OSMOREGULATION - how a fish regulates itís water

by Steve Hopkins

 Koi Talk,  the newsletter of the Hawaii Goldfish and Carp Association, November, 2005

Osmoregulation is vitally important to the fish as it controls water going into and out of itís body. A basic understanding is useful to the fish keeper because it will help avoid quite a few problems.

To understand osmoregulation, we need to understand semi-permeable membranes. A semi-permeable membrane is like a screen partition which lets some things through more easily than other things. When two solutions (for example, two bodies of water with different amounts of salt dissolved in each) are separated by a semi-permeable membrane the weaker solution (the one with less dissolved salts) will always dilute the stronger solution (the one with more dissolved salts).

To visualize this, imagine the semi-permeable membrane to be like a screen. The water molecules and dissolved salts are constantly moving about in the solution and hitting the screen. The salts are larger so they usually bounce off and do not go through the screen. The water molecules are smaller so they usually go through the screen. On the side with the stronger solution there are more salts and fewer water molecules. Therefore, there are fewer water molecules hitting the screen and going through. On the side with the weaker solution there is less salt and more water molecules. Therefore, there are more water molecules hitting the screen and going through. This results in the net movement of water from the side of the membrane with the weaker solution to the side with the stronger solution. The greater the difference between the two solutions the greater the net movement of water from one side to the other.

So, what does this have to do with koi and goldfish? Well, the fish skin, intestine, and especially the gills are semi-permeable membranes. The fish body fluid is a stronger solution than the surrounding pond water. Fish blood (like our own blood) is about 0.9% salts while the pond water is normally almost salt-free. Therefore, there is a constant flow of water from the pond into the fish. The difference in the salt concentration between the body fluid and the surrounding water creates an "osmotic pressure" forcing water into the body. The fish must constantly pump water back out of itís body to regulate the blood salt content. This is osmoregulation.

When the koi keeper adds salt to the pond, the osmotic pressure is reduced, the amount of water entering the body is reduced, and the fish does not have to work as hard to osmoregulate. If the fishís osmoregulation process is not working properly, it swells with excess water and scales stick out instead of lying flat on the skin. We know this condition as dropsy.

Freshwater fish pump water back out of their bodies by constantly excreting a weak urine. They may excrete up to 30% of their body volume each day. The pump is the kidney which takes up fluid from the body, removes the essential ingredients, and excretes the water. Most of the ammonia is excreted through the gills so the main function of the kidney is to remove water. Some medicines and other foreign chemicals may damage the kidney as it tries to reclaim the essential ingredients of the body fluid and, thereby, impair osmoregulation.