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Coming to terms with currents – 2011 Elwandle barge monitoring trip
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The SAEON Elwandle diving team has been documenting and monitoring changes in the fish and invertebrate communities on the two large transport barges that were scuttled in the near-shore region of Cape Vidal in 2008. These annual surveys have been conducted since 2009.
On the 2011 monitoring trip the Elwandle divers were supported by an independent team of diving enthusiasts led by Dennis King (author of the popular Reef Fishes & Corals and More Reef Fishes & Nudibranchs).
Each annual survey involves several dives along fixed transects in each different habitat (decks, holds and hulls) on the shallower barge (26m) and along the decks only of the deeper barge (42m). These dives are usually very spectacular in that they offer a pleasant and colourful change to the murkier, colder waters of the Eastern Cape where the SAEON Elwandle Node is based.
Diversity and abundance
Without fail, each year brings more diversity and abundance. In 2009 a new species of blenny was found (Cirripectes heemstraorum) and the recording of the Harlequin sandperch Parapercis maculata was a first recording for South Africa, and possibly a first for Africa. Last year saw the first recording of the barnacle Balanus variegatus in South Africa.
That said, these dives most often are not typically gentle and effortless because the barges have been scuttled along a section of the KwaZulu-Natal coastline that is subject to powerful long-shore currents. These currents often persist even when the surface conditions are perfectly calm and there is no wind.
Powerful currents
Acting on experience from the previous two years and with a bit of fortunate logistics, we intended to get some actual current measurements by installing an ADCP (Acoustic Doppler Current Profiler) on each barge for the duration of the trip. The very first dive of the trip revealed - both fortunately and unfortunately - that the current was very strong. It was fortunate because we were able to record the high current speeds, and unfortunate because we were unable to place the second ADCP on the deeper barge.
Data obtained from the ADCP on the shallow barge showed an average current speed at the 4m depth level of 2.66 knots (4.93km/h) over the three-day deployment period with speeds peaking at an impressive 3.63 knots (6.7km/h). To put these numbers into perspective, a fully kitted scuba diver would have difficulty swimming into a current speed of one knot. However, determined to soldier on as best we could in spite of the demanding conditions, we managed to complete all the fish transects on the shallow barge by the end of the fourth day. After two attempts at the benthic transects it became evident that we were not going to win against the current this year.
It is interesting to note that in 2009, four of the ten days intended for diving were cancelled because of strong current. In 2010, four days were also lost because of strong current and most recently in 2011, five diving days were lost.
With such powerful currents it is little wonder that a 17 500 ton barge, initially lying almost parallel to shore, has now been shifted to lie almost perpendicular to it.
