Mysterious catfish cluster in Canberra Street canal: Experts weigh in on curious mass movement

“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.