Assessing habitat damage caused by the grounding of the Shen Neng 1 on Douglas Shoal, southern Great Barrier Reef, using towed video (Towvid)

Brief description

Towed video (Towvid) was deployed on eight occasions over a two day period (17-18 April, 2010) on Douglas Shoal. Sites included the initial grounding site of the Shen Neng 1, on the south east edge of the Shoal, the final grounding site and a randomly selected unimpacted site on the same shoal but a minimum of 1050 m from the ship's path.Initially, it was envisioned that the multibeam sonar bathymetry survey would generate real time high resolution topographical imagery to guide the Towvid survey. However, rough conditions on the voyage meant that post-survey analysis was required to reduce signal noise. Therefore, the path of the stern of the Shen Neng 1, together with the lay of the ship (provided by GBRMPA), was used and tows were planned to intercept and traverse areas where damage was likely to have occurred from contact of the ship's hull on the shoal along with areas of shoal not affected by the grounded ship.A towed camera system incorporating a standard definition video camera and digital stills camera was deployed to recover imagery of the benthos. A computer-based application (Towvid), developed by AIMS, allowed for real-time classification of substrata, benthos and individual organisms. Data points were recorded at 2-second intervals or on demand when a new substrate, benthos or organism was observed.Habitats were classified based on the substrata (4 categories) and overlying epibenthos (8 categories) and individual organisms (4 categories) viewed on the tapes. Towed video (Towvid) was used to assess the benthic habitat and biota in undamaged areas of Douglas Shoal and in areas of Douglas Shoal damaged by the grounding of the Shen Neng 1. On 3 April 2010, the 225 m bulk carrier Shen Neng 1 ran aground on Douglas Shoal in the southern section of the Great Barrier Reef, fully laden with approximately 65,000 tonnes of bulk coal and 977 tonnes of fuel oil (ATSB 2010, MSQ 2010).Douglas Shoal is a large, flat reef reaching between 9 and 15 m of the surface. The vessel was underway at a speed of approximately 8 knots when the grounding occurred and she sustained severe damage to her hull, and to the propeller and rudder. The grounding coincided with the afternoon low tide and the last of the spring tides. By half tide the ship had moved further onto the shoal and with the continuing tidal rise undertook a further westward drift of ~ 1.3 km before coming to rest on the falling tide. By the afternoon of the 2nd day, the ship moved ~ 1 km south towards the edge of the shoal where it remained for a week. During spring tides on 12 April she was re-floated with the aid of tugs following removal of approximately half of the fuel oil. A minor oil spill (~ 2 tonnes) that occurred earlier in the grounding was treated with chemical dispersants. During this 9 day event, the ship ground its way across the top of the shoal, rotating as alternate fore and aft sections of the hull resisted the drift. As major oil and coal spills did not occur, the most important impacts to Douglas Shoal were considered to be physical damage to the reef structure and its biota as well as potential contamination by antifouling paint which can affect invertebrate recruitment and reef recovery.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: Statement: Sea surface conditions and strong currents due to spring tides, made it difficult to maintain a Towvid course in the prevailing sea conditions and resulted in a meandering track intersecting the known positions of the Shen Neng 1.The visual imagery of the benthic habitat was captured using a 1/3 inch single CCD colour video camera mounted on a tow frame and controlled by a winch with 320 m of electromechanical cable. The video signal was recorded on a shipboard miniDV tape recorder. In addition to the visual imagery, the miniDV tape recorder received GPS and sounder data (latitude and longitude, ground speed, true heading, date, depth and time), which was recorded on the audio track. The computer-based application developed by AIMS for the analysis of tapes is described in:Speare PJ, Johansson C, Depczynski M, Meekan MG (2008) Marine habitats of Southern Melville Bay using towed video and aerial photography. Final Report to Charles Darwin University/Alcan Gove Pty Ltd. Australian Institute of Marine Science. 25 p.Substrate and benthic categories employed for towed video to classify the habitats and individual organisms recognised:Substrate Class (abiotic substrate components):GroundingLimestoneRubble (5-50mm)SandBenthos Class (overlying benthic organisms):Hard Coral, MediumHard Coral, SparseMacroalgae and Heterotrophs, MediumMacroalgae and Heterotrophs, SparseMacroalgae, DenseMacroalgae, MediumMacroalgae, SparseNo BenthosIndividual Organisms:Solitary CoralSea PenWreckageNoneThe percentage cover of benthic communities was classified as follows:S - sparse (0 - 30% cover)M - medium (30 - 60%)D - dense (>60%)

Notes

Credit
Speare, Peter J, Mr (Principal Investigator)