Professor Nadine Strydom – winner of Nelson Mandela Metropolitan University’s Researcher of the Year Award – is Africa’s leading expert on “little fish”, and a regular pioneer of new technologies in her field.
Her speciality – a scarce skill worldwide – is identifying fish larvae so tiny they can only be viewed under a microscope. She tracks the early life stages of fish – monitoring their movement from the sea (where fish spawn) to the surf zone (where fish larvae are part of the plankton) to estuaries (where they grow into juvenile fish) and even up-river (where marine fish make use of unique nursery areas). She also monitors the age and size at which fish start reproducing.
Professor Strydom is contracted to complete the book on our two estuaries initiated by Sustainable Seas Trust (SST) and partly sponsored by Estuary Care.
On Sunday 26th February 2017 members of the Estuary Care Committee and some friends were privileged to join Professor Strydom on an educational cruise up the Bushman’s Estuary.
Professor Strydom explained that an estuary is the term for the meeting place of river water and sea water, and is where a change of salt content occurs in the water. This becomes the interface of nutrients and fosters the growth of plants starting with tiny micro-organisms, for hundreds of species. Most fish spawn at sea and our estuaries are the nursery areas for baby fish. Sea water (and typically the water in the mouth of an estuary) has about 35gms of salt per 1000gms of water whereas at the head of an estuary the salt concentration can be as low as 10gms per 1000gms. There is often a gradient of salinity in deep pools, especially further up the estuary.
The colour of the water is determined by the inflow of fresh water from up the river where dams have become a problem. (Environmental water laws were introduced long after the dams had been built).
Regarding sewage pollution from e-coli in our estuaries, Professor Strydom said sewage pollution does break down and does not have long lasting effects whereas heavy metal and chemical pollution is absorbed and only begins to break down after 50 years. Radio tags for measuring heavy metal pollution in fish have been installed at most estuaries. Too much sewage in the water does however change the water profile and is therefore detrimental.
Estuaries are divided into Reaches: the Lower Reach in which it is difficult for fish to live because of the constantly changing dynamics of the estuary, (e.g. prawns cannot live there) the Middle Reach where the estuary widens and the current slows down, sandbanks are formed naturally and where mud is found, and the Upper Reach which occurs after the end of the Nature’s Landing boundary.
She pointed out the importance of the salt marshes which lie along the tidal zone below some of the Nature’s Landing houses. Baby fish live in the eel grass (Zostra) and rice grass (Spartina) beds regulated by the tides. One should be very careful about removing natural detritus from the Zostra because this detritus needs to return to the estuary (for nutrition stabilisation) on the high tide. The presence of Phragmites reeds indicates the entrance of fresh water into the estuary flow. The river/estuary interfaces – where the salinity is very low, and which used to be marked by prolific mullet breeding, between the river water and the inflow of sea water have already been lost to both estuaries.
Because the natural method of scouring sandbanks which have become too big has been removed due to the advent of too many dams diverting the strong water inflow after a big rain, it may well be necessary to revert to dredging to clear the estuary sandbanks. However, Prof Strydom is not in favour of dredging.
Prof Strydom pointed out the desalination plant in the Lower Reaches and told us that the brine effluent it produces is very toxic, and we, as Estuary Care, should monitor that it is only released on the ebbing tide as required in the plant contract rules.
Regarding plastic pollution Professor Strydom pointed out that an ordinary plastic bottle takes 50 years to start degrading. Nevertheless micro plastics are being absorbed/eaten by sea creatures in estuaries and the sea and consequently get transferred to our human food chain.
Concerning boating on both estuaries Professor Strydom suggested that boating zones need to be redefined in various places because the edges of our estuaries are crumbling badly. Fish that live on the estuary edges cannot see to eat when the water is constantly turbulent from high boating activity. Estuary Care should write out estuary zone plans for both the Kariega and Bushman’s Estuaries. People should not be allowed to take boats into the Zostra areas.
Professor Strydom feels strongly that the total population must become environmentally aware as “water rights are for everyone”
A big thank you to Rem Gilfillan for these notes.