Diving Aliwal Shoal

The Aliwal Shoal is a fossilised sand dune lying roughly parallel to the coast, about 5 km from the shore. It comprises a narrow northern part (the crown, some 250 m wide, becoming about 800 m wide and finally with a wider landward ridge to the south, greater than 2 km in width. The crown varies in depth from about 6 m (around the Northern Pinnacles) to around 30 m on the seaward side. The mean depth of the crown area is 12.5 m. The southern wider area includes several protrusions that reach from a depth of some 30 m to around 15 m (e.g. Howard's Castle, Landers Reef). In the crown area, many dive sites, e.g. Raggie Cave and Chunnel have a large amount of uneven topography with small caves, ledges and swimthroughs rich in sea life, both fish-wise as well as in invertebrates, e.g. octopus.

The Aliwal Shoal has a subtropical climate witch mean monthly air temperatures ranging from 17.0°C (Jul) to 23.9°C (Feb). The sea temperatures tend to be somewhat warmer, due to the warm, tropical, southward-flowing Mozambique sea current, resulting in a balmy 21-26°C throughout the year. Mean monthly weather statistics at or near the Aliwal Shoal:

The period May to July has the least wind (18 to 19 days per month less than 20 km/h, i.e. small waves with breaking crests and fairly frequent whitecaps), while September to November is more windy (10-11 days less than 20 km/h).

Depending on sea current conditions, underwater visibility is sometimes strongly affected by effluent from the Sappi-SAICCOR paper mill at Umkomaas, released through a pipe that runs some 6 km into the sea just north of the shoal, venting along a considerable length of the pipe.

The shoal often has strong currents that can be from the north or from the south. This makes for fantastic drift diving.

The Aliwal Shoal is a fossilised dune upon which coral and other sessile animals have established. It is therefore not a true coral reef comprising the calcareous remains of dead corals upon which other corals grow. It is a coral site surprisingly far from the equator, made possible by the warm southwards-flowing Mozambique current. Therefore it contains many of the subtropical and tropical species found further north. However, the hard corals are relatively rare compared to those on tropical reefs and soft corals are much more common. The same applies to the tropical fish encountered on the shoal: they are relative more sparse than on tropical reefs. However, a surprising variety of tropical fish are still found on the shoal, including many species of surgeonfish, goldies, rockcod, triggerfish, cardinals, scorpionfish, lionfish and representatives of most other tropical fish genera. The wreck of the MV Produce houses the endemic Harlequin goldie Pseudanthias connelli.

Aliwal shoal is well known for the ragged-tooth sharks Carcharias taurus mostly encountered during winter. They pup in the cold water of the Cape during summer. During May to August they migrate up the east coast of southern Africa and mate in the vicinity of the Durban on the KwaZulu-Natal coast, including the Aliwal Shoal. After mating they spend a few months along the subtropical coast of Mozambique before returning to the Cape in the early summer. They prefer to rest in holes, caves or depressions large enough to contain sharks. They frequently swim slowly in a semi-sleep condition and can bump against divers.

The shoal commonly hosts blacktip reef sharks Carcharhinus melanopterus, bull sharks Carcharhinus leucas and tiger sharks Galeocerdo cuvier, the latter often found at deeper depths. Blacktip reef sharks are the most commonly encountered animals during baited dives.

The shoal has large numbers of invertebrate species such as sponges, coral (mostly soft corals), molluscs such as octopus, cowries and nudibranchs, crustaceans such as shrimps and lobsters, as well as sea anemones.

At more than 30 degrees south, the Aliwal Shoal lies at the extreme latitude at which tropical corals have been observed worldwide. The biodiversity on the shoal is shaped mainly by the amount of light that penetrates the sea water and by the physical wave action.

The shallow, northern part of the shoal with depth 6-10 m, is dominated by hard corals and sponges. Since hard corals are generally dependent on plant cells within the coral body (zooxanthellae), they are critically dependent on light for the photosynthetic products produced by the zooxanthellae. The most common hard coral species at Aliwal Shoal are the knob-horned and smooth-horned corals (Pocillopora sp. and Stylophora pistillata, respectively). The hard corals define a distinct community of animals along the top of the crown, found at dive sites such as the Northern Pinnacles and Pinnacle Ridge. The encrusting sponge Subites kelleri is also common in this area. The corals and sponges on these parts of the reef are physically robust and can withstand the wave action in shallow water.

A second distinct community on the shoal is characterised by specific soft corals, e.g. the Dead Man's Fingers (Eleutherobia aurea). These corals do not have zooxanthellae, are mostly dependent on water movement for feeding on small free-floating organisms, and cannot survive where the wave action is very strong. This community also has many sea worms (Polychaeta) and is found along the seaward as well as landward edges of the shoal at dive sites such as Raggie Sands, Blowhole and Inside Edge.

The third community is poor in distinctive corals and sponges, situated at greater depths, especially towards the southern end of the crown. However it still contains a large array of sea animals and it is easy to locate colourful nudibranchs. This area has a large abundance of algae and is found at dive sites such as North Wall and Burn Rock.

Possible threats to the shoal include:

• Pollution from the Sappi-SAICCOR paper mill effluent pipeline into the sea. Although it is claimed that the effluent is non-toxic, it reduces the light penetration to the coral on the reef and therefore affects the symbiotic nature of the corals and their zooxanthellae.
• Pollution from the coast and Mkomazi River. The Mkomazi river arises in agricultural areas, the polluted runoff of which flows into the sea at Umkomaas. However, for this water to reach the shoal, a special combination of rainfall and sea currents is required and it is therefore not expected to be a frequent event. Usually this runoff is close to the shore and not far in the sea.
• Structural damage by scuba divers. It has been claimed that the large number of scuba divers on the shoal damage the coral. However no strong evidence for this has been presented.
• Fishing, affecting the numbers of fish on the shoal. Fishing line also entangles and damages corals. Within the crown area this has largely been alleviated by the proclamation of the Marine Protected Area (MPA).

Although the above threats are real, we know very little about the relative importance of each of them.

The Aliwal Shoal Marine Protected Area (MPA) was proclaimed in 2004 in terms of section 77 of the Marine Living Resources Act of 1998, managed by Ezemvelo KZN Wildlife. It extends 18 km along the coast from the mouth of the Mkomazi River at 30°11.92'S; 030°48.29'E to the mouth of the Mzimayi River at 30°20.80'S; 030°43.60'E south of Scottburgh and with a width of some 5-12 km into the sea. It comprises a control zone in which limited fishing is allowed and two restricted zones where no fishing is allowed. The Crown area restricted zone, delimited the 25 m isobath around the crown, includes about 2.5 km². The Produce restricted zone includes the wreck of the MV Produce at around 30 m. The rest of the MPA is the control zone. The MPA was expanded in 2019, but this is not expected to affect diving.

• 🌍 Northern Pinnacles
• 🌍 North Sands
• Inside Edge
• Raggies Cave
• Chunnel
• 🌍 Cathedral
• South Sands
• 🌍 Eelskin ridge

• 🌍 MV Produce wreck
• SS Nebo wreck

• Tiger Ledge
• Howard's Castle
• Fern Reef
• Landers Reef
• Canyons