How Fish Metabolism Works

By PetMD Editorial on Nov. 15, 2010

What Makes Up Fish Metabolism?

“Metabolism” is the word used to cover the system of chemical processes that keep something alive. For a fish, that means providing energy to power critical body processes or building and maintaining the body parts needed to function.

Metabolism itself relies on three main things:

  1. Respiration and nutrition to supply metabolites (the products it uses, built out of both inorganic and organic matter)
  2. Osmoregulation for a stable working environment
  3. Excretion to get rid of all the poisons and other waste products produced as side-effects

In fishes, the metabolism covers two processes: catabolism and anabolism. Catabolism is the process of breaking down metabolites to produce active energy, while anabolism uses those same products to build new body tissue for growth, maintenance, and reproduction.

The metabolism can work at different speeds, depending on environmental conditions, and is controlled by hormones produced in the fish’s body. The metabolic rate can change with a variety of factors:

  • Size – bigger fish have relatively slower metabolic rates
  • Age – young fish grow more but don’t need the reproductive side yet
  • Activity – busy fish need a faster rate
  • Condition – fish in poor condition need more tissue maintenance
  • Environment – temperature, oxygen levels and salinity all affect the rate

If everything is normal in a fish’s environment, it produces energy by oxidation. This requires a constant supply of enough oxygen. If there isn’t enough, the fish will produce energy in white muscle tissue using “glycolysis” -- adrenaline stimulates the tissue and causes glycogen to be converted to glucose and energy without the need for oxygen. Unfortunately, this also produces poisonous lactate, so glycolysis can only be sustained for short periods. Oxygen and energy will also be needed to break down the lactate, so it’s a sort of “oxygen debt” in times of emergency.

If the fish’s environment is low-stress, stable, free of disease and supplied with everything required, excess energy can be used for growth and reproduction. In general, only the excess is used for these purposes, so good growth and active reproductive behavior are positive signs that favorable living conditions are being maintained.

At the other end of the process, waste products generated by using metabolites are excreted from the fish’s body. All waste is toxic, whether produced in energy creation or tissue growth and maintenance. Most of this waste consists of carbon dioxide and ammonia (both of which are ejected through the gills by diffusion), water and some larger molecules like purine, which eventually becomes urea and is removed with water by the kidney.

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