The death of 8000 fishes discovered in Sungai Kampung Enam in Bachang here last Monday was believed to have been caused by oxygen depletion in the river.
Chief Minister Datuk Seri Idris Haron said oxygen depletion occurred due to slow water exchange rate between the river water and the seawater.
“The gates at the barrage door control center is only half open during low tides. A long term solution to increase oxygen level in the river would be to flush out river water completely into the sea during low tide, so that new seawater can be flushed in during high tide to ensure higher oxygen content,” he said.
Idris said another cause of low oxygen content in the river was the high amount of sludge in the river.
“Thorough maintenance must be carried out to reduce the amount of sludge in the river, and this requires a massive clean-up at the river-bed,” he said during a press conference at Seri Negeri here, yesterday.
For this reason, Idris said the Malacca River Cruises would be stopped for a day or two.
“As long as we provide notice to customers, explaining to them that the river is undergoing scheduled maintenance, it should not be a problem,” he said.
Fishes that died were mostly tilapia hitam (Mozambique Tilapia)(Oreochromis mossambicus, jelanak (jenahak?) (Snapper) (F. Lutjanidae) and keli (Catfish) (Siluriformes).
A partially eaten tilapia discarded by the otters on the road beside the reservoir. Note that apart from the head, the rest of the body is intact. Another partially eaten fish with the same condition lies a few metres away in the background. Photograph by Tan Yee Keat
Location, date and time: Singapore Island, Marina Reservoir near Marina Barrage, along eastern shore; 12 September 2014; around 1830 hrs.
Observation: A group of seven otters left two dead tilapias of at least 20 cm total length on a road about 5 m away from Marina Barrage. The otters had retreated to the reservoir to avoid the observers, but stayed close to the shore. Only the head and cheeks of both fish were observed to be consumed.
Remarks: This group of otters is believed to be the same family that has recently become resident in the Marina Reservoir (Ee, 2014). Their apparent readiness to part with partially eaten prey (instead of carrying these with them) seems to imply that food is abundant and easily obtainable in the habitat. The present observation
suggests that alien species (such as the Mozambique Tilapia) do benefit certain native piscivores (such as the Smooth-coated Otter) as a source of prey (Yong et al., 2014). It is not clear why only the head and cheeks of the tilapias were eaten. Perhaps due to the abundance of food, the otters can afford to be wasteful, and eat only the ‘choice’ parts of their prey.
Ee, D.. 2014. Wild otters raise family around Marina Bay. The Sunday Times. 28 September 2014: top news, 10.
Yong D. L., B. W. Low, A. Ang, M. Woo & C. Ho, 2014. Multiple records of aquatic alien and invasive species in diets of native predators in Singapore. BioInvasions Records. 3 (3): 201-205.
The Tilapia species commonly encountered in ponds, rivers, reservoirs and canals, and even in mangroves, estuaries, and coastal waters, is likely to be the Mozambique tilapia (Oreochromis mossambicus).
Although reddish-coloured mutants are known to occur in the Mozambique and Nile Tilapias, the so-called Red Tilapia raised for aquaculture in Asia are predominantly hybrids descended from crossing Mozambique Tilapia and Nile Tilapia. Red-coloured hybrid Tilapia strains have also been produced by crossing Nile Tilapia with Blue Tilapia (Oreochromis aureus), as well as from crossing Mozambique Tilapia and Zanzibar Tilapia (Oreochromis urolepis). These strains have also been crossed with one another, with other hybrid strains of unclear parentage, as well as with wild-type Tilapia, which means that the ancestry of some Tilapia stocks may be unclear.