St Lucia's Skimmers: A return or temporary visit?

African skimmer at the St Lucia estuary mouth.

The return of the African skimmers to St Lucia estuary has caused quite a stir in recent weeks and for good reason…

African skimmer used to occur (and even breed) at the St Lucia estuary until 1943. Their disappearance coincided with an increase of human disturbance on the fragile estuarine system and could hardly be seen as coincidence…

Early photographs of the St Lucia estuary mouth area show a network of channels, separated by a mosaic of exposed sandbanks. At times there were multiple mouths and the estuary mouth area was much more vast than today stretching all the way to Mapelane bay at times. The estuary area would have supported a big variety of estuarine fish species including huge numbers of surface-feeding mullet. On the high tide clean ocean water would have pushed in submerging some of the sandbanks and also the mangrove forests at the estuary edge whereas on the low tide dirtier lake water would push out to to the ocean and all sand banks/islands would have been exposed. There were very few (if any reeds) due to the tidal action, salinity and unstable sandy substrate.

The Lower stretches of the Umfolozi river at the time would have been a vast swamp/floodplain. The floodplain would have acted as a natural filter/ silt trap, capturing silt from upstream. The water entering the estuary mouth area via the Umfolozi river would therefore have been very clean and the estuary mouth would have been predominantly sandy bottom.

This mosaic of exposed sand banks accompanied by an abundance of surface-feeding fish would have been perfect African skimmer habitat so no wonder they were breeding residents.

Unfortunately, the situation changed quite dramatically with human interference. The Umfolozi floodplain/swamp was converted to agricultural land (predominantly sugarcane farms) and the river converted to a canal. As a result the natural silt trap was lost and instead of being deposited in the floodplain, the silt was instead deposited in the estuary. Burgeoning human populations upstream of the river meant an increase in livestock and hence overgrazing. With the land overgrazed and bare exponentially more topsoil washed into the river during rains. An increase in agriculture (planting of crops) on non-terraced hill slopes had the same erosion effects and added to the sediment problem.

Along with the increased sediment/silt entering the system, there was also a big reduction of water. This was due to water extraction for agriculture, mining and to service the growing human population. The Lake itself gains a huge proportion of its water through groundwater seepage (derived from rainfall). Vast commercial plantations of exotic Eucalyptus and pine were planted at the edges of Lake St Lucia (on the Eastern and Western shores) and surrounding catchment areas. These thirsty trees consume exponentially more water than indigenous terrain and dramatically reduced the groundwater input into the lake/estuary.

The combined effect of more sediment and less water resulted in a diminished ability for the estuary to scour. A strong outflow on the low tide (derived from high lake levels) is needed to push deposited sediment from the river and also marine sediment (sand pushed in from the ocean on high tide) back out to the ocean. With this diminished ability to scour, the estuary mouth area became clogged with sediment and closed more frequently.

After recognising the sedimentation and water-loss problems a long list of management practices (which would require a 20 page article to explain fully) were implemented at the estuary mouth over the next 80 years with the intention of helping the estuary mouth to function better. Many of these were controversial and contradictory to each other, and in the opinion of many, did more harm than good. 

Anyways to cut a long story short, by 2018 the estuary mouth area and narrows looked very different to the mosaic of water channels and sand banks of the past. The siltation/sedimentation, combined with more frequent and prolonged closures of the mouth resulted in the estuary becoming a freshwater, shallow, muddy-bottomed system. Reeds thrived and most of the narrows and estuary mouth area were totally choked leaving very few (if any exposed banks). Not good African skimmer habitat at all.

With exceptionally good rainfall in recent years, the estuary mouth has re-opened twice (and is currently still open). With the mouth migrating Northwards (as it does), a sand spit developed on the seaward side of the channel as well as a sand spit curling into the estuary itself. The seaward sand spit was subsequently cut in 2 due to the flooding river leaving an isolated sand island opposite the mouth. The sandspit curling into the estuary was also sheltered by an impenetrable 50m wall of waterlogged reeds. The current conditions at the mouth therefore provide the skimmers with perfect habitat of exposed sand banks/spits sheltered from human disturbance and surrounded by calm (fish-rich) waters to forage in. 

The estuary remains fickle however and there are no guarantees current conditions will persist. With most of the estuary narrows being reed-choked there are virtually no exposed banks/ sand banks besides directly at the mouth. In my opinion, the longevity of the Skimmers stay at the St Lucia estuary is therefore almost totally reliant on the mouth remaining open and the availability of sandbanks on which the skimmers can perch- which are sheltered from human disturbance. One bout of Cyclone swell or easier accessibility for humans to the sandspit(s) could be all it takes to force the Skimmers to search for an alternative home. 

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The Lake St Lucia estuary mouth as it once looked