Raw sewage or natural causes?

Durban — There are mixed views on the possible cause of the mass death of fish that washed up on uMhlanga Beach on Sunday.

A concerned resident recorded several dead dead, of varying size, on the beach. The video has since gone viral on social media.

An environmentalist believed the fish died of poisoning in the uMhlanga Estuary, while another expert points to the build-up of freshwater on the estuary side.

The uMhlanga Estuary. Picture : South African Association for Marine Biological Research

However, uShaka Sea World education executive manager, Jone Porter, provided insight into the possible reasons that led to hundreds of fish appearing on shore.

“Before the weekend, the uMhlanga Estuary mouth was closed and this led to a build-up of freshwater on the estuary side,” Porter said.

“Due to the heavy rain experienced in the city recently, the mouth was opened and the freshwater ended up mixing with the salty seawater.

“At the weekend, there were cases where tilapia were affected due to the rapidly increasing and decreasing rate of salt water,” Porter said.

She said tilapia were able to live in both freshwater and saltwater, but weren’t able to cope.

Porter, who also represents the SA Association for Marine Biological Research (Saambr) said that it’s part of their mandate to monitor marine life to try to ensure that incidents like these do not occur: “It is important for the estuary mouth to open naturally as hundreds of marine life are reliant on the climate and habitat there.”

There are mixed views on what the possible cause of death of several fish that washed up on uMhlanga Beach on Sunday.

Environmental activist Pete Graham thought this could be due to a sewage spill. Graham provided insight as to how the spill could have possibly been caused by sharing how the pump station works.

“At the Ohlanga Pump Station, which is upstream from the uMhlanga Estuary, there are two rising mains. One of them has a crack. This is causing a relatively small sewage leak – and I say relatively small because we are coming from a time when more than 30 million litres of sewage were pumped into the Ohlanga River per day. With the rising main, it fills and then it gets pumped out. The pumps do not run all the time; they only operate in the morning and afternoon.”

Graham said that along with other engineers, they think that the spill is minor. From the environmental impact perspective, an important concept is Dissolved Oxygen (DO), he said.

“When sewage goes into a non-open lagoon, it leads to the tertiary effects of pollution, phosphate and nitrate build up. When the vegetation dies, it significantly reduces the oxygen in the water – which kills the fish – but there was no indication of that in this case as oxygen tests were done with no indication of levels dropping,” Graham said.

When sewage enters a river system, the fish are able to live in such an environment, so there is no way the fish could have been killed due to the spill, he said.

“The dead fish is the symptom, but the problem is the ongoing pumping against the conditions of their licences by UPL Limited of the highly toxic sludge into the water system. Their licence said they’re allowed to discharge 400 cubic metres, but they discharge double,” Graham said.