Aquaculture is one of the fastest-growing animal production sectors in the world and in Türkiye. From Black Sea and inland trout farms to sea bream and sea bass cages in the Aegean and Mediterranean, feed costs make up the biggest operating expense, about 50-70%.
Fish meal, the traditional foundation of fish feeds, has become scarce and expensive worldwide. Formulators have therefore turned to plant protein sources such as soybean meal, corn gluten and canola. But the digestive system of carnivorous species such as trout and sea bass cannot easily tolerate plant proteins. When limiting amino acid gaps are not closed, both feed conversion ratio (FCR) suffers and excess ammonia and nitrogen load builds up in the ponds. In this guide we look at the ideal amino acid profile and its link to water quality.
Limiting Amino Acids in Plant Proteins
Fish meal has an amino acid profile that matches fish muscle almost exactly. When plant proteins are used, gaps appear in 3 critical places:
- Methionine and cysteine (sulphur amino acids): Low in soy protein. Deficiency can cause cataracts of the eye lens, stunted growth and muscle disorders in fish.
- Lysine: Limiting in corn gluten and wheat by-products. It is one of the main building blocks of muscle protein synthesis.
- Threonine and tryptophan: Needed for gut lining integrity and immune response.
Liebig's law of the minimum (barrel theory): A fish can only build as much protein as the scarcest amino acid in the ration allows. For example, if methionine is only 70% of the requirement, the other amino acids in the ration cannot be used at full efficiency for muscle growth; they are broken down and released into the water through the gills as toxic ammonia (NH3).
Water Quality and the Environment: Reducing Nitrogen Discharge
Fish fed unbalanced protein release ammonia through the gills and undigested nitrogen in their manure. In flow-through and recirculating ponds, this nitrogen load raises the biochemical oxygen demand (BOD) of the water, leads to eutrophication (excess algae and nutrient build-up) and creates risk under environmental regulation.
With the right synthetic amino acid supplementation (L-lysine HCl, DL-methionine, L-threonine), crude protein in the ration can be lowered from 48% to 42% while growth performance is maintained, and nitrogen released into the water can be cut by about 22%.
Nutritional Needs by Fish Species
Example requirement values by species and age are as follows:
- Rainbow trout (portion size): Crude protein 40-44%, lysine 2.40-2.80%, methionine 1.10-1.30%, target FCR 0.95-1.05.
- Rainbow trout (fry): Crude protein 46-50%, lysine 2.90-3.20%, methionine 1.30-1.50%, target FCR 0.85-0.95.
- Sea bream (Sparus aurata): Crude protein 44-46%, lysine 2.20-2.50%, methionine 1.00-1.20%, target FCR 1.35-1.50.
- Sea bass (Dicentrarchus labrax): Crude protein 45-48%, lysine 2.40-2.70%, methionine 1.15-1.35%, target FCR 1.40-1.60.
These values are general guidelines and vary with water temperature, fish weight and feed energy level.
Frequently Asked Questions
Is it possible for FCR in fish feed to fall below 1.0?
Yes. Especially in trout farming, FCR values of 0.90-0.95 can be reached with extruded feeds that have an ideal amino acid balance and are enriched with highly digestible premixes. Results vary with water temperature, management and fish weight.
How much is the leaching loss in water?
Uncoated free amino acids can leach into the water at 20-30% within the first 5 minutes after the pellet falls into water. MCB Vital's coating technology aims to bring this loss below 3%.
Does using synthetic amino acids harm fish health?
No. L-lysine, DL-methionine and L-threonine are amino acids that fish already need. Used at the right dose, they close the gaps in plant protein sources and raise protein efficiency.
Why does nitrogen discharge matter?
Excess nitrogen enters the water as ammonia, degrades water quality and harms the gill health of the fish. It also creates risk for the farm under environmental regulation.
