As I stepped into the control room for my first morning watch, Isis dive 161 had been going on for 14 hours, and was still a little more than halfway through. This dive served two main purposes. The first one was to conclude a series of predetermined 500-m transects along the sea floor at the NW station. The second one was to poke the amphipod trap that had been deployed 36 hours earlier and could not be retrieved after triggering the acoustic release.
Video transects are typically carried out while flying at a constant speed of 0.4 knots and an altitude of 2 metres. Isis is equipped with a downward-looking HD colour camera for continuous recording of the seafloor. Video footage will later be analysed for a quantitative characterisation of the surveyed biotopes. During the transect any relevant observation is logged, and a video frame is grabbed using a specific software. Dive logs and preliminary annotation done in real time are very important since they enable reconstructing the sequence of events at a later time, thus facilitating the analysis of the huge amount of footage produced during ROV missions.
Two very different landscapes were surveyed during this dive. First, Isis performed a series of transects along a vertical wall at around 2500 metres depth. These were inhabited by a wide variety of fauna typically associated with rocky substrata, such as sponges of all shapes and sizes, black corals, crinoids and brittle-stars. The second part of the dive was dedicated to surveying a flat area of soft mud, which was literally covered with traces and tracks produced by an abundant benthic fauna comprising sea urchins, sea cucumbers, seastars and acorn worms. Scattered throughout the sea floor we saw a few xenophyophores, strange giant single-cell protozoans with an even stranger name. During the exploration of this flat area, Isis also came across numerous bony fish that roamed just above the sea floor, such as the rat-tail (Coryphaenoides spp.), the blue hake (Antimora rostrata) and one fearsome-looking lizardfish Bathysaurus ferox.
All transects done, we just had to retrieve the amphipod trap. So, we headed for the next waypoint, which coincided with the spot where the trap had been previously located from the surface. After some search the trap was picked up by the sonar, and minutes later it was shining on our screens. Swimming around the trap was a few chymaera that had been attracted by the mackerel placed in the trap to be used as bait for the amphipods. A close-up scrutiny of the structure showed that there were hundreds of amphipods swarming around, letting us anticipate a good catch. A few pushes with the manipulator arm set the trap free from its weight and it quickly lifted up towards the surface. Mission accomplished for Isis, it was time to bring it up for servicing before the next dive.
As I write these lines, Isis has just finished its descent towards the next sampling area. This time the objective is to obtain high-resolution digital photographs of the benthic fauna and to collect voucher specimens. The ROV is well equipped for the task, but judging by the size of the wish list, a long night awaits.