Febuary 2013 Expedition - Recording coral and other underwater life
My main scientific task on this expedition is to assess whether coral and other life form cover have changed since 2006. This includes assessing soft coral, calcareous algae and macroalgae amongst others.
During my first visit to Chagos in 2006, I recorded a series of underwater video transects at reef sites both in the lagoons and seaward sides of the atolls. The video is valuable because it records a detailed overview of what the reefs look like at the time. It can then be archived and compared with future recordings. I’m therefore trying to revisit the same sites in 2013 and re-record the same areas of reef.
Of course video technology has moved on since then. Back in 2006, I was using an analogue camera in a large housing unit and recording onto tapes. Battery power was always a limitation, especially for the halogen lights. Today, I’m using the latest Sony CX550 High Definition camera which records onto a 62Gb card. This piece of kit lacks moving parts making for a lighter, smaller and more reliable system. And much less demanding on battery power.
The underwater housing (Light & Motion Blue Fin), has electronic handles that control the camera remotely, minimising the risk of leaks, since there are no levers passing through the sides of the housing. I invested in a wide-angle lens for the housing to ensure a broad area of coverage, even when close to the reef. The most impressive part of the system are two very strong LED lights. These are excellent at picking up the pink and red colours of calcareous algae. However, one disadvantage is that sharks appear to be taking an interest in the activity – maybe they can sense the camera electronics!
Needless to say, it can be a challenge to find the same sites that I visited in 2006. A Global Positioning System (GPS) gets us to within 10m or so of a site and then it’s a case of recognising the reef. This is actually easier than it may seem. Because either the locality of the sites is very homogenous, for example, a stretch of outer reef with the same aspect tends to be very similar, so it really doesn’t matter where I record. Or there is an obvious feature such as a knoll or bommie of coral sticking up like a small mountain from the lagoon floor, which is easily recognisable. To help me find the sites in future years, we are embedding some metal stakes into the reef at 25m, 15m and 10m depths.
However, one thing that has prevented me revisiting 3 sites so far has been the weather and lunar cycle. Our last day in Salomon and first day in Peros Banhos brought strong winds from the North West. These generated big waves and long swell making it difficult to access the seaward side. Our dive boats are small - about 4m - and when loaded with 4 divers and their kit, we are slow and necessarily cautious.
We’ve also been experiencing spring tides. When combined with large waves, they pour water over the reef into the lagoons, and later, the water pours back through passes and runs along the reefs creating tidal rips that we can’t swim against.
It’s even more difficult to swim with a video camera, and especially to zigzag up the reef from a 25m depth to a 5m depth to record the reef community structure. I swim the camera around in a circle at 25m depth, and then progressively record the reef in swathes until I reach 20m depth. I then record another circle to record the ‘seascape’ and then record up to 15m depth and so on.
Once back from a dive, the video is downloaded from the camera onto a hard drive, and all the equipment is put onto charge for the next excursion.
A great addition to this expedition is a containerised laboratory mounted on the top deck of the Pacific Marlin. Many of us spend many hours in this lab processing samples and data, and are very grateful to the Marlin crew for fixing us up with air conditioning, electricity and water.
Before departing from Diego Garcia, we spent a day moving in benches, shelves, fridge, sink and all our scientific equipment. It’s an enormous improvement from trying to work under tarpaulins stretched across the deck in 450 temperatures!
We have genetic fish work happening in one corner: extraction of small shrimps from coral, clams, sea cucumbers and so on. And in another, lots of cameras, GPSs and ship-to-dive boat radios charging.
When I return to the School of Ocean Sciences at Bangor University, with the help of some MSc research students, I will have the long job of working through the video data.
We use a software programme called CPCe (National Coral Reef Research Institute, Florida), which allows me to analyse around 50 random sections from each depth range of the video.
I then record what lays under about 20 random points at each depth, and input the data via tabs. This generates a spreadsheet which calculates the percentage of cover from each category such as coral type, substrate, calcareous algae etc.
The data will be compared with the data analysed for 2006. Back then, the reefs were recovering from the 1997/98 bleaching event, and possibly other events in later years. The corals were mostly separate, having not really formed much of a canopy.
I had expected the coral canopy to have joined together and developed substantially, but I’m seeing many corals on the shallow seaward terraces appearing to be battered by storms. And many corals deeper than 15-16m in the lagoons to be dead.
The lagoons here are very unusual, being nearly 100% coral cover, rather than sand. It is early days in the expedition, but it looks as though the reefs are in worse condition than 2006, which is very alarming since recovery has been seen in the years since.
The dead corals in the lagoons may have died due to a warming event, since the same species seem fine in shallower water where they may be more resilient to higher temperature, or perhaps cooled by water over spilling the reef.
There are plenty more sites to revisit, and we will also record some new sites – I just hope that these are better than some of the sites surveyed to date.
Jon Turner