Inner Hauraki seafloor acoustic mapping

The seabed of the inner Hauraki Gulf is mapped to provide a general overview of the types of habitats present and their spatial extent within the survey area. This information can then be used to help identify potential marine reserves in the area, and as a general resource inventory. This work builds on a similar survey undertaken for the Firth of Thames. Purpose: To provide a general overview of the types of seabed habitats present and their spatial extent within the survey area.

Survey of the area was undertaken in May - December 2002 using the acoustic mapping system QTC View. The survey area covered ~ 2200 km2 of seafloor. Main habitats and their spatial extent are provided at a relatively broad spatial scale. Six acoustic classes were detected with the QTC View system, and these fell out as discrete areas within the survey area, strongly related to depth and distance offshore. Descriptions of epifaunal assemblages and overlays of fish usage from the MFish database are included in Morrison et al. (2003).

Inner Hauraki Gulf – south of Waiwera and due east to Colville. Excluding estuaries and the Firth of Thames (south of Coromandel Harbour).

Inner Hauraki Gulf – E2725589 N6453054, NZMG,1:20000, AREA

May – December 2002

The survey was run in two phases; the first consisting of parallel west-east survey run-lines at 1500 m intervals. Boat speed averaged approximately 3 metres per second during the survey runlines. A differential Global Positioning System was used to ensure high spatial accuracy in navigation, and recording of the spatial data. Two data collection systems were run simultaneously during the survey. QTC View was used to collect bottom habitat classification data, while HYDROTM (Trimble Ltd) was used to record spatial positioning information and depth measurements. Two different survey vessels were used; the University of Auckland's research vessel 'Hawere', and the Department of Conservations vessel 'Hauturu". Both video footage and sediment grab samples were collected from each station. Video footage was collected using a low-light underwater camera, suspended on a sledge held just above the seafloor. The survey vessel was allowed to drift with prevailing wind and tidal currents for a period of ~10 minutes, or the engines used to hold the vessel on station / move it slowly, depending on current speeds. Video footage usually covered 100–200 m of seafloor. A Smith-McIntyre grab sampler was used to collect three sediment samples from each of the ground-truth stations. Each sample was visually assessed for primary substrate type (muds, sands, shells, rocks), and for secondary cover such as larger dead shell, sponges, and rubble. A sediment scraping ~3 cm deep was taken from the center of each of the cores, and archived for potential later sediment analysis (a 200 kHz sounder can be expected to penetrate 2 to 4 cm into the soft substrates, depending on hardness). The remaining sediments were run through a sieve of 1 mm mesh size, and preserved in formalin for later analysis of the infauna. Then the preliminary results from the first phase were discussed with DOC's S&R unit, and phase 2 sites chosen to cover smaller seafloor blocks that represented both high and low habitat acoustic variability areas. These sites were; low acoustic variability, proposed Te Matuku marine reserve (Waikehe Island) (7 km2), North of Whangaparoa Peninsula (3 km2); high acoustic variability, southern Motuihe Island (9 km2) and northern Ponui channel (9 km2). Acoustic data ground-truthing (video and Smith-McIntyre grab-samples) was achieved using sediment grab sampling and video imagery from a minimum of two ground-truth stations assigned to each of the acoustic classes identified. Frequency of collection: One off survey

Data quality: QTC and HYDRO raw data files are groomed for obvious outliers based on depth. Attribute accuracy: Phase 1 misclassified some non-mud areas as mud classes, due to data swamping. The more detailed Phase 2 mapping resolved these misclassifications. Completeness: Not reported

electronic Digital Format: QTC IMPACT, GIS (ArcView)

Placement of marine reserves General resource inventory

Summary info/maps freely available. See NIWA/MFish for specific invertebrate or fish data. Sensitivity/Confidentiality: See Department of Conservation or NIWA

still being completed

Associated benthic sampling from this project is still to be processed (separate DoC contract). Additional comments: Unexplained variability in the phase 1 QTC View plots, suggest that additional habitat/acoustic types were present within the sixth QTC acoustic class. Trawl survey results indicate that snapper and trevally appear to have an affinity for acoustic class 6. Epifauna were in general quite sparse, but were more common in the higher acoustic diversity areas, more intensively mapped in the second phase. Digitised shape files for the historical extent of mussel beds in the Firth of Thames are held by DoC - Jason Roxburgh (FOT only) and Clinton Duffy (HG and FOT combined).