Why We are Here
Known as the rainforests of the sea, coral reefs are perhaps the most threatened of all marine environments and are critically important to the communities that have been intrinsically linked to coral reefs for their survival for thousands of years.
The South Western Indian Ocean is a biodiversity ‘hot-spot’, with a large number of endemic species. Madagascan coral reef ecosystems are extremely rich and diverse with an estimated 752 fish and 380 coral species, the highest recorded coral diversity of the Western Indian Ocean and Red Sea. Of the 3,540 km of reef systems surrounding the island of Madagascar the majority are found along the west coast, which has 90% of the island’s coral reefs and 98% of its mangroves
The 32 km reef system in the Bay of Ranobe forms part of the Toliara Barrier Reef Complex. This is a 400 km system of nearly continuous shallow water reef, one of the most extensive coral reef systems in the Western Indian Ocean region, and the third largest in the world. This system, however, is under enormous pressure from a variety of anthropogenic stressors.
Outlined below is a brief explanation of the main issues affecting the marine habitats within the Bay of Ranobe:
1. High sedimentation rates
High sedimentation within the Bay of Ranobe is a result of the prolonged deforestation of terrestrial forests and mangroves. Over the years the cyclone and rainy seasons have resulted in increased levels of sediment being washed down the rivers and out to sea and, due to the loss of the mangroves along the coastline, sediment has been allowed to flow over the coral reefs. This increase in sediment in the water column causes disruption to feeding, reproduction and settling mechanisms of the reef’s fauna. Sediment increases can be tolerated to a certain point after which the fauna is killed, although the extent to which species can tolerate this increase does differ. However, the large scale and prolonged deforestation of the surrounding area has caused the release of sediment loads well above most species’ tolerance levels, namely that of the corals.
2. Sustained high seawater temperatures
The extent, severity and frequency of coral bleaching is increasing globally and mass bleaching is fast becoming one of the most important determinants of the future of these diverse and productive ecosystems. The rise in water temperatures in the Bay of Ranobe is a result of increasing global sea surface temperatures and the topography of the bay; with only two major but narrow passes for the lagoon water to escape, there is a low rate of flushing (cooler open sea water replacing warmer lagoon waters) so temperatures are allowed to rise and remain high for sustained periods during the summer months (October to March). In addition, climate change over the last five years has also caused the exterior waters to rise above their seasonal averages, so that the replacing waters are having little effect on lagoonal water temperature. Elevated seawater temperatures within the region over the last decade has led to mass bleaching events and, as large areas of shallow water corals affected have not recovered, these bleaching events have resulted in significant coral reef habitat degradation within the Bay of Ranobe.
3. Intensive fishing activities
4. Local population increase and influx of migrant fishermen
5. Mass algal blooms