Sunday, October 17, 2010

Introduction to the reefs close to Ras Mbisi


The mid-west coast of Mafia Island is characterised by small patch reefs surrounded by large expanses of sand and mud, occasionally dominated by seagrass in the shallower areas. The whole area is relatively shallow, generally being less than 30m deep. As guests are most likely to be interested in seeing coral and the associated wildlife, it is the patch reefs that are of the most value and therefore covered below.

Eleven reefs are highlighted, having previously been identified as possibly dive / snorkel sites within reach of Ras Mbisi.

The closest reef is Kasilani at approximately 4km NNE from Ras Mbisi. The furthest is Al Hadjiri to the WSW at 17km, just outside the Mafia Island Marine Park border. GPS coordinates and distance from Ras Mbisi of all sites are given in the table below.

Reef GPS coordinates Distance from Ras Mbisi
Kasilani S07°47.15 E039°41.73 4km
Ras Murundo patch reef S07°47.51 E039°44.87 4km
Halfway reef S07°45.65 E039°42.59 6km
Mbakale ya mwamba S07°47.91 E039°39.35 7km
Mwamba Mkuu S07°45.05 E039°44.72 8km
Mbakale ya fungu S07°45.39 E039°39.38 9km
Tirene reef S07°52.08 E039°39.23 9km*
Barakuni island S07°43.52 E039°45.60 11km
Shungu-Mbili S07°42.32 E039°40.84 14km
Sefo S07°49.95 E039°34.25 16km
Al Hadjiri S07°53.11 E039°34.81 17km*
* These reefs cannot be reached in a straight line, add approximately 2km for actual distance.

Reef Details

Kasilani Reef - This patch reef is approximately 300m by 200m in size. The reef flat is 2 to 5m deep depending on the tide. On the periphery, the reef slopes down to approximately 7 to 9m, forming a small but still noticeable wall. Beyond this is sand that gradually becomes deeper. This latter area is fairly devoid with only the occasional coral lump. Visibility is generally good but can be reduced in stormy weather or at mid tide. Hard coral cover on this reef is generally very high and fairly diverse but it does lack the complex topography (i.e. ruggedness with caves, nooks and crannies) that is often present on reefs. Subsequently, the fish present tend to be small with fewer large fish such as groupers. Having said this, many different reef fish species are found here including unicorn fish. Hawksbill turtles are occasionally seen here. Given its shallow reef flat, high diversity of hard corals, general good visibility and range of fish life, this is an ideal reef for all levels of snorkellers at low or high tide. The best area is the north east part of the reef flat (S07°47.115, E039°41.789). In terms of diving potential, it is most suitable for relatively inexperienced divers at high tide due to its limited depth. Experienced divers may find it a bit shallow. The best area is on the small walls in either the east (S07°47.167, E039°41.787) or north west (S07°47.099, E039°41.701).

Ras Murundo Patch Reef - Exploratory dives have been unable to locate any reef, the area appears to be predominantly seagrass, despite how it appears on the chart. It is therefore not recommended for either snorkelling or diving.

Halfway Reef - This is a fairly large reef with a distinct and steeply sloping wall in some areas. This wall, however, mainly consists of a sand and rubble bank. The reef flat has a fair amount of hard coral in some areas but in general, the entire reef seems to have a low diversity of coral species. In addition, this reef regularly suffers from very poor visibility and because of this, it is not ideal for either snorkelling or diving.

Mbakale ya mwamba - This reef varies greatly in quality. The reef flat is largely rubble, with some small patches of coral bommies. The north eastern wall (S07°48.091, E039°39.233 to S07°47.877, E039°39.419) drops to a maximum of 16 to 18m and consists of large patches of coral interspersed with fairly devoid patches of sand and rubble. Where present on the wall, the coral is dominated by large submassive forms which attract a high diversity of fish life including angelfish, butterflyfish, groupers, spadefish and large shoals of sweetlips. Other wildlife resident on this section of reef includes many bluespotted rays, nudibraches, octopus and occasionally turtles and dolphins. Visibility tends to be ok but can occasionally be reduced due to tide or weather. Because of the relatively barren reef flat, this reef is not suitable for snorkelling, however the north eastern wall makes for a good and interesting dive.

Mwamba Mkuu - Despite appearing as an undulating fringing reef on the chart, much of this reef is in fact seagrass. Small patches of good coral reef do exist (e.g. S07°44.990, E039°44.413) so the potential is there for both snorkelling and diving, but further exploration is needed to find the best areas. Visibility can be reduced here at times.

Mbakale ya fungu - This reef is approximately 600m long by 400m wide and there is a small sandbar at the southern end which dries at mid to low tide. The reef flat is approximately 2 to 5m deep, and consists of a wide diversity of hard and soft corals. The eastern wall (S07°45.400, E039°39.470) is the deeper of the two walls, extending down to around 20m in places and is quite varied in the quality of coral present. There is evidence here of previous destructive fishing practices. The western wall is shallower (10-15m) but has a much healthier coral cover especially towards the northern tip (S07°45.309, E039°39.368) On the western wall there is a high diversity of fish life, often with large shoals of parrotfish and moorish idols. Turtles (both green and hawksbill) are regularly seen here. Other wildlife of note sighted at this reef from the boat include dolphins and humpback whales at certain times of the year. The reef flat and the shallower sections of the reef wall make for very good snorkelling and both walls are good for diving especially the shallower western wall.

Tirene Reef - Despite it’s appearance on the chart Tirene mainly consists of a shallow sand and rubble bank, with a few large coral bommies (e.g. S07°52.068, E039°39.283) Depth here varies between 3 and 10m. Large sections that were previously reef have been reduced to rubble, presumably by destructive fishing practices. Wildlife tends to be concentrated on and around the bommies and can appear to be completely absent elsewhere. Near this reef is where many whale shark sightings tend to occur at certain times of the year. Snorkelling at this reef is very hit and miss, depending on the number of bommies encountered. At low tide in good visibility the bommies can be seen from a boat and are easy to find. Similarly diving is very variable here but because of the lack of depth, this reef is probably more suitable for snorkelling.

Barakuni Island - The main reef is found on the eastern side of the island and extends a fair distance offshore. While much of the reef close to the island is shallow, further out the coral extends down to a depth of 12-15m (e.g. S07°42.669, E039°45.539). The reef here consists of a diverse range of both hard and soft corals and is home to many species. Giant grouper and hawksbill turtles have been seen on this reef along with unicorn fish, batfish and large parrotfish. Visibility is variable but generally good. This reef can be considered good for both snorkelling and diving.

Shungu-mbili Island - Limited trips to this area have suggested that the main reef can be found on the north east side of the island but is fairly shallow. High coral cover and diversity, along with a good range of fish and invertebrate species make for good snorkelling, but the reef is probably too shallow for divers. While better dive sites on the reef may exist, they have yet to be found.

Sefo Reef - One of the two reefs just north of the Mafia Island Marine Park (MIMP) border, Sefo is a very large reef with sand bars that dry out at low tide. The reef itself ranges in depth from the surface to around 10-15m and is punctuated by coral sand gullies. A typical area can be found at S07°49.919, E039°34.212. Very high and diverse coral cover provides habitats for a very large number of fish and invertebrates including angelfish, butterflyfish, large parrotfish, along with many other species. Green and hawksbill turtles are also regularly seen here and visibility is consistently good. The reef provides an excellent site for both snorkelling and diving and is arguably the best dive site on the mid west coast of Mafia Island.

Al Hadjiri Reef - The second of two reefs just north of the MIMP border, Al Hadjiri has generally high coral cover and consequently a high diversity of fish and invertebrate life. Shallow but distinct walls can be found in places that drop from a few metres to around 8m, whilst in other areas the coral consists of large coral bommie fields down to approximately 10m. Due to high wildlife diversity, good visibility and a range of depths, this reef offers good potential for both snorkelling and diving.

Good environmental practice

 Anchors and anchor lines dragging over the reef can damage large areas of coral which may take years to recover. Always drop anchor on a sandy area well away from any coral.

 Many coral species are fragile and easily damaged. Snorkellers and divers should always avoid touching, standing or resting on the reef and should never remove any ‘souvenirs’, living or dead.

 No litter, especially plastics, should be thrown in the water from a boat ever, as it persists in the environment and can kill birds and turtles that swallow items mistaking them for food.

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