THE IMPORTANCE OF DISSOLVED OXYGEN IN PACIFIC WHITE SHRIMP CULTURE
THE IMPORTANCE OF DISSOLVED OXYGEN IN PACIFIC WHITE SHRIMP CULTURE
At present, industrial scale marine shrimp producers in almost every country produce the same kind of shrimp - Pacific White Shrimp, or White Leg Shrimp. Production of Black Tiger Shrimp is dropping every year in proportion to the increase in Pacific White Shrimp production. The main countries that still produce significant amounts of BlackTiger Shrimp are Bangladesh, Vietnam, India and the Philippines. Other than that, all the leading shrimp producing countries, such as Thailand, China and Indonesia, produce almost exclusively Pacific White Shrimp. In Vietnam and Indonesia production of Pacific White Shrimp is also rising every year while production of Black Tiger Shrimp is dropping. The main reason for the popularity of this variety is that long years of genetically breeding have resulted in a variety that is easy to raise, even in intensive conditions, grows quickly and has consistent size. By contrast, Black Tiger Shrimp culture produces inconsistent results, even though it is now possible to obtain breeding shrimp domesticated brooders rather than from the wild. The economic performance of Black Tiger Shrimp cannot compare with Pacific White Shrimp.
Almost every country produces the same kind of shrimp and for the same objective - export sales. The only exception is Brazil, which Pacific white shrimp mainly for domestic consumption, because the price is quite high. China also produces significant amounts of shrimp for the domestic market. In the future it is likely that China will have to import more shrimp due to the high rate of economic expansion in China. By comparison, most of the world's other major economic powers, such as the USA, the EU and Japan, are all experiencing economic downturns. It will probably take a long time before their economies grow strong again. These countries are the major purchasers of imported shrimp. It is unlikely that there will be a strong increase in demand for shrimp and that prices will rise in the near future, unless there are some unexpected problems, such as disease outbreaks or extreme weather, in some of the major shrimp producing countries, leading to reduced supply of shrimp in the world market. For now, shrimp producers need to find ways to insure that their yield is certain, within the capacity of their production system, and that they can control production costs to remain competitive.
It is well known that feed costs attribute 40-50% of the total costs of shrimp production. Thus, efforts at cost reduction often focus on controlling the amount of feed given to the shrimp so that there is no excess or unnecessary waste. Proper feed management will maintain the good water quality in the cultured ponds leading to a successful yield. However, good food management is not the only factor required to insure that the shrimp grow well. If the amount of dissolved oxygen (DO) in the pond is too low, the shrimp will eat less and there will be more left over food. This will affect other parameters of water quality, and if the water quality drops the shrimp will get weak and more susceptible to bacterial infection. Such conditions lead to low survival rates. When the DO of the water is low, the shrimp's growth rate is slow and the feed conversion ratio (FCR) is very high. It takes longer to raise a batch of shrimp than when the DO is sufficient. You can see that in 2010-2011 there were many more problems with bacterial diseases than before. In the past, bacterial infection was seldom a problem with Pacific White Shrimp culture. This indicates that there is something wrong or something lacking with the aquaculture management practices used at present. Usually it is a problem with overfeeding. Excess feed causes a build up of waste materials in the pond, which encourages the growth of disease-causing Vibrio bacteria. When the oxygen or DO level in the pond drops over the course of the culture period, the shrimp are likely to get sick.
The most appropriate DO for shrimp health and normal growth is from no less than 4 mg/l (ppm) up to the saturation point. The amount of oxygen that can remain dissolved in the water depends on the water temperature. When the temperature is higher, the water can hold less oxygen. For shrimp culture it is best to maintain the DO at 6-8 mg/l, or no less than 4 mg/l all the time. In reality, on most farms the DO only stays this high during the first month or two of the culture period. On some farms, once the shrimp are 30 days old the night time DO, from about midnight to early morning before dawn, goes down to less than 3 ppm. This situation is found on extensive culture farms with no aerators and on semi-intensive culture farms or sometimes on intensive farms or sometimes on intensive farms.
On most shrimp farms, during the day time the DO level is suitable at over 4 ppm. On sunny days, the phytoplankton in the water photosynthesize and give off oxygen. On farms with large populations of phytoplankton in the water, the DO may rise to over 10 ppm in the afternoon. This is often the case on intensive farms with low salinity and good plankton bloom, where the aerators are turned off during the day when it is sunny. However, even on farms where the day time DO is quite high, after about 21:00 at night the oxygen level starts to drop because of the respiration of the plankton, the shrimp and the micro organisms that are decomposing organic matter. The DO drops to very low from about midnight to early morning. In the pre-dawn hours the DO may be down to 3-4 ppm, 2-3 ppm or even less than 2 ppm on some farms. This is also the case on extensive farms with no aerators and low density of shrimp, and on semi-intensive farms with 30-60 shrimp per square meter and about 6-18 hp per hectare of aeration. The DO usually drops to about 2-3 ppm at night, or even lower at times. The shrimp grow slowly, the survival rate is low and the FCR is high. The shrimp grow better on farms where the oxygen level is higher.
