Biological Modification of the Culture System in Aquaculture

Biological Modification of the Culture System

Biological Filtration

Biological filters facilitate the purification of the water in high density, semi-closed or closed aquaculture facilities by the oxidation of ammonia to nitrite and nitrite to nitrate. Ammonia oxidation is accomplished by Nitrosomonas while nitrite oxidation to nitrate is completed by Nitrobacter. Both nitrite and ammonia are highly toxic compounds. The well being of the animals in the culture system depends on the ability of the biofilter to rapidly convert ammo-nia to nitrate which is relatively less toxic.

Ammonia is a ubiquitous by-product in an aquatic environment. It is the main excretory product of water-breathing animals. It is also the end product of the decay of organic matter. In newly established closed system the accumulation of dangerous levels of ammonia and nitrite might occur since it takes time for the biofilter to become completely colonized by Nitrosomonas and Nitrobacter. Convenient sources of nitrifying bacteria are rich garden soil (for freshwater facilities) and gravel from existing well-established biofilters. Tap water or un-filtered seawater contains these bacteria but only in small numbers. Commer-cial bacterial inoculates for the biofilter are also available.

Probiotics

Historically, probiotics are a group of food and feed products for both human and animal consumption and are also known as direct fed microbials. A Rus-sian scientist who attributed the longevity of a group of Bulgarians to their consumption of fermented milk products (yogurt) first wrote the concept of probiotics about in 1908. In 1960, an Oregon microbiology professor first used the term “probiotic,” meaning for life as opposed to “antibiotic,” or against life.

In aquaculture, commercially available probiotic products contain bacterial in-ocula not for consumption of the fish but for the environment. Species of Bacil-lus are most commonly used, but species of Nitrobacter, Pseudomonas, Enterobacter, Cellulomonas, Rhodopseudomonas, and photosynthetic sulfurbacteria or their combination has been used as inocula. Some probiotic prod-ucts contain enzymes or plant extracts without live bacteria.

Manufacturers of probiotics for aquaculture claim that the mode of action of their products is to enhance natural processes such as organic matter degrada-tion, nitrification, ammonia removal, denitrification, sulfide oxidation, and degradation of toxic pollutants. They further claim that increasing the abun- dance of useful bacteria, competitive exclusion of undesirable species, includ-ing pathogenic ones, occurs.

Polyculture

The practice of keeping several of different species in the same pond is polyculture. One of the objectives of using polyculture is to better utilize avail-able foods in the pond. Another objective would be to control unwanted com-peting offspring of a cultured fish by providing a predator species. Polyculturing fish and prawns with filtering organisms to reduce phytoplank-ton, bacteria, and organic particles had been done on the experimental scale. Oyster, clams, macroalgae, and bloodworms are often utilized to accomplish this clean up. It is also suggested that a pond or tank containing a more varied population of fish or shellfish will also harbor a more diverse bacterial flora. This diversity may prevent the dominance of any bacterial species specially the opportunistic facultative pathogens.

Phage Therapy

Many antibacterial agents have through time aided the human race in fighting bacterial diseases. In the last part of the 1800’s and in the early 1900’s scien-tists have discovered viral particles that had a killing effect on bacteria. These particles were named bacteriophage, or phage, for their ability to “eat” bacte-ria. Phage therapy is the use of bacteriophage to treat bacterial disease. The development of antibiotics in the 1940’s directed attention away from bacte-riophages as a mode of treatment. However, the recent concern regarding bac-terial resistance to antibiotics has prompted a renewed interest in bacterioph-age as biological control of bacterial pathogens.

Each kind of bacteria has its own phages, which can be isolated wherever that particular bacterium grows - from sewage, feces, soil, the sea, ocean depths and hot springs. Phages are specific to their bacterial host because of a particu-lar protein antibody on the surface of the bacteria that a phage will specifically bind to. Since phages are specific they cause much less damage to the normal microbial balance of the host. Moreover, phages replicate within the body of the infected animal thus requiring only a single dose of phage that multiply only as long as the target bacteria are present.

Most of the work on phage therapy was conducted in Eastern Europe and all these concentrated on common human bacterial disease. Although phage therapy is still not practiced in agriculture and aquaculture, it shows great po-tential as a safe, specific and cheap alternative to chemotherapeutics.