AUV exploration of the mesophotic reef zone in the South Ryukyu Islands

2016 Fiscal Year Research-status Report

• Project/Area Number: 16K17819
• Research Institution: Nagoya University
• Principal Investigator: HUMBLET Marc 名古屋大学, 環境学研究科, 特任准教授 (40623616)
• Project Period (FY): 2016-04-01 – 2018-03-31
• Keywords: Coral reefs / Mesophotic habitats / Benthic communities / Seafloor topography / AUV / ROV

Outline of Annual Research Achievements

A new lightweight autonomous underwater vehicle (AUV) called Highly Agile Terrain Tracker for Ocean Research and Investigation (HATTORI) was built by Dr Toshihiro Maki's laboratory at the Institute of Industrial Science, The University of Tokyo. This new AUV is designed to record high-resolution images of the seafloor and acquire topographic data while remaining at a constant preset altitude above the seafloor. It is especially suitable to track the rugged topography of coral reefs and will be used to explore the seafloor at our target area located along the southern margin of the Sekisei Lagoon southwest of Ishigaki-jima.
Initial testing of the new AUV was successfully carried out in the Tonoshiro port at Ishigaki-jima in November 2016. At the same time, in order to select the location of transects along which the AUV will operate, a small remotely operated vehicle (ROV) equipped with a camera was used to explore the seafloor at the target area along two transects at depths ranging from 30 to 115 m. A super short baseline acoustic system (SSBL) was used to determine the position of the ROV during each dive. Based on the ROV images and a high-resolution topographic map of the target area, eight 100 m long transects roughly parallel to contour lines ranging in depth from 30 to 65 m were selected for the AUV survey scheduled in May 2017. These transects were selected so as to encompass a wide range of substrate types and topographic features in order to map the distribution of benthic organisms in various mesophotic habitats.

Current Status of Research Progress

3: Progress in research has been slightly delayed.

Reason

At the start of the project, it was decided to use a new lightweight AUV instead of the larger and heavier model that was originally chosen for this study. The initial plan was to carry out a first survey in September 2016 using a ROV only. Instead, the first survey took place in November 2016 after completion of the new AUV and included a preliminary phase of testing of the new submersible. With its relatively small size and weight, this new AUV will be an important tool to explore the marine habitats down to mesophotic depths at a lower cost than was previously possible.

Strategy for Future Research Activity

For our second survey scheduled in May 2017 four locations were selected along a 900 m bathymetric profile at depths ranging from 30 to 65 m situated on the southern margin of the Sekisei Lagoon southwest of Ishigaki-jima. At each location, the AUV will perform continuous high-resolution imaging of a 2-3 m wide strip of seafloor along two 100 m long transects roughly parallel to isobaths (eight transects in total). These transects were selected so as to include a wide variety of mesophotic habitats. The AUV will be equipped with a SSBL device that will enable its localization during underwater operations. The AUV will also carry sensors to measure salinity, temperature and light attenuation in order to characterize the oceanographic conditions of the different habitats surveyed. In parallel, a small portable ROV will be used to visualize the operations of the AUV during each dive. The AUV will be tested first in shallow protected waters before operating along the selected transects in the study area.
The seafloor images acquired along each transect will be used to identify live and dead benthic organisms and map their distribution as well as that of the different substrate types. The 3D imaging capability of the AUV will be used to match the distribution of organisms and substrate types to the seafloor topography. The results will be used to evaluate the role of factors such as depth, salinity, temperature, light attenuation, substrate types and topography on the distribution of benthic organisms at mesophotic depths.

Causes of Carryover

The second survey initially scheduled in March (FY2016) will take place in May (FY2017). The reason is that it was decided at the start of the project to use a new AUV to carry out the survey. Therefore additional time was needed for completion of the submersible and testing before conducting the survey at the study site.

Expenditure Plan for Carryover Budget

The remaining amount will be used together with the amount initially allocated to the fiscal year 2017 mainly to cover the cost of the second survey in Ishigaki-jima. This includes the transportation cost of the material, the rental fee of a small fishing boat for a maximum of five days, the rental fee of a car for one week, and the travel cost of three collaborators and myself.