Giant Freshwater Prawn (Macrobrachium rosenbergii), locally known as Udang Galah or Udang Jepit, commands some of the highest market prices in Sarawak’s seafood sector (RM 70 to RM 120+ per kg). However, unlike hardy pond fish species, prawns are bottom-dwelling organisms (benthic) that are exceptionally sensitive to fluctuations in dissolved oxygen, pH drops, and toxic ammonia build-up. Successful production relies heavily on maintaining stable water chemistry and ensuring bottom-layer oxygenation.
Ideal Water Quality Parameters for Udang Galah
To support healthy growth, fast molting cycles, and low mortality rates, maintain your earthen or lined ponds within these target ranges:
| Parameter | Target Range | Critical Threshold | Management Impact |
| Dissolved Oxygen (DO) | 5.0 – 7.0 mg/L | $< 3.0\text{ mg/L}$ | Bottom layer DO is vital; low oxygen causes mortality and cannibalism. |
| pH Level | 7.2 – 8.5 | $< 6.5\text{ or } > 9.0$ | Acidic water soft-shells prawns and delays molting cycles. |
| Total Alkalinity | 80 – 150 mg/L | $< 50\text{ mg/L}$ | Buffers pH swings and provides carbonate for shell hardening. |
| Water Temperature | 28°C – 31°C | $< 24°C\text{ or } > 34°C$ | Regulates metabolism and feeding rates. |
| Transparency (Secchi Disk) | 30 – 40 cm | $< 25\text{ cm (bloom risk)}$ | Indicates healthy phytoplankton levels for shade and natural balance. |
| Un-ionized Ammonia ($NH_3$) | $< 0.05\text{ mg/L}$ | $> 0.1\text{ mg/L}$ | High levels burn gills and trigger mass mortality. |
1. Oxygen Management in the Bottom Zone (Benthic Layer)
Prawns spend 90% of their time crawling along the pond floor. Traditional surface paddlewheels often aerate only the top 0.5 meters of water, leaving the bottom layer oxygen-starved.
- Install Bottom Air Diffusers: Pair surface paddlewheels with bottom aeration systems (micro-porous diffuser tubes or Venturi injectors) to drive oxygen directly to the pond bed.
- Night-Time Aeration Schedule: Run aerators continuously from 10:00 PM to 6:00 AM. During the night, microscopic algae switch from producing oxygen to consuming it, creating a severe oxygen crash just before dawn.
- Prevent Sludge Accumulation: Position paddlewheels to create a gentle circular current. This pushes unconsumed feed and fecal waste into a central collection zone, preventing toxic anaerobic sludge across the rest of the pond bottom.
2. Countering Sarawak’s Acidic Soil & Rain Drops
Sarawak’s high rainfall and acidic soil profiles (particularly near peatland zones or acid-sulfate clay regions) present constant risks of sudden pH drops during heavy monsoon downpours.
- Regular Liming: Broadcast Agricultural Lime (GML – Ground Magnesium Limestone) or Dolomite at $200\text{–}300\text{ kg per acre}$ after heavy rain events to prevent the water pH from plummeting.
- Maintain Calcium Levels: Prawns require calcium and magnesium dissolved in the water column to absorb immediately after molting. Low alkalinity causes “Soft-Shell Syndrome,” leaving freshly molted prawns vulnerable to predators and cannibalism.
- Dike Perimeter Bunding: Build raised earth dikes around the pond perimeter to prevent acidic land runoff from washing directly into the water during heavy storms.
3. Managing the Molting Cycle & Reducing Cannibalism
Udang Galah grow by shedding their hard exoskeletons every few weeks. In a soft-shelled state, a prawn cannot defend itself against larger tankmates.
- Install Submerged Shelters: Provide ample hiding spaces by sinking vertical shade netting, plastic mesh, or bundles of clean PVC pipes throughout the pond. This increases the total living surface area and gives molting prawns safe refuge.
- Never Aerate or Feed Irregularly: Sudden stress from low DO or skipped feedings triggers aggressive cannibalism, where hard-shelled dominant males (Blue Claw males) prey on soft-shelled prawns.
4. Pre-Harvest Flushing & Purging
- Purge Off-Flavors: 48 hours prior to harvest, transfer prawns to clean holding tanks supplied with continuous freshwater flow and aeration to remove any muddy taste (geosmin) absorbed from earthen pond beds.
- Cool Morning Harvest: Harvest during cool early morning hours using seine nets. Place live prawns immediately into chilled, oxygen-infused holding water (20°C–22°C) to slow their metabolism during transport to buyers.
Source Aquaculture Equipment & Post-Larvae in Sarawak
Are you sourcing high-quality Udang Galah post-larvae (PL), paddlewheel aerators, DO testing meters, or water treatment lime? Connect directly with verified hatcheries and aquaculture equipment suppliers across all 12 Sarawak divisions on SarawakAgro.com!