Possible culprits: Catfish, terrapins or rocks
The PUB suspended kayaking, dragon-boating and canoeing in Bedok Reservoir from Dec 17 to Jan 16 after complaints of injuries that could have been caused by bites from freshwater turtles or fish such as the toman.
A PUB spokesman told The Straits Times it had advised water-activity operators to tell participants to exercise caution and to put on proper footwear.
The public is also reminded not to release animals into reservoirs, she added.
During the suspension, several fishes, including a tarpon, African walking catfish, armoured sucker catfish and peacock bass, were caught, said the PUB.
An expert told The New Paper that none of these fishes are indigenous to Singapore and they had probably been released into the reservoir illegally.
Dr Tan Heok Hui, a lecturer at the department of biological sciences at the National University of Singapore, said: "All these are non-native species that have been introduced into Singapore's waters. Introduction into urban reservoirs is usually via release by humans, either intentional or accidental."
Nature conservationist and advocate Ben Lee said: "For religious purposes, some people believe that releasing fish into water is a good deed since the fish will be saved from human captivity.
"I have witnessed a group of more than 10 people, each with a bucket of hundreds of fish, wanting to release them into one of our reservoirs."
The introduction of new species could affect the ecology of the reservoir, he added.
"If you put more alien species into the reservoirs, it will result in increased competition and affect biodiversity. There will be a lack of food and space for our local species, so it's not good for the overall ecosystem," said Mr Lee.
"The catfish can be a menace to humans because it has poisonous spikes that can pierce and cause puncture cuts on the feet," he added.
After viewing photographs of the wounds of Temasek Polytechnic students Nicholas Ong and Kian Wai Seetoh, Dr Tan said: "The small scrapes could also be from rough rock edges or small teeth plates that some catfish have.
"(As for) the larger gashes, (they) may indicate a wider cutting or biting edge, but the size is exaggerated by the human pull-away response. This could be caused by either fish or terrapins."
Those who illegally release animals or fish into reservoirs can be fined $50 for the first offence and $200 for the second. Subsequent offences will be prosecuted and offenders can fined up to $3,000.