Feeding Affects Pond Water Quality

Article : Claude E. Boyd, Ph.D.
Professor, Department of Fisheries
And Allied Aquaculture
Auburn University
Auburn, Alabama 36849
E-mail : boydcel@auburn.edu

Article copied from : Global Aquaculture
June 2004, Pg.29-30

Feed is the most important and costly ingredient in most types of pond aquaculture. Feeding allows much greater production of shrimp than possible from natural feed webs within pond ecosystems. However, the nutrients in feed are not converted completely to shrimp flesh.
Uneaten feed, feces, and metabolic wastes pollute pond water and cause its quality to deteriorate. The capacity of ponds to assimilate wastes can be increased by mechanical aeration, but there is limit to the amount of feed that can be used efficiently in ponds. Exceeding this limit impairs water quality, stresses culture animals, increases mortality, and lessens feed-conversion efficiency.

Fate of feed

Only a portion of the feed applied is eaten. Shrimp nibble on feed pellets, and up to 40% of their feed may not be consumed directly. About 70-80% of the feed eaten by shrimp is absorbed across the intestine, and the remainder becomes feces.
Absorbed nutrients and used in metabolism and, to a lesser degree, growth. Water, carbon dioxide, ammonia, phosphate, and other basic compounds are excreted as metabolic wastes.

Effects of Feeding Wastes

Wastes from feeding have a tremendous influence on pond water quality. Microorganisms decompose organic matter in uneaten feed and feces easily. However, the decay process removes dissolved oxygen from the water and releases carbon dioxide, ammonia, phosphate and other mineral nutrients. The culture species (shrimp) also consumes dissolved oxygen and excretes minerals similar to those generated by decomposition.
Ammonia and phosphate stimulate the growth of plankton and benthic algae. These organisms produce dissolved oxygen through photosynthesis in daytime, but consume oxygen continuously. Algae have a life span of few days, so they die continually and contribute organic matter to ponds.
Algal abundance in ponds increases as feed input increases. Ponds with dense blooms of plankton algae typically have high dissolved-oxygen concentrations during the day, but at night, lower dissolved oxygen can stress or kill the culture species.
When feeding rates exceed 30-40 kg/ha/day, dissolved oxygen concentrations often fall to less than 3-4 ppm at night. Low dissolved oxygen stresses most culture species, so at high feeding rate, mechanical aeration is necessary. Dissolved oxygen levels should be monitored, even in ponds with mechanical aeration, to avoid excessive feed input.
Water exchange sometimes is used to flush plankton and nutrients from ponds and improve water quality. However, this practice can lead to water quality deterioration in receiving water bodies.
Ammonia can increases to toxic concentration in ponds with high feeding rates. Moreover, if sediment in ponds becomes highly enriched with organic matter, it can become anaerobic. Nitrite and hydrogen sulfide originating in anaerobic sediment also can be toxic to aquatic animals.

Waste Assimilation

Ponds have a large capacity to assimilate the wastes that result from feeding. As stated above, bacteria mineralize organic matter to carbon dioxide, ammonia, and phosphate.
Ammonia is lost to the atmosphere by diffusion. It also is oxidized to nontoxic nitrate by nitrifying bacteria (Bacway). Nitrate can be denitrified to nitrogen gas, which diffuses in to the air.
Carbon dioxide is converted to organic carbon by photosynthesis or diffuses from pond water to the atmosphere. Bacteria can transform carbon dioxide in sediment to methane, which also diffuses to the atmosphere. Sediment usually has a large capacity to fix phosphorus in insoluble iron, aluminum, and calcium phosphates. Some of the organic matter in ponds resists microbial decay and accumulates in sediment as stable or…………………