JBL Expedition Indian Ocean: part 2 - the coral reefs near Madagascar

Between the whale shark trips we had time to visit the reefs near the shore, to take some water measurements and to assess the biodiversity (variety of life forms). It was noticeable that the reefs at Nosy Be were in widely varying conditions: Parts were intact, showed no coral bleaching and had a species-rich and individual-rich fish density, while other areas looked like cemeteries. There were metres of stone corals (mostly Acropora) lying around dead at the bottom!

Why for these reef areas were existing side by side to each other was completely unclear. An increased water temperature in the past would have caused the entire reef area to die to a certain depth, but never partially. Since the dead and intact areas could be found at the same depth, storms could not be the reason. Their wave troughs can still "clear away" the corals from the reef at depths of up to 10 metres, but then everywhere and not just in one place!

We have no solution to this riddle. Everything was lively in the intact reef sections and there were a lot of turtles enriching the underwater life.

In the shallow water above sandy ground we found the biggest shoals of fish. This reminded us of what we learned at the shark workshop in the Bahamas ( Taller 2018 Bahamas ): Shark accidents often occur at sandbanks, because the sharks look for food there, but the shallow water creates a stressful situation for them. I was able to take some good pictures of the shoals of fish, but didn't meet a single shark. For us aquarists it was fantastic to see large shoals of full-grown, 20cm long, silver moonyfish (Monodactylus) under the boats seeking for shelter.

The water values measured: conductivity 53.1 mS/cm; density 1.025; pH 8.2; carbonate hardness 8 °dKH; calcium 460 mg/l; magnesium 1340 mg/l