The Bay of Ranobe

Outlined below is a brief explanation of the problems ReefDoctor and other scientists believe are affecting the reef systems within the Bay of Ranobe.

1. High sedimentation rates
High sedimentation within the Bay of Ranobe is a result of prolonged deforestation of terrestrial forests and mangroves by generations of Vezo (fishermen) and cattlemen in the area, and villages up and down the coast for agriculture, construction of homes and fuel.

Over the years the cyclone and rainy seasons have seen increased levels of sediment 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 water temperatures
The rise in water temperatures in the Bay of Ranobe is a result of world wide climate change 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 in 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.

During the months from the end of December to the end of March for the years 2001, 2002 and 2003 ReefDoctor recorded average sea surface temperatures of 30ºC and 31ºC down to a maximum depth of 15m for a prolonged period of time. Also, during the months of December and January the area is hit by cyclones causing water temperatures to fluctuate over many degrees during and after the cyclone (usually a five day period).

The combination of these factors, including the high sedimentation rates, has increased the stress factors to both hard and soft coral species in the region. In 2001 this led to a mass bleaching event and large areas of shallow water corals affected did not recover.

This event, and subsequent similar events in the following years, caused a loss of habitat and associated reef organisms in many patch reefs within the lagoon. This change to the ecosystem means that fishermen need to increase their fishing effort to catch the same or smaller amounts of fish, resulting in them adopting new techniques, some of which lead to even greater habitat destruction.

3. Intensive fishing activities in localised areas and physical damage
The topography of the lagoon and the limited fishing materials available to Vezo fishermen due to cultural practices and low economic status, means that the majority of the Vezo from Ifaty and Mangili remain within the lagoon to fish. This in turn has meant that specific areas of patch reef within the lagoon and the leeward parts of the barrier reef have become known as good fishing locations and are always visited.

Over successive generations the pressure on these areas has increased so that the diversity, structure and populations of targeted fish species have declined. The increased pressure is the result of an increase in the number of Vezo fishing in these areas, and the fact that some of the fishing techniques used, directly and indirectly destroy the corals and reef substrate causing loss of habitat for fish.

The resulting loss of habitat and reduction of diversity and population has led to certain fish species moving to other areas, increasing the pressure on other healthy reef areas of the Bay of Ranobe.

4. Local population increase and influx of migrant fishermen
The population of Madagascar has been increasing at a rate of 3.2% annually. This has led to an increase in native Vezo populations living along the coast of the Bay of Ranobe and also of inland populations in the region.

Poor agricultural practices of slash and burn (Tavy) have led to larger areas of land being unusable for crops, and many farmers from the surrounding areas are starting to move to the fishing villages of Ranobe. These migrants do not know about Vezo cultural fishing practices, such as seasonal fishing bans on certain species (e.g. lobster), and tend to utilise techniques that are more destructive.

This population increase is causing higher sustained pressure on the remaining resources, and increasing the pressure on areas of healthy reef within the lagoon.

5. Mass algal blooms
During the rainy seasons it is not only sediment that is washed into the sea but also agricultural fertilisers, which provide increased nutrients that cause algae to bloom out of control during the summer months.

Human effluent probably plays a bigger role in providing increased nutrients to the water, as the town of Toliara (population 400,000) and all villages (pop. estimate 20,000) along the bay have no sewage facilities. This means that the input of nutrients from human effluent has been both consistent and prolonged.

Sustained nutrient loading, coupled with the increased fishing pressure and poor flushing of the lagoon water, is causing a shift in competition between algae and corals. With no cropping and a continued increase in nutrients, the algae smother the corals blocking out light and killing them.

Now many shallow reef areas within the lagoon are dominated by macro algal species that do not provide the proper habitat and environment for reef fish, causing them to migrate to other areas and putting more pressure on remaining healthier areas.

The above factors affecting the reefs within the Bay of Ranobe are putting a huge amount of pressure on the remaining healthy reef areas and affecting resources available to the local communities. Therefore, information about what is occurring, why it is occurring and what can be done to slow down the degradation must be relayed to the local population, who must be encouraged to undertake appropriate measures to safeguard their resources for future generations.

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