Charles Lutz | 10/4/2004 4:24:34 AM
Heavy rainfall and winter feeding tend to add up to conditions that can lead to brown blood disease in the spring. Brown blood is caused by nitrites (one of the results of winter feeding). Growers can avoid the fish losses caused by nitrites through diligence and the application of chlorides.
To understand why nitrite levels are a problem in the late winter and early spring, we need to understand where nitrites come from.
Catfish producers are continuously adding nitrogen to their ponds by feeding their fish. Not all of this nitrogen stays in the fish. A large portion enters the pond as fish waste, in the form of ammonia. Ammonia is broken down in ponds by two separate bacteria. One converts ammonia into nitrite, which is toxic. The other bacteria convert nitrite into nitrate, which is non-toxic. During the winter, ammonia tends to accumulate in ponds because the water is too cold for these bacteria to function. In extreme cases, this ammonia can contribute to winter kill problems. As water temperatures begin to warm in the spring, the bacteria that turn ammonia into nitrite “wake up” long before those that convert the nitrite into nitrate. As a result, until pond temperatures are fairly warm, nitrite levels may increase to a point where brown blood becomes a problem.
Over the years, farmers have found that adding salt to production ponds helps prevent or reduce problems with brown blood.
This practice is based on the idea of adding enough chlorides to the ponds (salt is just sodium and chloride) to protect fish from taking too much nitrite across their gills and into their blood. This is important because once a nitrite ion enters the bloodstream, it binds with a red blood cell and prevents it from carrying oxygen to the rest of the body. The result is similar to carbon monoxide poisoning, although more gradual. Only so many ions can cross a catfish’s gill surface and enter its bloodstream, and chloride ions in the water compete with nitrite ions for the available “spaces” along the gill surface.
So, adding salt to a pond can work well if a grower can get the ratio of chlorides to nitrites high enough. How high is high enough? Well, over the years the answer to that question keeps going up. At first, researchers recommended a 3:1 chloride to nitrite ratio. In no time at all, though, stocking and feeding rates (and, therefore, nitrogen levels) increased substantially. The new recommendation went up to 6:1. Further studies in laboratory and real-world settings now suggest that a 10:1 ratio is required to sufficiently protect catfish from brown blood. A ratio of 16:1 completely suppresses brown blood formation, but the low levels of brown blood found at rations of 10:1 or higher can be tolerated by healthy fish.
So, back to the question: How much salt is really needed?
It is not uncommon for nitrite levels to reach 10 ppm, so a safe level of chlorides to maintain would be 100 ppm. Unfortunately, nitrites can jump to levels as high as 20 ppm for the reasons described above. If this happens in one of your ponds, it may be possible to add more salt. Another option is to keep the fish in a stress-free environment and wait for the nitrites to drop back to more normal levels. That means no seining and no D.O. levels below 4. Remember that even if brown blood does not kill fish directly, it can weaken their resistance to disease, especially during ESC season.
Of course, if you don’t know what your chloride and nitrite levels, none of this discussion will do you any good.
Remember that chloride levels may drop substantially from their fall values due to dilution and flushing by winter rains. To avoid being caught off guard, find out what your chloride levels are now, and figure out how much salt you need to reach 100 ppm in your ponds. Test kits for chlorides and nitrites, or consultants willing to do the work, are readily available to just about anyone growing catfish. If you need advice on calculating the volume of salt you’ll need, ask your County Agent or Fisheries Agent for