SWIOFP Acoustic and Pelagic Trawl Survey around Mauritius and the Mascareigne Plateau

Two weeks at sea, and the SWIOFP survey of the Mascarene Plateau in the Western Indian Ocean is nearly over. Much has been achieved, but as always, much, much more still needs to be done. Unfortunately the RV Dr Fridtjof Nansen will now leave the Indian Ocean and return to the Atlantic. This is mainly as a result of the piracy threat in the Indian Ocean, which has sadly led to the cancellation of SWIOFP surveys on this vessel along the coasts of Tanzania and Kenya.

One of the most startling results of the survey of the Mascarene Plateau was the virtual absence of small pelagic fishes, such as scads, sardines and anchovy. This result confirms the finding of a 2008 survey, which took place during a different season of the year, and which also found very few small pelagics. Bottom trawling showed another picture, however. With relatively few bottom trawls, we identified a large biodiversity of fishes, ranging from typical coral reef inhabitants, such as butterfly fish, wrasses, angel fish, parrot, trigger and file fishes to larger predators such as emperors, groupers, carangids and sharks (see pic).

The trawl net had to be handled with great care, to minimize damage to coral reefs, which cover much of the Nazareth Bank. Even so, we did manage to snag and damage the net on, wait for it, trawl number 13! Looking at what fishes do occur in this area, and the obviously good condition of reefs here, one can only applaud the Mauritian Government’s prohibition of bottom trawling in this area!

A number of fishing vessels were observed on Nazareth Bank. One of them was a mother-ship with about 6 dinghies dispersed over many miles of ocean surface, each with 3 line-fishers busily hauling their bottom drop-lines. How do they manage not to get lost in this vast blue ocean, hundreds of miles from the nearest land?  

In conjunction with the biological sampling, the ship did a hydrographic transect across the top of the Nazareth Bank. This transect repeated a similar transect done in 2008, and will serve as a reference of the hydrographical conditions encountered in this very isolated area of the ocean. Sampling was done by lowering the CTD (which measures profiles of temperature, oxygen, fluorescence and salinity over a depth gradient) at each of 7 stations across the Bank. The CTD was lowered to a maximum depth of 1000m at the edges of the Bank, and to the seafloor at 50m depth on top of Nazareth Bank. At each station water samples were taken to determine chlorophyll concentrations, and a plankton net was deployed to collect plankton samples at the surface, 80 m depth and approx. 150 m depth.

For the past 24 hours, the RV Dr Fridtjof Nansen has been steaming southwards, towards Port Louis in Mauritius, which we should reach by tomorrow morning. There, a reception by the Mauritian Minister of Fisheries and Rodrigues awaits us. And that will be the end of our journey, which I think will remain for long in the fondest memories of the scientists onboard. We all saw and learnt much, and doubtlessly the skills learnt on board the Nansen will echo throughout our respective careers as marine scientists! In fact, I have the feeling that most of us are now enjoying this particular survey so much that we are not too keen to leave the ship at all  (see pics)!!

 We had some heavy seas on the first few days out of Mauritius, and many of us were somewhat off-colour, to say the least. But the last few days were plain sailing, and the ship has slowly made her way northwards in a zig-zag pattern, to cover the slope around Mauritius, the Soudan Bank, St Brandon, and now the Nazareth Bank. All of these exotically named banks are actually way below the sea surface, so alas, all one can see from the deck is the very clear blue and wide Indian Ocean. Well, not quite all of it. We did get to the minute Frigate and Pearl islands - each about the size of two football fields, and sporting their very own clusters of palm trees. Not surprisingly, they are uninhabited, and definitely in danger of being submerged by rising sea-levels (see pic). Around these islands were large flocks of seabirds, including shearwaters, several species of terns, boobies and even four large frigate birds. Some flocks numbered more than 400 birds diving on a school of fish close to the surface. While anchored near Frigate Island, we lowered a HD video camera to the reef at about 25 m deep, and shot about 1 hour of superb footage of the reef, showing many damselfishes, butterflyfishes, wrasses and even a large trevally passing by. Definitely a non-destructive sampling method with good potential for the future!

Talking about fishes, the echograms have so far shown just about no pelagic fishes along our transects - and definitely no concentrations worth trawling on! Echograms have shown mostly plankton possibly with some lanternfishes - too small and scattered to trawl on. To compensate, weve done about 7 surface trawls for fish and crustacean larvae, and a couple of CTD stations and plankton trawls to look at the depth of the thermocline and the layer of phytoplankton. The latter lies surprisingly deep, at about 80-130 m, below the warm surface waters and right at the deepest penetration of sunlight. No fishes there though - but many other strange organisms such as the beautiful little squid,

Stoloteuthis sp. photographed by Oddgeir Alvheim (see pic)! 

The echograms did however show some concentrations of bottom fishes. A slight hiccup was that our research permit allowed only for pelagic trawls - no bottom trawling!! So special permission had to be obtained from the Mauritian government, and yesterday the new permit arrived! Within 10 minutes, the demersal trawl net went over the side and down to 300 meters. And it did not disappoint! When lifted aboard about 45 minutes later, we had caught a giant of a grouper, nearly 1.5 metres long and weighing in at about 125 kg (see pic).

Seabird counts are done each day, and so far at least 10 species have been identified, including white and sooty terns, red-footed boobies, common noddy, and wedge-tailed and Audobon shear-waters, and of course the greater frigate birds.

No tropic birds yet, but we are holding thumbs. No whales or dolphins so far, possibly reflecting the absense of pelagic fish (their food) that we are also seeing on the echograms. Today we are approaching an area where pelagic and demersal fishes were present two years ago, possibly as a result of local upwelling, so we are looking forward to doing some trawls a bit later on! For now, greetings from the Nansen, where all is well at present, with an ocean as flat as a mirror - or almost -, sunny skies and warm waters.