Selangor DoE director Nor Aziah Jaafar said the dead fish, mainly suckermouth catfish (Hypostomus plecostomus) and Mozambique tilapia (Oreochromis mossambicus), were found over a 5km stretch of the river.

She also said that an in situ test at Sungai Kayu Ara near a log boom revealed foam in the river and the dissolved oxygen concentration was 4.5mg per litre. Healthy water usually has dissolved oxygen concentrations above 6.5-8mg per litre.

Nor Aziah said the DoE scoured the upstream and downstream stretches of the river for potential sources of pollution within a 10km radius from where the dead fish were found.

This involved inspections at several food and beverage-based factories, commercial centres, stormwater discharges from residential areas and two public sewage treatment plants, she added.

Nor Aziah advised people who had information on the incident to come forward and assist the DoE in the investigation, saying the department will take firm action against environmental criminals.

Complaints of pollution can be reported via the DoE toll-free line at 1-800-88-2727 or email to [email protected] or the DoE e-complaint portal here.