Bioenergetics and Energy Utilization in Fish

Bioenergetics, or nutritional energetics, is the study of how animals, including fish, utilize dietary components by studying the balance of energy intake, expenditure, and gain. All living organisms require free energy from their environments to sustain life processes. While non-biological systems can utilize heat energy, biological systems are essentially isothermal and utilize chemical energy to sustain life. Heterotrophic organisms, like fish, obtain this energy through the breakdown of organic molecules in their environment. Bioenergetics investigates the energy changes associated with these biochemical reactions.

Instead of focusing on the complex utilization of all dietary components, bioenergetics simplifies this by examining the partition of dietary energy-yielding components. These components are used for various purposes

• Catabolism (breaking down molecules for fuel)
• Anabolism (using energy to build and store molecules in tissues)

 This approach helps understand how efficiently fish utilize dietary components for energy and growth.

Figure. Showing an alimentary canal of a typical fish

Energy Utilization and Requirements in Fish

The energy partitioning scheme proposed by the U.S. National Research Council (NRC, 1981) provides a framework for understanding energy utilization in fish. Here's how it relates to fish:

• Intake of Energy (IE): Represents the total energy content of the food consumed by the fish.
• Fecal Energy (FE): The energy lost in the feces, indicating the energy from the food that was not digested or absorbed by the fish.
• Digestible Energy (DE): Calculated as IE - FE, representing the portion of energy from the food that is actually digested and absorbed by the fish.
• Urine Energy (UE): Energy lost through excretion in the urine.
• Branchial Energy (ZE): Energy lost through excretion via the gills, primarily as ammonia.
• Metabolizable Energy (ME): The energy remaining after accounting for losses in feces, urine, and gills (ME = IE - (FE + UE + ZE)). ME represents the energy that is physiologically available to the fish for metabolic processes.
• Heat Increment of Feeding (HiE): The energy expended due to the process of digesting and metabolizing food. It represents the energy cost associated with utilizing the nutrients from the diet.
• Recovered Energy (RE): This is the energy retained in the body as new tissue constituents. It represents the energy from the diet that contributes to growth and other anabolic processes.

Factors Affecting Energy Requirements

Several factors influence the energy requirements of fish:

• Water Temperature: As poikilotherms, fish's metabolic rate is significantly affected by water temperature. Higher temperatures lead to higher metabolic rates and increased energy demand.
• Body Weight: Larger fish have a lower metabolic rate per unit of body mass compared to smaller fish. This means that smaller fish require more energy per unit of body weight.
• Activity Level: Fish that are more active, such as those in fast-flowing streams or those that swim for long durations, have higher energy requirements.
• Diet Composition: The composition of the diet, particularly the protein, lipid, and carbohydrate content, influences how efficiently energy is utilized. High-protein diets result in higher HiE compared to high-lipid diets.
• Physiological State: Energy requirements vary depending on the physiological state of the fish, such as growth, reproduction, or starvation. For example, fish require more energy during periods of rapid growth or during reproduction.