Call for St Lucia Estuary Mouth to be Opened

Written by Nick Bates - The Mercury

Monday, 29 August 2005

Waiting for heavy rains to cause a natural opening of the estuary appears to be a shortsighted solution, writes Nick Bates. The quiet hamlet of St Lucia faces a tough test in future weeks as pressure to open the St Lucia estuary mouth mounts, both from local residents and ecologists. The mouth of the estuary closed naturally in June 2002, just before the grounding of the ship Jolly Rubino in September that year, and additional sand was bulldozed into place to ensure that no hazardous chemicals reached the estuary. It was expected to reopen with the equinox tide in September that year, but this never occurred, possibly due to the excess sand at the mouth that was never removed. In the past, the channel near the mouth had been dredged to ensure the free exchange of sea water into the estuary, but on this occasion the closure was ignored. The tidal influence was thus terminated, and so, too, the promise of new life was interrupted. The term estuary was in reality annulled, and it has now been transformed by catastrophic events into a massive freshwater pond. Initially, the loss of water through evaporation and other means saw the pond reach salinity levels almost five times higher than the Indian Ocean just 400m away from the sea, but the salinity levels have dropped to around 10 ppt (parts per thousand) measured at a mere 6km from the mouth, whereas sea water concentration is generally 35ppt. Tilapia and barbel have thus become common residents, and become the upper trophic level feeders. Tilapia (kurper) were recently reported as being the dominant catch, and held a generous 45% margin over other species following a commissioned survey conducted by the Greater St Lucia Wetland Park Authority. This is typical of a species invading an area that has lost traditional predators such as shad, kob and kingfish after the 2004 die-off. Their presence would have prevented such an intrusion. Further, the appearance of kurper can be interpreted as an indicator species - that the system is disturbed or out of balance. Nursery The estuary, which forms part of the Greater St Lucia Wetland Park, is a critical nursery for countless marine species, while its socio-economic value is significant to St Lucia and this World Heritage Site comprising some 3 280km2. Ichthyologist Kian Barker is adamant that it is not only the various fish species that are under serious threat, but also the surrounding fauna (birds, small mammals and invertebrates) thanks to the low salinity levels, and which may not immediately be visible to Ezemvelo KZN Wildlife and its consultants. "The estuary was last closed in 1997, and although it appeared to have regained stability, this might not be the case," says Barker. "Bear in mind that we are now approaching our third year in depriving certain species' young access to a natural nursery, and that there is now a potential threat of a progressive decline of species within the waterway. "We're effectively three generations (cohorts) behind, and could be in for a nasty shock once things return to 'normal'." Thus, the ability to breed in meaningful numbers, thanks to gene reduction and sheer population numbers, might only be apparent when the mouth is either opened or opens naturally, and the loss of a specie or species will be a very surreptitious event. "We simply won't know about it," Barker says. The now ever-present sandbanks are in turn colonised by freshwater grasses, which in turn changes the habitat for mangroves, mud skippers, fiddler crabs and crocodiles. Everybody wants nature to follow its natural course, but being a World Heritage Site, we need to protect and manage its future by reducing the negative effects caused by man and protecting its future does not mean measuring salt levels now, or issuing communiques boasting about the abundance of pelicans, kurper and hippos. Dynamic It means looking further than that. Salt levels play an important role in the distribution of species within the lake and the fauna shifts in response to a salt gradient. During periods of high salinity, generally ichthyofauna (a fish population made up of a variety of fish) will shift towards the mouth. And even out to sea. Although it is believed that it is beneficial to leave the mouth closed during periods of low rainfall, this needs to be questioned. Sandy soils are dynamic and so are the plant species associated with them. High salinities are assumed to damage vegetation, however this could be managed by reseeding or a little landscaping vegetation affected by salinities (if proven necessary). In fact, plants that are not protected by legislation, are being favoured for a protected and threatened fish population. So the mass mortality of fish in 2004 affected the largely protected or managed fish species, and quite possibly endemic genetic fish populations. No one has bothered to conduct an offshore study of the juvenile fish, waiting through hereditary instinct, to enter the estuary either. The livelihood of "Africa's official call", the fish eagle, must be considered too, as their menu becomes more limited (barbel and tilapia), while hippos remain in the low level water at half mast during the day, thanks to inadequate water levels, Barker says of recent developments. On the point of desalinification, if this lake system is on average 90cm deep, then 1 000mm in a year could effectively reduce the salinity rapidly, after normal rainfall returned. Rain cycles or drought cycles featured are relatively short. Therefore from record the collective effect might not be as catastrophic as anticipated if the estuary mouth remained opened. The inevitable will happen, nature will succeed where man has failed. Barker says the saving grace here will be global warming, and the resultant rise in sea level to bring about the effective functioning of an ecosystem that appears under pressure from an ineffective management policy. Barker gives another classical example of the damage being done thanks to the estuary closure. "Prawns such as swimming and tiger prawns, which normally inhabit the system are unlikely to be present either, as they have a life span of approximately 18 months. We are almost three years down the line, so it can be safely assumed that they're almost non-existent now." Prawns play a vital role in the estuary in that they not only feed on the estimated 60 tons of hippo dung deposited each day into the system, but are also a valuable food source to the normally resident vertebrate species. The signs to the experienced environmentalist who visits the St Lucia estuary must be activating alarm bells, while the tempers of local residents are frayed too, thanks to the town's dwindling popularity and attraction. As the water level in the estuary continues to drop, so the chances of revival lessen. The current level is too low to attempt opening of the mouth, and one solution is to either pump water from the Black Umfolozi just a few kilometres away, or dig a channel between the two bodies of water, once a mean sea level has been reached with the incoming fresh water, the mouth could be opened. This would avert a catastrophic influx of sea water, bringing in large quantities of silt laden sea water. Waiting for heavy rains to fall into the catchment areas of the five rivers flowing into the estuary, which would see a natural opening of mouth, appears to be a shortsighted solution. It would take an estimated 15 years for this ecosystem to reach its full reproductive potential, from this very depleted state. Barker has strong views on the current scenario. "This magnificent area comprising five ecosystems, which can be viewed from one various single point near Mission Rocks, needs a much more sensitive and intelligent input, but foremost, from those that have a broader, long-reaching vision should they want it want to survive. "A permanent management plan is required in order to maintain a healthy balanced and productive system - indefinitely." - Nick Bates is a freelance writer