zoology, University of Kashmir
Hazratbal, Srinagar 190006, India.
Email: imtiazamu1@yahoo.com

Reference (open access)
Ahmed, I., Ahmad, I., & Malla, B. A. (2024). Effects of dietary tryptophan levels on growth performance, plasma profile, intestinal antioxidant capacity and growth related genes in rainbow trout (Oncorhynchus mykiss) fingerlings. Aquaculture, 740710.