Koh Bon is a remote uninhabited island which was recently added to the Similan Islands National Park. It’s located 20 kilometer north of Similan Island No 9. The island has no beaches and has the shape of a horseshoe. Both above and below the surface the waves surge through holes that they have cut right through the island. The breathtaking scuba dive underwater terrain on the southwestern point has a step-down ridge that carries on to depths of over 45m.
The western side of the ridge is more of a gentle slope with coral bommies forming mushroom-like formations out of the finger corals. On the ridge itself, seafans of different sizes, shapes and colors grow, and schooling fish swim into the current . Particularly this perfect feeding ground for manta rays, gives you your best chance of seeing and diving with them in the whole of the Andaman Sea.
All about the dive at Koh Bon West Ridge
Divers more or less have to decide how to dive this site, whether they plan to hang around for Manta Rays, or enjoy a nice and interesting deep dive here at Koh Bon. The bay area in the south is quite sheltered and most of the time used as a drop in point. From there, ascend down to maximum depth heading in a due westerly direction. Zigzag back up the ridge in an easterly direction. Depending on current, either continue on around the ridge to the east into the coral gardens, or return along the wall back to the drop in point. The wall itself is best viewed in the afternoon, as the sun shines brilliantly on the anemones and soft coral gardens.
Koh Bon is the only limestone island in Mu Koh Similan National Park, which is much softer thus the water composition may sometimes be dense with particles and plankton. This plankton is the very reason that draws numerous manta rays to the site regularly. They are filter feeders and eat large quantities of zooplankton, which they swallow with their open mouths as they swim. They also use this site as a cleaning station for the removal of parasites, let us say they come here to have a spa. The cleaner fish, like cleaner wrasses and gobies will then remove and eat the parasites from the skin, even swimming into the mouth and gills. This is a form of cleaning symbiosis.
Most of the time approaching manta rays will get you what you in the least wanted; it not only shortens your dive time, they usually swim away to never been seen again. So it gets the others annoyed as well. When banging tanks or other signs of exited divers warn you their presence, just hang around calmly as mantas often swim towards or past divers and fly by with such grace, you will never forget.
In addition to mantas, many kinds of different sharks including Leopard sharks and White and Black tip sharks can be found. Sometimes bamboo and nurse sharks are hiding under rocky outcrops so keep your eyes open. This is a good location to find the elusive purple fire goby. Napoleon wrasse and off course cleaner wrasse are present. Several schools of barracuda, trevallies and tuna are in mid water as well. You are likely to see Sea snakes, Octopus and Cuttlefish. Shrimps and lobster are numerous and varied which makes night diving here exceptional.
The majority of the corals here are hard corals including staghorn coral and brain coral that are interspersed with a few big coral heads.
All about the dive at Koh Bon Pinnacle
Best to drop in and ascend quickly to maximum depth at the sandy bottom of the south pinnacle, the bigger of the two. While down in these deeper area, you can look for sleeping leopard sharks and blotched fantail rays. Slowly spiralling up around and through both of the colourful coral covered peaks, there is a good chance of Napoleon wrasse and hunting trevallies. Moray eels peep out of their holes and many more fish, smaller or bigger, swim around. In nooks and crannies and on the rocks invertebrates like crabs, lobsters, snails, octopuses, starfish and sea-urchins are common here.
Heading into the second half of the Similan diving season, as from January, giant manta ray sightings are frequent with sometimes up to four or five individuals circling around the divers in the strong flow of water. While feeding with their mouth open, they graciously glide over the top of the pinnacle swooping around the different dive groups. The thought of coming face to face with giant filter-feeding manta rays, evokes different reactions. From paralysed to bold bravado. Some do approach real close which may make them most of times panic and swim away. This ‘chasing’ will also waste your air supply and reduce your and your buddies’ changes to encounter more of them. Let them decide how close they want to be.
Off course everyone hopes for ‘fly by’s”, but sometimes, as a special treat, slow-moving whale sharks pop up here too. They are in fact the largest known extant fish species – 12 meter - with a lifespan of about 70 years. Whale sharks have very large mouths (1.5 m wide!), and as filter feeders, they feed mainly on plankton but also fish. The head is wide and flat with two small eyes at the front. The whale shark is an active feeder, opening its mouth and swims forward, pushing water and food into the mouth through the gills, or sucking in volumes of water by closing its mouth. In both cases, the filter pads serve to separate food from water. Whale sharks migrate to feed and possibly to breed. They are docile sharks. While some divers may be unnecessarily worried about these harmless giants, sometimes they allow divers to catch a ride without any danger. Younger whale sharks are gentle and can play with divers. But spectacular it will be, so keep your eyes open into the blue.