However, if there is an excessive occurrence of algae in shrimp farming ponds, it will cause fluctuation in water environment, affecting the health of shrimp.
Harmful effects of toxic algae
Toxic algae are types of algae that directly or indirectly affect aquatic animals such as Cyanophyta, dinoflagellate, euglenid, etc. These types of algae often produce toxins causing necrosis of shrimp liver. At the same time, they also affect quality of water in shrimp farming ponds, making shrimp more susceptible to pathogens.
Cyanophyta is a type of algae that is big with length of several millimeters. When cyanophyta appears much in the pond, it is often seen by visual observationsthat the pond is dark green like the colorof green paint. There will be green scum on pond’s surface. Under hot and strong sunlight, it floats on the water surface and at the end of wind flow. When it floats, it is easy to identify cyanophyta type, filamentous or granular algae. If cyanophyta appears much in the pond, it will make the shrimp smell bad, block shrimp’s carapace as algae releases mucus in the cell membrane. Because of cyanophyta, shrimp may get white feces as algae in shrimp intestinal tract are not digested. Filamentous and granular algae are equally poisonous. Nonetheless the filamentous form is often considered more toxic due to as it is easier to block shrimp’s carapaceand more difficult to digest if shrimp ingests. In addition to causing mentioned situations, cyanophyta can also cause disease in fish, especially fish, which are feed with algae, such as silver carp, large-scale silver carp, mollusc, crustacean and for humans if eaten.
About euglenid, when euglenid develop, pond water has dark brown color like the color of centella. Euglenid can move quickly in water thanks to its flagellum hair at the front of the unicellular body with red eye spots. The appearance of euglenid in farming pond is a signal that the pond started to be contaminated. In intensive farming, it might be due to excessive feed. With other less feeding models, it might be due to unclean water source or pond’s bottom has been contaminated since the beginning euglenid also grow well in conditions of organic ponds. Thereby, it affects dissolved oxygen content in the pond’s water, contaminating the pond environment. When euglenid grow in large amount and high densities, the pond water will get the color of centalla green and even dark brown in some cases.
As for dinoflagellate, when they grow with high density in pond, water in pond will turn into reddish brown color. There will be a lot of reddish brown scum appearing on water. Shrimp do not digest this type of algae because they have hard cell walls. There is some case that shrimp intestinal tractgetting obstruction or disruption when there are too many dinoflagellate cells in shrimp intestine. Another harmful effect of dinoflagellate is that shrimp have to go the surface for breathing due to lack of oxygen in the water at night time and water glows in the dark, affecting the behavior of shrimp in farming pond.
Reasons and solutions to overcome
The main reasons for strong growth of algae are various. It might be because of poor food management, which causes residual food to accumulate on the bottom of the pond, shrimp’s feces during the whole crop, dirty bottom of pond due to improper construction process of the pond. It might be due to unpredictable weather which changes very fast between hot sunshine and nonstop rain. Continuous rain causes salinity of farming pond to decrease quickly and stratify water surface to create best conditions for cyanophyta to grow. Prolonged hot weather together with sudden thunderstorms changes the environmental factors in the pond swiftly. As a result, the process of decomposing organic residue increases. It produces many nutrients to facilitate harmful algae in farming pond.
Handling this situation, farmers need to have solutions to overcome and prevent toxic algae in shrimp farming ponds. Required conditions for algae to grow are nutrient in water and light. And so, lessening nutrient in water is a methodto limit excessive algae growth. There are always some useful techniques to prevent algae growth such as removing of dead algae, changing water in farming pond in case the sedimentation pond has been treated. It is recommended as it helps to reduce the density of algae in farming pond. At the same time, it is necessary to control food without excess feeding and treat algae by probiotics ferment with molasses afterbeing incubated from 3 to 6 hours over night. Combining with algae treatment using lime at night with an allowable dose of less than 20 kg for every 1,000 m3 of water.After applying lime, it is recommended to add zeolite with an amount of 20 kg for every 1,000 m3, suck up mud, regularly siphon of farming pond’s bottom and use algae killer based on CuSO4. Particularly for cyanobacteria, one of the most effective method is increasing salinity for farming pond water by supplying sea water for farming pond or adding salt into water with the amount of 10 kg of salt per 1,000 m3 hanging at propeller. Another technique to handling algae is stocking tilapia together with shrimp in same pond. Tilapia often lives in the middle and bottom level of water in farming pond. It can digest 30 - 60% of protein in algae, especially cyanobacteria and green algae. It helps to stabilize water quality in pond.
In addition, there are some algae preventive methods as followings: farmers should not use phosphate to control water color for shrimp ponds to prevent cyanophyta. Before stocking, it is recommended to treat the pond carefully, destroy presented algae at the bottom soil and around the bank of farming pond, avoid taking water from sources where algae blooms. At the same time, farmers should regularly use metal chain to release nitrogen gas accumulating atthe bottom of farming pond. Pulling chain from bottom eliminates conditions that allowing the algae to develop strongly. It is advisable to take care farming pond carefully daily to detect the appearance of cyanobacteria at the earliest possible. Doing so, the processes of algae treatment will become easier and faster. Handling dinoflagellate, farmers should avoid taking water to raise shrimp during the period when algal blooms from nearby water sources. They are not recommended to change pond water if nearby water sources have algal blooms. If farmers observe that there is a source of water nearby that does not have an algal blooms, it is recommended to select the appropriate water source to supply for farming ponds.
Thien Binh