Measuring and calculating nutrient utilization in cultured fish/shellfish

What is nutrient utilization?

• How efficiently fish absorb, digest, and metabolize nutrients from their diet to support growth, reproduction, immune function, and overall health

• It involves the processes of ingestion, digestion, absorption, assimilation, and excretion of macronutrients (proteins, lipids, carbohydrates) and micronutrients (vitamins and minerals)

Key aspects of nutrients utilization in fish?

1. Protein utilization

2. Lipid utilization

3. Carbohydrate utilization

4. Energy utilization

Protein utilization?

• Essential for muscle growth and enzyme production

• Digestibility varies varies (animal vs plant based proteins)

• Excess protein is deaminated, with nitrogen excreted as ammonia

Lipid utilization

• Primary energy source; essential for cell membranes, hormone production, and immune function

• Marine fish require essential fatty acids (EPA, DHA)

• Lipid metabolism affects energy efficiency and growth rates

Carbohydrate utilization

• Fish have limited ability to digest carbohydrates compared to terrestrial animals

• Limited digestion in carnivorous fish; better in herbivores/omnivores

• Excess leads to fat deposition in the liver

Energy utilization

• Derived from protein and lipids

• Energy balance affects feed conversion efficiency (FCR) and growth performance

Model Components

Feed conversion ratio

Feed efficiency

Dry matter (DM)

• The portion of a sample that remains after removing all moisture

• It includes proteins, lipids, carbohydrates, ash, and other nutrients

• Unit = %

Moisture content

• Represents the amount of water present in a sample

• It is crucial in determining the shelf life and stability of fish and fish feed

• Unit = %

Ash

• Represents the total mineral (inorganic) portion left after a sample is burned at high temp (>500 degree Celsius)

• It provides information about the mineral composition in fish or feed

• Unit = %

Crude protein

• Total protein content in a sample, estimated based on nitrogen content using the Kjedahl or Dumas method

• Unit = %

Crude Lipid

• The total fat content extracted from a sample using organic solvents like ether

• Unit = %

Carbohydrate content

• Estimated as the remaining portion of dry matter after subtracting protein, lipid, ash, and fiber

• Unit = %

Gross energy

• Represents the total energy content in a sample

• calculated based on the caloric values of macronutrients

• Unit = MJ/kg

Nutrient digestibility

• Includes protein, lipid, and energy digestibility

• Measured directly from differences between the feed intake and fecal output

Crude protein digestibility

• The proportion of dietary protein that is digested and absorbed by the fish, rather than being excreted as feces

• Key parameter in evaluating feed efficiency

• Unit = %

Lipid digestibility

• The proportion of dietary lipids (fats) that are digested and absorbed by the fish, rather than being excreted in the feces

• It is an important measure of feed efficiency, particularly for evaluating fat utilization and its contribution to fish growth

• Unit = %

Energy digestibility

• The proportion of the energy from the feed that is absorbed and retained by the fish, as opposed to being lost in the feces

• It is an important indicator of how effectively the fish convert the energy for growth, maintenance, and reproduction

• Unit = %

Protein efficiency ratio (PER)

• Measure of protein quality that evaluates how efficiently a given protein supports growth

• Defined as the weight gain of an animal per unit of protein consumed

• Commonly used in nutrition studies, especially in animal feed and food protein evaluation

Nutrient retention

The proportion of consumed nutrients that are retained in the fish’s body for growth, maintenance, and physiological functions, rather than being lost through excretion or metabolism

1. Protein retention

2. Lipid retention

3. Energy retention

Protein retention

• The proportion of dietary protein consumed by the fish that is retained in its body for growth and tissue development, rather than being lost through excretion or metabolism

• it is an essential measure of protein utilization efficiency in aquaculture

• Unit = %

Lipid retention

• The proportion of dietary lipids (fats) that are stored in the fish’s body for energy reserves and growth, rather than being metabolized or excreted

• It is a key indicator of fat deposition efficiency

• Unit = %

Energy retention

• Measures how effectively the fish converts dietary energy into body energy for growth and maintenance

• A higher energy retention percentage indicates better feed efficiency and utilization

• Unit = %

Factors influencing nutrient utilization

1. Feed related factors

2. Fish related factors

3. Environmental factors

4. Feeding management

Feed related factors

• Ingredient composition - highly digestible proteins (e.g fishmeal) improve absorption

• Feed processing - extruded feeds with water stability enhance nutrient availability

• Antinutritional factors - Tannins / phytates / protease inhibitors in plant-based feeds reduce protein digestibility

• Feed additives - enzymes (e.g. phytase, protease) can enhance nutrient availability and absorption

• Protein-to-energy ratio - imbalanced protein-energy ratios lead to excessive nitrogen excretion and lower retention

Fish-related factors

• Species-specific digestive physiology - carnivores digest proteins better; herbivores process plant efficiently

• Age and growth stage - Juveniles have higher feed efficiency

• Metabolic rate - fish with high metabolic rates require more energy, affecting retention efficiency

• Gut microbiota - beneficial bacteria in the gut aid complex nutrient breakdown, improving digestibility

Environmental factors

• Water temperature: optimal ranges enhance enzymatic activity and digestion efficiency

• Dissolved oxygen (DO) levels - higher DO improves metabolism

• Water quality (pH, salinity, ammonia levels) - poor water conditions stress fish, reducing digestive efficiency

• Stocking density - overcrowding increases competition and stress, affecting feed intake and utilization

Feeding management

• Feeding frequency and ration size - overfeeding increases nutrient wastage, while underfeeding limits growth potential

• Feeding method - proper feed distribution minimizes feed loss and ensures uniform nutrient intake