Hauraki Gulf mapping

Seabed mapping by NIWA has revealed a range of seafloor habitats, fine scale structures and the impacts of human activities beneath the waters of the Hauraki Gulf.

The mapping covered four areas of the Gulf - Port Fitzroy (Great Barrier Island), the large island-enclosed embayment just north of Coromandel Harbour, the east side of Ponui/Chamberlains Islands, and the area between Rotorua and Tarakihi islands (east of Waiheke Island). Three of the locations are high value juvenile snapper (<90 mm length) nurseries; while the fourth holds a diversity of seafloor habitats and associated species assemblages (and a juvenile blue cod nursery area).

The NIWA Wellington multibeam sonar team, in collaboration with the MBIE “Juvenile fish habitat bottlenecks’ programme and R.V. Ikatere skipper and crew, collected high resolution bathymetry (the measuring of underwater depth - the underwater equivalent of topography); and back-scatter (how the sound bounces off different types of surfaces, e.g. mud, hard substrate) across the locations.

Along with the range of habitats and structures, the mapping also revealed clear ‘foot-prints’ of multiple mussel farms in two of the areas.

These data will be processed into maps and used to help further describe and understand the dynamics of fish nurseries. In particular, they will underpin juvenile fish tag-recapture experiments in 2020, as part of the juvenile fish habitat bottlenecks programme, which will quantify mortality/survival rates, growth, and fish movement across different habitats and habitat landscapes. The final seafloor maps will also be used by Foundation North (a Trust fund for Auckland not-for-profit groups), with assistance from NIWA, to help better inform people about the importance of seafloor habitats, and how their functions can be adversely affected by human activities. 

The work was carried out through funding from Foundation North’s GIFT Fund, with a matched contribution from Auckland Council, Waikato Regional Council and the NIWA Vessel Fund.

Initial low-resolution field map of multibeam backscatter in the large embayment just north of Coromandel Harbour. Light grey indicates harder seafloor substrates; the rectangular features are mussel farms, with both the farm structures and the seafloor below contributing to the backscatter return. High juvenile snapper (< 90 mm) densities occur in this area; it contributes a significant part of one of four key Hauraki Gulf snapper nurseries. [Image:NIWA]