Mysterious catfish cluster in Canberra Street canal: Experts weigh in on curious mass movement
The sagor catfish is a marine species native to Singapore and other countries in the region, and it can sting when threatened.
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SINGAPORE: An unusual sight in a quiet canal along Canberra Street in the Yishun-Sembawang district drew the attention of passers-by in the middle of last week.
A large black mass was seen moving inland from the sea along the bottom of the canal last Wednesday (Dec 10) morning as water levels remained low.
A closer look revealed distinct shapes of fish, each one crowding over the other in a dense, moving cluster.
Experts told CNA that the fish were likely sagor catfish, or Hexanematichthys sagor, a marine species from the Ariidae family native to Singapore and other countries in the region.
Canberra resident Regine Tan, who filmed a video and posted it on the Singapore Wildlife Sightings Facebook page later that day, was astonished by the phenomenon.
The 41-year-old financial adviser said: "We were so amazed at how the fish queue up to swim from one place to another in such an orderly manner. And we have never seen such a huge number of fish all at one go."
Her video quickly drew similar reactions, with some people quipping that the fish were leaving en masse due to the country's cost of living.
Other residents said that the phenomenon was not new.
Inventory assistant Mohamad Salleh Saidali has seen this happening on occasions over the past few years, having observed the canal's biodiversity from the window of his flat.
"I have been watching them from time to time. They usually come when the tide is going up," the 43-year-old said.
The smaller canal along Canberra Street is connected to a larger canal along Yishun Avenue 8 and Simpang Kiri Park Connector that opens out to sea, from which the school of fish could have entered.
Residents have also seen otters romping and hunting fish in the canals.
CATFISH SPECIES COMMON BUT "POORLY UNDERSTOOD"
The sagor catfish is a coastal species found near river mouths and tidal rivers with muddy beds.
Dr Karenne Tun, group director at the National Biodiversity Centre of the National Parks Board (NParks), told CNA on Monday (Dec 15) that it is amphidromous, which means that it migrates between freshwater and saltwater environments.
"Its ability to survive in both environments provides it with access to a wider range of food sources, and opportunities to escape from predators that are unable to tolerate changes in salinity," Dr Tun added.
A study on its reproductive biology in Vietnam showed that the black fish grows to more than 44cm.
The species is classified as being of "least concern" in the Singapore Red List published in the third edition of the Singapore Red Data Book, which documents Singapore's biodiversity to support conservation efforts.
This means that the sagor catfish has a stable population and is not considered close to threatened status.
Asked why the fish were in the canal, independent researcher Ng Heok Hee suggested that the school of subadult catfish could be feeding.
The ichthyologist who has studied catfish for about 20 years at the National University of Singapore said: "This species largely feeds on shrimps and crabs (with the occasional small fish thrown in), so canals may be a rich hunting ground for them.
"Despite being a relatively common species, the biology of H sagor is poorly understood. Results from some studies have suggested that the juveniles prefer higher-salinity waters.
"If so, this may be a large school that transiently enters freshwater for food. Another possibility may be that they are just following the tide as they swim up canals."
Dr Ng added that fish swim in schools for protection to avoid being singled out by predators.
Fellow ichthyologist, Dr Tan Heok Hui, said that the canal's hard, straight walls and lack of hiding places made it a "sterile environment" compared with the catfishes' natural riverine habitat.
On whether the species is edible by humans, Dr Tan said that anglers sometimes catch and then throw the catfish back because they "don't taste good".
"(Anglers) call them 'ah seng' and say that they smell and taste of dirty drains," Dr Tan added.
The sagor catfish has a venomous spine on its dorsal fin and on each of its pectoral fins, which are used to protect themselves against predators, Dr Tun from NParks said.
"Their stings can be excruciating and long-lasting. Members of the public are advised to take caution if they encounter catfish in the wild and to avoid handling them," she added.
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