Surveys of the marine habitats of southern Melville Bay, Northern Territory, using towed video (Towvid) and aerial photography

Brief description

Towed underwater video, supported by sophisticated real-time data capture, geo-referencing and classification software (TOWVID) was used to survey and map benthic habitats throughout Southern Melville Bay and waters adjacent to and seaward of Gove Peninsula. The field work was carried out towards the end of the dry season (30 October to 8 November), in light to moderate sea conditions and neap tides after the full moon.A total of 36.117 km of seafloor were mapped from 63 camera tows at an average spatial resolution of 1.5 m. Camera tows were conducted in the following locations: Gove Harbour (5 tows, 4.097 km); Wharf (6 tows, 1.855 km); Wanaka Bay (1 tow, 1.476 km); Inverell Bay (2 tows, 1.642 km); Halftide Bay (3 tows, 0.657 km); Middle Bay (5 tows, 2.447 km); Drimmie Arm (11 tows, 2.577 km); NoName Bay (3 tows, 3.385 km); Melville Bay (19 tows, 8.387 km); West Woody Island (3 tows, 2.955 km); and Northern Beaches (5 tows, 6.639 km). The low visibility and turbid water conditions constrained surveys in many areas to relatively shallow (Classification of habitat included one schema for the abiotic substrate components and another for the overlying benthic organisms. A limited suite of individual organisms were also recorded as point events. The percentage cover of benthic communities was also subjectively quantified.Maps of mangrove areas surrounding the Bay were to be extracted from aerial survey images provided by CDU. However, these images were found to be incorrectly rectified and for this reason were not included in the GIS database.
The aim of this study was to support the establishment of a monitoring program to address potential impacts of the cooling water discharge of the Alcan Gove refinery on the marine environment in Melville Bay. The study was designed to address this issue by providing baseline mapping of marine habitats within and around Melville Bay. These baselines will allow the establishment of a long term monitoring program that can document any changes in sensitive habitats such as mangroves, seagrasses and corals.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: Statement: Towvid:Visualisation 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.The 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.In 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.All 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.The habitat classification scheme used in this study follows:Speare 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.Habitat Classification:Substrate Class (abiotic substrate components):SludgeMudSilt (sandy mud)SandCoarse sandRubble (5-50mm)Stones (>50-250mm)Rocks (>250mm)No substrateBenthos Class (overlying benthic organisms):Seagrass, DenseSeagrass, MediumSeagrass, Sparse Algae, DenseAlgae, MediumAlgae, SparseHard Coral, DenseHard Coral, MediumHard Coral, SparseIsolates, DenseIsolates, MediumIsolates, SparseWhips, MediumWhips, SparseGorgonians, Sparse Burrowing Animals (Bioturbated Mud)NoneIndividual Organisms:Solitary Hard CoralSolitary Soft CoralSea PenNoneThe 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)