By Dr Ed Butler
The Eastern Cape (EC) province of the country has been gripped by prolonged drought for the past several years. This has meant that many estuaries have been starved of precious freshwater input, creating toxic environments.
Many EC estuaries are classified as temporarily open/closed. These systems are ordinarily closed off from the ocean by sand spits but are breached at the mouth and flushed during heavy rainfall. Familiar examples include the Boknes, Kasouga or Kleinemonde Estuaries. During closed phases, when the estuary is cut off from the ocean, freshwater largely enters the estuaries via rainfall and leaves through evaporation. If the amount of water leaving the system is greater than the amount of water entering the system, the water becomes increasingly salty. Large seas and spring high tides may also introduce more salt into these estuaries during ‘overtopping events’. Continuous evaporation without rainfall means that the salt content of the water becomes higher and higher. Fishes and other aquatic life remain trapped inside of these estuaries with no opportunity to exit or escape to the sea until rainfall returns, and the mouth is breached.
Fishes naturally need to ‘osmoregulate’. This process maintains salts and fluids within certain concentrations inside their bodies. Under freshwater conditions, fishes need to absorb salts and excrete excess water to maintain homeostasis (the stable conditions needed for survival). Under salty conditions, the opposite holds true, and fishes need to get rid of salts and retain water. Fishes are able to do this using organs such as their kidneys and gills. Many of the fishes found in our estuaries are particularly good at doing this, as they need to be able to move from the sea, into estuaries and sometimes into freshwater. These adaptations allow them to withstand extremely salty water for periods of time. However, this process has obvious costs for the animal.
Years of extremely hypersaline conditions have left fishes in many Eastern Cape estuaries in a state of ‘osmoregulatory distress’. As salt concentrations have increased, so to have the costs of osmoregulation. On top of this, many natural food sources have also been impacted and have likely died out or become less available, compounding the issue. Weakened by prolonged salt tolerance and relative lack of food, the fishes in many systems are in poor condition and the onset of other stressors, including cold weather snaps, are now likely enough to kill many populations.
This was experienced in the mid-year period of 2021 which saw numerous largescale fish-kills across a number of estuarine systems including that of the Birha, Mgwalana, Mpekweni and Kasouga. Kob and grunter suffered the largest fatalities, numbering well into the thousands. Extensive sampling of the Birha Estuary in July 2021 revealed other deaths as well – catface rockcod, Natal stumpnose, bartailed flathead, leervis, blacktail, zebra and even a single white musselcracker. On the other hand, and quite interestingly, certain fish species were glaringly absent from many of the kills, including Cape stumpnose, a number of mullet species as well as the somewhat bulletproof Mozambique tilapia.
Salinity readings in the impacted estuaries were remarkably high at the time, with salt concentrations higher than double that of the ocean in some systems. Other estuaries, such as the Kleinemonde East and West for example, remained fairly unaffected, with salinity similar to that of the ocean. More recently, good rainfall across the EC has provided some reprieve. However, hypersaline conditions prevail and many of the most severely impacted estuaries have not yet been breached.
It is important to point out that estuarine hypersalinity, and the fish kills that come with them, are not unnatural nor uncommon. These events have been widely reported across many South African estuaries, since people began reporting such things, and often occur in estuaries which are otherwise healthy and free from human pollution or disturbance. Therefore, these events are not something that require human intervention – they are part of natural environmental cycles. What may be changing, however, is where these cycles take place and how long they last. Therefore, they provide opportunities for understanding and for research.
Interestingly, a number of fishes do still prevail within many of the impacted EC estuaries, albeit in poor condition. For example, spotted grunter are still caught occasionally within the same estuaries which experienced kills of thousands over the recent past. The ability of some fishes to survive, while others perish has caught the eye of researchers from Rhodes University’s Southern Africa Fisheries Ecology Research Laboratory. Currently, we are aiming to study these survivors to better understand what allows them to withstand these conditions. As global climate change progresses, drought and extreme weather will become more common. Hypersaline estuaries may drive the evolution of fishes as they are forced to adapt to a changing world. By studying these events, we may be able to better understand how fishes will respond and adapt in the future.