Towed Video - Benthic survey of Gove (30-Oct-2007 - 08-Nov-2007 )

Statement: Towvid: \n \nVisualisation of the habitats of Southern Melville Bay and adjacent areas was done using a low-voltage underwater video system towed behind a vessel. This equipment was powered by 12V lead-acid batteries delivering power to the deck winch for flight control and to a video camera, via the electro-mechanical tow cable, mounted on a tow frame and directed towards the seafloor. The camera returned the imagery to a topside LCD display and simultaneously to a video recorder. In addition to enabling real-time classification of the imagery as it was captured, the display had a dual role of also providing the operator with visual evidence of the position of the tow body with respect to the seafloor, thereby facilitating height adjustments via the variable speed winch control in response to changes in seafloor topography and the relative movement of the vessel. \n \nThe imagery was recorded to miniDV tape that simultaneously acquired serial data from onboard instruments. A 3 dimensional GPS fix (latitude, longitude and time) and the water depth were combined and written to one of the audio tracks of the tape receiving the video stream. In this way, all imagery was retrievably attached to space and time and, together, could be moved to other media, such as digital electronic files. This facility supported post-processing and full geo-referencing of the imagery and, additionally, review and reclassification as required. \n \nIn turbid conditions such as those encountered in Melville Bay, the towed video traverses close to the bottom so that the resultant imagery passes the camera, and consequently the observer, quite rapidly even at slow boat speeds. For this reason, taxonomic resolution to genera or species is usually restricted to readily identifiable organisms, some of which may be less important as keystone species in the environment. However, the most crucial ecosystem components, such as seagrasses are readily identifiable by morphology and tend to occur in mono-specific assemblages, enabling the technique to differentiate at fine taxonomic scales. Similarly, it was also possible to differentiate between Acropora dominated and massive (Porites, Goniastrea, etc) coral communities. In any event, issues with fine scale taxonomic resolution using this technique must be balanced against the ability of towed video to sample large areas of bottom in a cost-effective manner. \n \nAll data records were fixed in 4 dimensions (latitude, longitude, depth and time) and georeferenced to the World Geodetic System, WGS84, at a resolution typically less than 2m horizontally, 0.1m vertically and 2 seconds temporally. Each record had 2 positional fixes, one the nominal position of the tow vessel (that is the GPS) and the second the position of the tow body (=imagery). The latter was calculated using offsets, laybacks and ship heading and the accuracy of this estimate was inversely related to water depth. Excluding any vagaries in the accuracy of the GPS positioning device, in the shallow waters on the margins of Southern Melville Bay, position estimates were probably within 2 m of actual locations of the towed camera. \n \nThe habitat classification scheme used in this study follows: \nSpeare P and Cappo M (2006) Fish populations on inshore Shoals: A pilot study and lessons for monitoring the effects of zoning in the Great Barrier Reef Marine Park. Report to the Australian Government Department of the Environment and Heritage. Australian Institute of Marine Science, Townsville. 29 p. \n \nHabitat Classification: \n \nSubstrate Class (abiotic substrate components): \nSludge \nMud \nSilt (sandy mud) \nSand \nCoarse sand \nRubble (5-50mm) \nStones (>50-250mm) \nRocks (>250mm) \nNo substrate \n \nBenthos Class (overlying benthic organisms): \nSeagrass, Dense \nSeagrass, Medium \nSeagrass, Sparse \nAlgae, Dense \nAlgae, Medium \nAlgae, Sparse \nHard Coral, Dense \nHard Coral, Medium \nHard Coral, Sparse \nIsolates, Dense \nIsolates, Medium \nIsolates, Sparse \nWhips, Medium \nWhips, Sparse \nGorgonians, Sparse \nBurrowing Animals (Bioturbated Mud) \nNone \n \nIndividual Organisms: \nSolitary Hard Coral \nSolitary Soft Coral \nSea Pen \nNone \n \nThe percentage cover of benthic communities was classified as follows: \nS - sparse (0 - 30% cover) \nM - medium (30 - 60%) \nD - dense (>60%) \nThis data consists of 25752 records derived from 63 tows covering 36.114 km at 1.40m resolution, over a depth range of .9 to 18.1 meters. Classification of the benthos and substrate was based on 17 benthic classes and 6 substrate classes. Summary of the primary benthic categories recorded Gorgonians 0.98% Hard Coral 11.57% Macroalgae 7.78% NoBenthos 39.82% Seagrass 16.32% Other 23.50% Summary of substrates recorded Rubble 0.63% NoSub 0.88% Rock 13.64% Sand 25.38% Silt 27.55% Sludge 5.91% Mud 23.23% Stone 1.30% SandCoarse 1.44% A custom interface has been developed by AIMS staff for reading Towed Video tapes