Aquaculture in rainy and flood season

During rainy season, floods often occur creating unfavorable weather conditions for shrimp farming: temperature changes during a day and widespread thunderstorms making abrupt changes in water environment, causing shock in shrimps and possible disease outbreaks.

1. Abrupt changes in environmental factors and aquaculture pollution causing shocks in farming species and leading them to stop eating, or even death due to infectious diseases:

- • Water temperature drops suddenly.

- • Rainwater is acidic or leaches out alum from embankments into ponds, which makes pH in pond drops suddenly.

- • Sudden drop in salinity (in brackish water farming)

- • Reduction of dissolved oxygen in the water

- • Reduction of alkalinity of water

- • Rainwater and flood water can leach out alum, fertilizers, plant protection chemicals and toxic waste from fields, orchards or residential areas into rivers, canals and farming ponds.

2. Farming species, ponds and cages can be lost or damaged due to:

- • Rising water level can overflow and destroy embankments.

- • Hurricanes, strong winds, big waves and flows can damage cages.

1. Must have a settling pond and water treatment before pump water into farming ponds:

Settling pond should be about 1/3 to 1/2 farming ponds.
Farming ponds can be changed alternatively after each farming season. Farming ponds can be settling ponds and settling ponds can be switched into farming ponds.
Reserve enough water to replace water in farming ponds if necessary.
Do not breed shrimp with too shallow water levels.

2. Increase aeration, oxygen in the pond bottom, reduce stratifications of temperature, salinity and oxygen in ponds:

Install aeration system: an aerator is estimated to provide enough oxygen for the 2800 shrimps from the release until the harvest.
Install aerators in accordance with technical requirements:
When aerators operate, water has to swirl in the middle of the pond to collect organic humus to the centre.
·The speed of the aeration must reach 80-85 rounds/minute.
·How to test: Run aerators, pour 5-10kg saponin into the pond. If bubbles are concentrated in the middle of the pond, aerators are properly installed.
Install oxygen system in the bottom of the pond with plastic tube or pumice stones.

1. Regularly check water in the pond:

The pH in shrimp ponds is always needed to be 7.5-8.5.
Rainwater is acidic. It can wash out alkaline from edges of the pond and reduce pH of water in the pond.
To limit the decrease of pH when it rains, use lime along edges of the pond before it rains (apply it dryly) with 10 kg lime/100m2. After the rain, use lime with 10-20 kg lime/ha.
Combine aerators to prevent water stratification.

4. Methods of increasing alkalinity in ponds

· Alkalinity suitable for black tiger prawn is from 90 to 130ppm, for whiteleg shrimp is 100-150ppm.

· Mix dolomite lime in fresh water for 24h, and then splash it into ponds at 8-10 P.M. at night.

· Each 1,655g of dolomite lime increases alkalinity of 1m3 water into 1 mg/ml.

· How to calculate the amount of dolomite lime: to increase alkalinity of a 5000m3 pond from 80mg/ml to 90mg/ml:

· The amount of dolomite lime used = 5,000 x 1,655 x (90-80) / 1000 = 82,75kg

· When increasing alkalinity of water in ponds, Viet Linh notices that only increase 1 time 10mg/ml then again, but not too much at 1 time because it will cause shocks in shrimp.

· If the method above does not increase or increases slowly alkalinity, we can use the following method:

· Combine 70% soda lime (NaHCO3) according to the above formula, 30% dolomite lime in fresh water for 24 hours; splash the mixture at 8-10 P.M. at night.

5. Algae management when salinity is lower than 8 ppt

When salinity of the pond is lower than 8 ppt, blue-green algae often appears. Situations happen during that point:

• Algae are often blighted.

• pH changes significantly during the day

• Shrimps are covered with algae.

• The lack of oxygen in early morning often occurs.

• Shrimps with black gill disease and other gill diseases.

Solutions:

• Reduce feed.

• Use BKC 800 with 0,5ppm concentration. Apply it in 1/3 area of the pond, downwind without aerators.

• Take out algae foam that remains after applying treatments for ponds.

• Repeat 2 or 3 times these methods.

• Use probiotic with a combination of zeolite to absorb toxic gases caused by dead algae at the bottom of the pond.

6. Managing turbidity in pond

In pond, turbid water is mainly caused by clay particles. Turbid water affects the following factors:

· Limit photosynthesis of algae in the pond; cause the lack of oxygen and high CO2 concentrations that increase shrimp suffocation.

· Algae are decimated suddenly

· Alluvium clinging in shrimp gills causes swelling or yellow gills.

Increase water changes in ponds and use probiotics.

07. Managing toxic gases NH3, H2S, CH4

Avoid excess feed.
There is a need of treatment pond to change water frequently in the last months.
Using probiotics regularly.
Siphon the bottom of the pond to take out waste.
Increase aeration systems.
Stabilize pH from 7.8 to 8.2
Measure the concentration of NH3, H2S, NO2- (NO2- often appear in ponds with salinity <10 ppt).

8. Check shrimp activity and water environment after the rain:

Check the appearance, color, and reflectivity; check shrimp gut and shrimp feed in the farm, etc.

Check pH, alkalinity, turbidity, and salinity.

9. Feeding according to schedule and reducing feed intake when it rains or when the rain is coming

As soon as it is cloudy and about to rain, the amount of feed should be reduced or even stopped if the rain approaches. Wait until the rain ceases and feed with 30-50 percent feed fewer than the normal amount.
To ensure shrimp resistance and prevent soft-shelled shrimp, synthetic vitamins, minerals and vitamin C can be mixed in feed every day.