Tonga Expedition Leg 2

Today was the last day of quarantine before departing to Nuku'alofa tomorrow where we  board our expedition vessel DP1 Seaworker for the second leg of the Inkfish Coastal Seas Expedition. Most of us were haunched over on the computer: either creating check lists, tweaking the analytical pipeline that will be used to guide sample collections in the field, reprogramming microcontrollers, or even writing a quick tool to help with site selection (based on the bathymetry that was collected by the science crew on the first leg). In the afternoon we got a message from Joe O'Callaghan (Oceanly) saying that they had started their journey back to port. We are all looking forward to meeting up with them tomorrow in person and hearing the stories of the past two weeks at sea.

We arrived to Nuku'alofa  after a short flight and boarded the vessel. We did a hand-over with Joe O'Callaghan and the rest of the science team, and then proceeded with safety briefings and inductions. After that we made the most of being in port by unpacking a lot of the gear (that we loaded on the boat in New Zealand) and setting up the containerized labs. There are four containers: the top two will be dedicated to the AUV team and CalAcademy team on this leg, with one bottom container for storage of all the scientific gear, and the fourth holds a decompression chamber and office for the medic.

Today we spent the day steaming to Ha'apai, with lots of whale sightings on the way. During the transit, we went through various dive safety briefings and emergency protocols. As soon as we arrived and went on anchor, the gear storage container was reopened, and there was a flurry of activity preparing gear for the next day.

Today the AUV team spent a lot of time on Skye running trials for the mowing-the-lawn patterns that will be used to do the large-area photogrammetric reconstructions we have planned. The rebreather team spent ~3 hours in the water completing all their safety training and doing rescue drills, all while enjoying at least 2 hours of whale song in the background. In the evening, our medic Laurence gave us a tour of the recompression chamber that is in one of the containers on board, and walked us through the emergency protocols.

With all the diving safety training and AUV trials completed, we had an ambitious day planned where we wanted to try and do a full sequence of both teams working together. Figuring out a way for the AUV and divers to be working together is something we had been planning for months, and so we were both excited as well as a bit anxious to see if we could pull it off. Based on the bathymetry collected during the first trip, we used the planning tool built by the AUV team to find an appropriate site where we could image a section of coral reef from 5-100 m depth. The day then started with the tender (Skye) going out to the site with both the AUV team and the shallow divers, with the latter splashing first to set up the shallow perimeter laying out 25m transect tapes (at 20, 10 and 5m) and deploying acoustic (USBL) tags at both ends. Based on the acoustic positioning, the AUV team then designed a mission based on that perimeter and deployed AUV. After multiple iterative AUV missions, the rebreather divers then splashed in the afternoon to set up the deep perimeter again with acoustic (USBL) tags, and complete high-resolution photogrammetry at 60, 40, 20, 10, and 5m depth. During their ascent, the rebreathers shot a surface marker buoy (SMB) to signal the shallow divers to come and meet them, and they started doing additional shallow-water mapping (5-22 m) to include the structurally complex sections of reef in the shallow that are more difficult for the AUV to map. Thanks to the amazing logistical support of the Seaworker and Inkfish crews, we managed to pull off the entire sequence without too many hiccups, and so there was a great sense of both relief and excitement that the plan had worked out.

Today was a scheduled dry day for the dive team, which is a safety measure after multiple days of deep diving. This allowed for the team to run 3D reconstructions for all the collected data from the first two days, and develop the sampling strategy for the next days of collections. For PhD student Phaedra Hernandez this meant scoring  the different coral species inside the high-resolution focal plots from mesophotic depths, and determining which of those will be collected. PhD student Dennis van Hulten focuses on highly abundant species in the shallow (5-25 m), where he projects a grid onto the 3D reconstruction, and then assesses which of the (~2,700) grid points hits his target species. He then constructs a numbered navigation path across those points, which gets uploaded to an Android phone in an underwater case, which will then guide the collecting diver. All in all, a lot of computer processing work with lots of moving parts (and the usual last minute tweaks and fixes) and pressure to get everything ready for the next day in the water. Meanwhile, the AUV team went out into the shallow waters around Ha'afeva, where they conducted trials for the broad-scale surveys (focused on habitat mapping rather than photogrammetry) and further fine-tuning of the vehicle's positioning capabilities.

We woke up to unfavorable weather with north-easterly winds met by southerly swells, making it likely that our Kotu sites may not be accessible to us today. We decided during the toolbox meeting to send out the tender (Skye) to verify, and they confirmed that the swell was too large for operations. After checking out several other potential "plan B" spots, the tender crew could not identify any alternative sites that go deep enough (~60-100 m depth) and that were accessible in this weather. Instead,  the AUV team and shallow divers went out to scout for any potential sites harboring large monostands (sections of reef dominated by a single species of coral) around Ha'afeva but unfortunately were not able to locate something suitable. Fingers crossed the wind changes direction somewhat tomorrow, so we can revert back to our Kotu sites.

Weather clearing up a bit in the early evening?

The weather continued to be unfavorable for work at Kotu, and so after a quick run to check conditions with Skye, we decided to relocate work to the more protected island Tungua. Using the bathymetric mapping data generated onthe first leg, Jackson reran his models to see if there were any sites that give the right terrain but perhaps do not go as deep as originally intended (60-100 m). We identified  a site to the south of Tungua, and decided to give it a go.  The shallow reef of the site was very beautiful and extended down to ~40 m depth. The rebreather divers explored the area between 40-60 m, however the area was very sandy with very few patches of hard substrate, and so photogrammetry transects were conducted from 40 m and up. Conducting the transects and experiencing the beautiful shallow reef, while hearing whales singing in the distance was an incredible experience. The AUV also had a successful mission mapping the reef down to 50 meters with plenty of diver-AUV encounters.

Still pretty surreal to be working underwater alongside an underwater robot

Same drill – the tender (Skye) went out to assess conditions on the outer reef of Kotu (Kopo) and this time we got the thumbs up. Still a bit roly for Seaworker on DP (dynamic positioning - a system that allows a the boat to maintain position and heading) but workable. So Skye filled up with both the AUV and dive team again, with the former focusing on filling some of the gaps in the 3D reconstruction of this site, and the latter spending the day conducting coral collections. The rebreather team went down to collect small tissue samples (~0.5cm2) from corals in the high-resolution photogrammetry transects at 60, 40 and 20 m depth. After shooting a surface marker buoy (SMB) they were met by the shallow divers who brought down the photogrammetry camera, so that the rebreather divers could continue with photogrammetry work. The shallow divers then took over collecting samples from 5-20 m depth, with Phaedra focusing on corals that are common at mesophotic depths and Dennis & Norbert sampling corals from the grid projections (using a phone in an underwater case). The evening was spent processing the coral samples, downloading the data, fixing some faulty gear, and preparing for the next day of collections.

AUV flying over and mapping the shallow 10 metre transect

After some weather contemplation, we decided during the morning toolbox meeting to make the crossing to the Tofua and Kao volcanoes. It was difficult to ascertain whether conditions were going to be workable but we decided it was worth the attempt, even though it meant that planned AUV trials of the secondary vehicle could not go ahead (the divers had a dry day). After loading the tender (Skye) back onto Seaworker and strapping everything down, we departed around 0900 and then arrived at the islands around 1400. It was an amazing sight watching the volcanoes (that we had seen on the horizon over the past week) slowly come closer and closer. Once we arrived at Tofua, there was great excitement to see this island with its rich history up close, smelling the sulfur and watching the smoke rise up from its crater. We cased out a potential mapping site for tomorrow, and Seaworker then retreated to a back-and-forth pattern for the evening (as conditions were not suitable for anchoring or DP).

Kao (left) and Tofua (right)

AUV documenting the beautiful reef of Tofua

Everybody was itching to get back in the water and revisit the site at Tofua that we had set up yesterday. As always, the AUV team were the first to be ready to go, and they were all prepped to fill in any gaps of yesterday's photogrammetric mapping. There was a lot of running around making sure we had all the sampling equipment ready to go, and walking through the dive sequence to ensure everybody knew what to do at which part of the dive. This becomes very important as the shallow divers spent the first hour in the water, with the rebreather divers splashing right when they surface and then spending another 3-3.5 hours underwater, with the shallow divers joining the second half of that dive. That means a lot of time underwater and a lot of different activities happening without having the ability to talk things through once the plan is in motion. Throw in the concurrent AUV operations happening from the tender, and hence why it is so important to talk through the dive sequence multiple times. Collections went very smoothly at 60 and 40 m depth with samples collected from ~70 different corals – the view at that depth is pretty astonishing with dramatic cliffs and schools of fish buzzing around. On the way up, the team suffered an oxygen sensor issue and so had to abort collections and move straight up to decompression - luckily they could communicate to the shallow divers and they were able to take over their shallower collections. In shallower waters, the sensor started behaving normally again, and the divers could enjoy the very shallow bouldery section of Tofua reef that was teeming with life. Given the very calm seas in our protected spot, the AUV team decided to continue mapping into very shallow waters, right up until the steep vertical wall that rises out of the water. Back on the boat, there was a frantic mission to get all the samples processed and gear washed, as the plan was to steam back to the Kotu area right after dinner.

The  steam from Tofua to Kotu overnight was a  lot bumpier than expected, and so there were some tired faces in the morning. However, nothing that could not be cleared up by yet another amazing breakfast served up by Trevor & Danielle! Then it was time for the last toolbox meeting of the trip, discussing a morning of AUV activities on Skye, followed by the departure to Nuku'alofa around 1500.  The AUV team decided to put their secondary vehicle in the water, and came back very happy as the vehicle performed great straight after assembly, and they managed to get some more mapping done. Meanwhile, the dive team had started packing up all their gear, and reshuffling everything in the storage container to ensure that gear for the next and final leg of the Inkfish Coastal Seas expedition in Tonga was easily accessible. At 1515 the anchor was raised and we started our journey back – what an amazing and productive trip it has been! We would like to thank the amazing crew of Seaworker for all the hard work and providing a safe and supportive platform for our research, and the Inkfish tender team (Gaby / Dario / Laurence) for meticulously ensuring our safety and lots of heavy lifting. Finally, we thank the people of Ha'apai and the Kingdom of Tonga for welcoming us to study their beautiful ocean world – "mālō 'aupito 'i ho'omou talitali mafana.".