Metabolism, Energy Use and Feeding Behaviors in Fish

Metabolism can be described as the collective term for the
chemical processes that give life. Metabolism uses products
called metabolites that include organic food and inorganic matter
such as oxygen. Metabolism is linked to all of the other body
processes by providing energy, or by building and maintaining the
structures necessary for them to function.

There are two types of metabolism. Catabolism (pronounced
ca-tab-o-lism) breaks down the metabolites that produce energy
for activity. This process releases energy by breaking down
complex molecules into simpler ones. Catabolism is also known as
destructive metabolism. Anabolism (pronounced a-nab-o-lism) uses
metabolites to build new tissue for healing, growth and
reproduction. This process uses energy to construct complex
molecules from simpler ones. Anabolism is also known as
constructive metabolism.

There are many similarities in fish metabolism and energy
usage to that of other animals. Some aspects are unique to
animals that spend their lives “under the sea”. After all, they
are depended on water for locomotion, respiration, maintaining
body temperature and blood chemistry among other things.
Understanding energy metabolism and the factors that influence it
is crucial to stress management and handling of fish.

Energy metabolism that uses oxygen is called aerobic
metabolism. Aerobic metabolism is highly efficient and
sustainable. Anaerobic metabolism does not require oxygen and it
quickly depletes energy reserves in the cell. Anaerobic
metabolism occurs in situations that require sudden bursts of
energy such as escaping a predator. Anaerobic metabolism is not
sustainable. Fish need a continual, sufficient supply of oxygen
to balance energy supply with demand.


Energy intake from food falls into three categories. Gross
Energy or GE is the total energy released by food as measured
with a calorimeter. Food can contain a high level of GE and still
not have nutritional value to an animal if that food is not in a
form that the animal can digest and utilize. The Digestible
Energy or the DE of food is the amount that is utilized and
digested, minus the portion that ends up in the feces. In fish,
some DE is lost through the urine and across the gill membranes.
The remaining energy actually used by the animal is the
Metabolizable Energy or ME.

Removing and/or reducing all sources of stress is essential to
how fish utilize their energy. Stress can disturb the normal
physiological equilibrium or homeostasis of the animal by forcing
a reallocation of energy within its system. Any response or
adaptation to stress requires energy that could otherwise be
utilized for maintaining normal body functions such as growth,
digestion, disease resistance, healing and reproduction (Barton
& Iwama, 1991).

What does metabolism in fish depend upon?

  • Nutrition and respiration for metabolites
  • Osmoregulation to provide a stable working environment
  • Excretion to remove useless or poisonous waste

Energy deprivation is the central concern. Sufficient oxygen
is required for cellular energy. Without enough energy to power
osmoregulation and other functions fish will die. This can take
the form of delayed mortality syndrome. A lack of sufficient
oxygen or food for fuel does not directly kill the animal; it is
the lack of energy and the inability to regain lost reserves.

Each species of fish should receive foods that immolate their
natural diet as closely as possible. It is the responsibility of
the aquarist to research the dietary needs of each animal prior
to purchase. Consider the natural feeding frequency and style.
Example: is the fish a predator, grazer or planktivore?

Water surface agitation and brisk current that varies in
direction is important for maintaining a high oxygen level,
removal of toxic waste and good gas exchange in our

Osmoregulation typically consumes 25 to 50% of the total
metabolic energy output in fish (Morgan & Iwama, 1999.
Laiz-Carrion, et., al, 2002). Osmoregulatory dysfunction
is an inherent part of stress in fish (Harrell & Moline,
1992. Weirich et., al, 1992). Epinephrine released during the
stress response increases blood flow to the gills to provide for
the increased oxygen demands of stress. The elevated blood flow
to the gills causes dilation of gill blood vessels and increased
use of vessels that are normally not used at rest. This increases
the surface area of the gills that is available for gas exchange,
but in saltwater fish this also leads to accelerated ion influxes
and water losses. In freshwater fish the reverse occurs, i.e.
water influx and ion losses are increased. This is the phenomenon
known as the osmorespiratory compromise (Folmar & Dickhoff,

Four important body functions are closely associated with
processes in the gills:


  • Gas exchange
  • Hydromineral control (osmoregulation)
  • Acid-base balance
  • Removal of nitrogenous waste

Two important byproducts of metabolism are carbon dioxide and
ammonia. Along with excreting wastes via digestive processes, the
gills play an essential role in the removal of useless or
poisonous waste products. The gills excrete eighty to ninety
percent of nitrogenous waste. Healthy gills are essential to
metabolism for normal gas exchange, osmoregulatory balance,
acid-base balance and the removal of nitrogenous wastes.

What affects the rate of metabolism in fish?

  • Hormones such as cortisol
  • Environmental conditions: temperature, salinity, oxygen
  • Level of the animals activities
  • Size of the animal: larger fish have a lower metabolism
    rate per unit of weight
  • Age because of growth and reproduction energy costs
  • Health or condition: repair consumes energy

A high level of cortisol (a stress hormone) in the bloodstream
increases metabolism as it accelerates the energy demand for
osmoregulation. It can also disrupt digestive processes and
feeding behaviors of fish.

The amount of oxygen available affects the rate of metabolism.
Osmoregulation requires energy provided primarily by oxygen in
aerobic metabolism. Metabolism and oxygen demand increases as the
water temperature rises. At the same time, the oxygen carrying
capacity of water declines as the temperature increases. Large
temperature changes slow metabolic recovery and lactic acid
removal (Kiefer, Currie & Tufts, 1994).

Age is a factor in metabolism, as young fish require a large
portion of energy for growth. Reproduction consumes a
considerable amount of energy as well. Larger specimens will have
a slower metabolism than their smaller counterparts will. Marine
fish do require a saline environment. However, the more saline
the environment is the more energy is required in osmoregulation,
thereby increasing the metabolism rate.

Species that are active swimmers consume more energy in
locomotion than inactive or sedentary fish. Keeping the lighting
low and providing a sufficient amount of hiding places can reduce
activity. Avoid increasing the metabolism rate when keeping fish
in an aquarium without a fully matured biological filter. This
will help control the amount of ammonia produced.

Fish that are ill or injured consume a portion of their energy
for healing and immune function that is not necessary for animals
in good condition and health. Compromises in the mucus/scale/skin
barrier are also believed to increase the amount of energy
required in osmoregulation.



The food energy requirements of fish are only about ten
percent of what is necessary for mammals and birds (Smith, 1989).
This is due in part to the fact that fish are exothermic (cold
blooded) so they do not expend energy for maintaining body

Feeding behaviors:

  • Appetite
  • Visual and chemosensory ability
  • Restricted area searching
  • Responding to and capturing prey
  • Handling and ingestion of food

Fish rely on their sensory abilities for cues that alert them
to the availability of foods. These sensory abilities include
olfactory (taste and smell), hearing and visual cues. Stress can
inhibit these sensory abilities and it has been observed to
disrupt any or all of the components of feeding behaviors
(Beitinger, 1990). Stress can also cause digestive processes to
cease temporarily.”(Mazeaud & Mazeaud 1981).

Fish normally search areas within their territory or aquarium
where they feel safe and have found food before. Stressed or sick
fish can have a reduced appetite and simply are not hungry
despite the need for nutrition. Toxins or other forms of stress
can impair the ability of fish to taste, smell or visually
recognize foods. Stress can inhibit the response to prey (or
other food) and the ability of fish, including swimming ability,
to capture prey. Have you have ever seen a fish take food into
their mouth only to spit it out again? This is an example of not
properly handling or ingesting food. This fish may or may not
have an appetite, it did taste, smell or visually recognize the
food, it probably was searching, it responded to and captured the
food, yet it did not handle or ingest the food properly.

Fish tend to recover feeding activities when they regain
normal homeostasis or equilibrium. The timeframe it takes fish to
recover feeding behaviors depends on the severity of the stress
and the physiological state of the fish. There is a correlation
between the resumption of feeding behaviors and the
re-establishment of normal physiological status (homeostasis).
When cortisol (stress hormone) in the blood returns to a
pre-stress level the fish usually begin to eat again.

Stress and high temperature increase oxygen consumption.
Digestion also requires oxygen and energy. Increased oxygen
consumption during digestion is referred to as a phenomenon
called “Specific Dynamic Action” or SDA (Yu, 2004). It is a good
idea to withhold feeding or at least to reduce the amount of food
offered when the water temperature is high, because the oxygen
demand could exceed the supply (Stevenson, 1987).

Factors influencing feeding behaviors:


  • Overall health
  • Security
  • Temperature
  • Photo-period
  • Osmoregulatory balance

Eating is an indicator of health and environmental conditions.
Digestion requires a lot of energy and it increases oxygen
consumption. Sick fish may be expending a great deal of energy
for healing with a limited amount of energy available. If they
are using a large portion of their metabolic energy for healing,
regaining normal homeostasis and osmotic balance, etc., then less
will be available for digestion.

Adjusting to captivity in a quarantine tank where the fish do
not have to compete for food or deal with less than peaceful
tankmates can help them recover feeding behaviors sooner. Fish
must feel unthreatened and safe from predation or other dangers
before they will risk coming out of hiding to eat. Providing
plenty of places to hide, keeping the lighting low and staying
away from the aquarium will help newly acquired fish to feel (if
feel is the right word) less threatened in a new environment.
Species that exhibit schooling behaviors may adjust and begin
eating sooner when they share an aquarium with a shoal. Seeing
other members of the shoal eating can help them to recognize
foods that are new to them.

Metabolism and oxygen consumption increase as the water
temperature rises. To complicate matters, the oxygen carrying
capacity of water falls as the temperature increases. Keep the
water temperature close to that found in the animal’s natural

Active fish are searching out food and aware of feeding
opportunities. Many species respond to changes in light intensity
and photo-period with behavioral modifications. This includes,
but is not limited to, seeking shelter or hiding and becoming
more or less active.

Fish expend a large portion of their metabolic energy in
osmoregulation. Stress induces osmotic dysfunction in fish.
During periods of osmotic disturbance, less energy is available
for digestive processes.

Tips for encouraging feeding behaviors:

  • Remove or reduce sources of stress.
  • Create an environment that imitates the natural habitat of
    the species as closely as possible.
  • Reduce the lighting for timid fish.
  • Consider each fish’s feeding style. Is it s top-level,
    mid-water, or bottom feeder?
  • Offer fodder that the fish may recognize as food by
    emulating the species natural diet whenever possible.
  • Fish are attracted to foods by movement (i.e. live foods),
    bright colors, smells, tastes and even sounds.
  • Adding fish oils, anise oil or garlic to the food and
    adding vitamins to the water may help stimulate a feeding


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  Advanced Aquarist

 Terry D. Bartelme

  (16 articles)

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