Biological and geological information about Sakalava reefs and islands.
On request, we can organize information sessions on reef biology, Nosy Be island geology, marine geology,
human impact on tropical reef ecosysteme in general and Nosy Be in particular.
Dangers to avoid to keep diving safe...
Just to replace dangers in the righ order, sharks are a lot less harmful than a lot of others dangers a diver can encounter on a tropical reef. Divers are in the water to explore and observe, not to touch. Willing to touch everyting can turn a dive in a bad souvenir. Scorpion fishes, urchins, star fishes can have poisonous spines. The most dangerous are stone fishes and lionfishes. Corals have an abrasive texture, and some corals (fire coral) and some alga are urticant and their contact remind the contact with nettles. Apparently so harmless and so beautiful, cones shells can send their dart 5m away to hunt their preys. About 10 species, like marmoreus/ textilus/ geographus.. are deadly, because the conotoxine concentration is very high. Rays can have a cutting or poisonous dart or some can produce electicity (torpedo) and their contact can give an absent-minded diver a several thousands volts electric shock to be waken up.
Of course, all those animals are replying to an aggression. To observe them is not one. But it's better to know which animal you're dealing with.
In all those animals, one is harmful for human but deadly for the reef. It's eating 10m2 coral / year : acanthascter plancii. this starfish is catastrophe, and a group of it can destroy several km2 of coral reef /year. Their only predators are collected by humans (seashells, shrimps) and to cut it in pieces will make as many starfishes. the only solution is to take it out of the water, and to let it dry under the sun.
Nosy Be geology context
The position Nosy Be is really particular, between Madagascar and Africa, which makes a various geology. There's an Influence from the main land (Madagascar), streched by separation of the Gondwana and the Mozambique canal ouverture. Nosy be has been created by several volcanic episodes, ancient to recent. The island has sedimentary limestone and sandstone from lias, with some plutonic intrusions presenting different grades of erosion.
Tropical Indopacific islands common environmental problem...
Normal erosion is helped by human impact, who's still using in a lazy way the fire to clean the land in order to plant vegetables. All the deep roots are dying and the grass, rice, manioc or any other culture are unable to hold the soil when the rain season and heavy rains are appearing, especially when the rock underneath is basalt in decomposition which produce a colloidal mud, clay-like. The soil become very poor, and after being washed several years, totaly sterile. Another problem is those cubic meters of mud, driven by rivers to the sea, and spread over the reef. Covered by those particules, the corals can't breathe anymore, and get weak and die. Coral's eternal enemy for the conquest of the hard substract is the alga which take advantage of the situation and covers the dying corals. The plancton is also take benefit of it, the mud is increasing the phytoplancton , then the zooplancton (including larva of acanthascter plancii) which will add a volume of particules falling down on the reef.
Erosion, with human rejections, destructive ways of fishing (small nets, poison)... reef is dying. "El niño", a warming water phenomenon in central pacific zone is very often indicted for the coral death worldwide, and to remove human responsabilities. But a global phenomenon can't explain condition differences between reefs near inhabited zones and reefs in natural zones, sometimes less than 30km away.
Example of soil burnt and washed several years by heavy rains which produce a totaly sterile colloidal mud, clay-like.
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