Feature Article: 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 products
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 aquariums.
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, 1980).
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
- 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 temperature.
- 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
- 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 habitat.
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 response.
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- Yu, S. Belokopytin. "Specific Dynamic Action of Food and Energy Metabolism of Fishes under Experimental and Natural Conditions," Hydrobiological Journal, 40, Issue 1, 2004