Fish die-off: A combination of factors are killing fish in estuaries along the Eastern Cape coast

Prof Alan Whitfield from the South African Institute for Aquatic Biodiversity (Photo: Supplied)

By Tembile Sgqolana

12 Aug 2021

Lower rainfall along the Eastern Cape coast has led to a change in salinity in the Mpekweni and Mgwalana estuaries. This is one reason why fish have died.

Persistent drought in several parts of the Eastern Cape is causing the die-off of spotted grunter and other fish species along the coast at the Mpekweni and Mgwalana estuaries.

Last month, the lack of rainfall in the Eastern Cape and Western Cape resulted in a national drought disaster being declared in some areas by the National Disaster Management Centre.

The fish die-offs at Mpekweni and Mgwalana come as the Garden Route town of Wilderness also saw fish, including spotted grunter, dying in the Touw River estuary in the past few weeks. In that incident, SANParks spokesperson Reynold Thakhuli said they were still investigating the cause.

Commenting on the die-offs in Mpekweni and Mgwalana, East London museum scientist Kevin Cole said one of the causes of the deaths was changed salinity conditions. There had been no good rain for a while and the Mgwalana estuary system had been closed for a number of years.

“Hypersalinity conditions prevail as salinity of the river is now greater than that of the adjacent sea. Due to the lack of rain there has been no substantial freshwater supply to the system. In addition, the situation could have been exacerbated by high evaporative rates over time from a river which has a relatively small catchment area,” he said.

Commenting on the die-offs in Mpekweni and Mgwalana, East London museum scientist Kevin Cole said one of the causes of the deaths was changed river salinity. (Photo: Supplied)

He said with decreased water volume in the Mgwalana estuary and increased salinity, it could be expected that the mortality of the biota would be evident.

“Other creatures will also feel the effects, such as the macrobenthic fauna (small creatures living on the sandy substrate), and will not be able to survive in the estuary,” he said.

Mgwalana resident Robyn Coleen Taylor said the fish were dying due to a lack of oxygen in the water and the salinity being so high.

“This drought has caused rivers to be smaller and water becomes stagnant, oxygen levels disappear and salt builds up. This shows the extent of the drought in the Eastern Cape,” she said.

Taylor said heavy rains upstream are the only thing that will help the fish.

Ichthyologist Dr Paul Cowley said fish die-offs have also been observed at the Kasouga River in the Sarah Baartman District of the Eastern Cape and Riet River near Port Alfred, but the Mgwalana seems to be the most heavily affected.

Cowley said that cold water could also play a role in fish mortalities.

Retired museum fish expert, Greg Brett, believes that deoxygenation of the system may also be a contributing factor — the large size of the spotted grunter that have died and the numbers reported hinted at this factor.

“To assist in interpreting the scale of the impact on other species of fish and animals which have died as a result, salinity is an important factor in the functioning of an estuarine ecosystem. It is evident from the reports this week that spatial and temporal changes in the chemical and physical parameters have taken place in the Mgwalana estuary due to a changed hydrological cycle, and this has contributed to noticeable fish mortalities,” he said.

salinity and cold water Prof Alan Whitfield from the South African Institute for Aquatic Biodiversity said a combination of high salinities and cold water temperatures is lethal to many fish species in estuaries. (Photo: Supplied)

Prof Alan Whitfield from the South African Institute for Aquatic Biodiversity said he was recently informed of fish deaths in estuaries between the Mpekweni and Birha estuaries on the Eastern Cape coast.

“These occurred after the recent very cold snap which plunged water temperatures in some estuaries of the region to below 100C in places. At the same time, salinities well above seawater salt concentrations were recorded in several temporarily closed estuaries along this coast. These estuaries are temporarily closed off from the sea by a sand berm and will only open when the rivers flood and breach a mouth through the sand bar,” he said.

Whitfield said a combination of high salinities and cold water temperatures is lethal to many fish species in estuaries, and this was the probable reason for the recent fish kills in the Birha and other estuaries.

“I do know that large spotted grunter (Pomadasys commersonnii) and dusky kob (Argyrosomus japonicus) were among the fish species to perish in these mortality events. Presumably this was also the cause of the fish kill in the Mgwalana estuary,” he said.

He said the extent to which the climate crisis was responsible for these fish deaths was largely unknown.

“If climate change is causing the prolonged drought that we are experiencing in the Eastern Cape, then a link could be made. Unfortunately, distinguishing climate change impacts from typical prolonged droughts and extreme temperatures, either very hot or very cold conditions that have occurred in the past century, are difficult to determine — so the jury is still out when it comes to pointing a finger at climate change as the indirect cause of these fish kills,” said Whitfield. DM/OBP

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