OPERATION - Immersion of an automated system of image acquisition: a better knowledge of the marine
Designed by Hervé Demarcq of the MARBEC UMR in Sète, it constitutes an experimental part of the pelagic mission, planned for the continuous acquisition of underwater images during 2.5 days or 5 days depending on the constraints of the benthic team.
In addition to conventional pelagic, oceanographic and acoustic observations, the principle is to observe at a fixed point the passage of pelagic or semi-pelagic species between the surface and the upper parts of the Walter Shoal.
The system consists in a flexible stainless steel anchor line with 6 cameras (gopro session 4) powered by one or two Lithium-Ion rechargeable batteries, all in sealed enclosures that have been previously successfully tested thanks to the hyperbaric chamber of the research vessel. A diving camera targets the bottom at about 3 m from the ground, three others are placed at about 10 m depth for a 360° viewing.
To establish links with the benthic operations, the bottom camera will allow a comparison between the pelagic and benthic compartments. The observations are coupled, on the one hand, between the pelagic observations and the bottom occurrence of benthic species, on the other hand between the photos taken every 5 seconds at the bottom and the divers sightings. The final contribution of this operation will depend in part on the results of both teams and how they can be combined. In the absence of an exhaustive sampling of the upper part of the Walter Shoals, a crown of about ten km in diameter, a first estimate of the biological diversity on a tropical seamount will be possible.
La mise en œuvre
Unfortunately Hervé could not be on board as originally planned. Since the electronic equipment is delicate to handle, adequate skills in assembling and putting into operation are required from the team. The mini-cameras were installed every time by Pascal Cotel and Florence Galletti (IRD), in their plexiglass cases after recharging the batteries. A delicate point lies in the sealing and greasing of the seal of the housings. The finalization of the anchorage was carried out on board, with the precious help of the crew. The flexible programming and the real time constraints require quick decisions. The operation is dependent, for the launch and recovery of the line, of the divers of the benthic team with their zodiac, of the weather and all time constraints from the other research operations or the board.
Theoretically simple, the practical implementation is not. It is depending on the usual hazards (for example, the surface box, dedicated to the observation of birds, has been broken during the first immersion) and the delicate sealing constraints.
The system was first immersed on a 26m bottom, relatively far from the boat, for about 2 durations of 24 hours, then with 4 cameras only and finally 3, which were able to take continuous images at regular intervals of 5 seconds. A single camera records about 35,000 photos in two and a half days, recorded on 200GB micro-SD cards, then transferred to a hard disk.Three out of six cameras and their associated external batteries are out of order at the end of the survey, which involves the collection and proper treatment of the waste generated.
The images obtained do not have the beauty of those from the divers, nor do induce the same immediate sensations, but they do not have the same scientific role. They already show rainbow fishes, a galapagos shark (Carcharhinus galapagensis), probably squids …
An extra part of the work will be the counting after the mission, with a relevant automatic recognition system, of the fishes on the images, in quantitative or even qualitative terms, depending on the quality of the information. This phase will be done in association with teams specialized in image recognition.
