Aliwal Shoal

About

The Aliwal Shoal is a rocky reef which is the remains of an ancient sand dune approximately 5 kilometres (3.1 mi) off the coast of KwaZulu-Natal, South Africa. The reef is inhabited by many kinds of hard and soft corals and other marine invertebrates, and a variety of tropical and subtropical fish species. Aliwal Shoal was named after the near-sinking in 1849 of the three-masted vessel Aliwal, captained by James Anderson. There are two wrecks near the reef that are popular recreational dive sites. The Norwegian bulk carrier MV Produce sank in 1974, and SS Nebo sank in 1884. Aliwal Shoal has diverse marine life, including large predators, and is popular as a recreational scuba diving destination. The Shoal is known especially for its abundance of Grey nurse sharks (known locally as ragged tooth sharks or "raggies") between July and November when the sharks congregate there to mate.

Aliwal shoal is part of a large offshore reef complex. The narrow ridge parallel to the coastline about 5 kilometres (3.1 mi) offshore, more specifically referred to as the Crown, is the part usually referred to as Aliwal shoal, and this area is inside the sanctuary zone of the Marine Protected Area. The shallowest part, extending to within 6 metres (20 ft) of the surface, is at the north end of the Crown. To the north the bottom slopes down relatively steeply, and to the sides the slope is less. The shoal is about 380 metres (1,250 ft) wide to the north, narrower to the south, and then spreads out towards the coast in the section known as the Ridge. Average depth of the Crown is about 12.5 metres (41 ft), and the Ridge is about 19.5 metres (64 ft) average depth, with a few isolated pinnacles.

The reef consists of aeolianite with subordinate beachrock. It is thought to be sunken coastline of the late Pleistocene. The type of rock was determined from samples recovered from the hull of the vessel Aimée Lykes, which struck the shoal in 1963, and made it to Durban for repairs.

Aliwal shoal is a subtidal, algal-dominated subtropical reef in the transitional region between the warm-temperate reefs of Pondoland and the tropical to subtropical reefs of Maputaland. The benthic communities are dominated by algae and sponges, but various soft and hard corals, echinoderms and ascidians are also present. The environment can be classified as marginal, in that it supports fauna of tropical, subtropical and warm temperate reefs. The diversity is comparable to similar latitude reefs of Australia. The adjacent coastline of KwaZulu-Natal has a humid subtropical climate with summer rainfall, and the freshwater and riverine sedimentary discharge into the region is significant and affects turbidity and plankton blooms. Most of this input is from the mKomazi River at the north of the MPA. A marine pipeline operated by Sappi Saiccor discharges just outside the MPA. The shoal is exposed to heavy wind-driven wave action, with an annual average swell height of 2.5 metres (8.2 ft). The following benthic taxa are recorded from the shoal:

Amphiroa ephedraea Callophycus condominius Champia compressa Dichotomaria diesingiana Hypnea viridis Hypnea sp. Laurencia brongniartii Meristotheca papulosa Osmundaria serrata (ex Vidalia serrata) Peyssonnelia capensis Phaeophyta (brown algae) Dictyota dichotoma var. intricata Exallosorus harveyanus Lobophora variegata Stypopodium multipartitum Zonaria subarticulata Porifera Dragmacidon sanguineum Cliona orientalis Chondropsis sp. Fascaplysinopsis sp. Forcepia sp. Geodia sp. Hemiasterella vasiformis Oceanapia aff. ramsayi Placospongia sp. Polymastia sp. Rhabderemia sp. Spheciospongia excentrica Spheciospongia globularis Spheciospongia vagabunda Suberites kelleri Octocorallia (soft corals)

Adapted from Wikipedia (CC BY-SA). Photography via Wikimedia Commons.