This famous stretch of Andaman Sea, lying in both the national marine parks of the Similan Islands and Surin Islands, is well renowned as whale shark liveaboard safari territory. It lies immediately to the north of the Similan Islands and covers Koh Bon in the south, up to Koh Tachai, and finally northwards on to Richelieu Rock, just below the Burmese border. It contains many of Thailand's top dive spots.
Although there are several sites in Thailand for diving with whale sharks, Richelieu Rock stands proudly at the top of the list. The surrounding deep sea upwellings of nutrient-rich water make for excellent diving where plankton blooms attract many visitors from the blue, most notably, the biggest one of all - the whale shark. It is a large site that can be dived several times over without the need to cover the same area twice. This aspect, plus the superb bio-diversity it boasts - both macro and large marine life - makes Richelieu Rock Thailand's top dive site and a hot favourite on liveaboard diving cruises.
Manta rays are also frequent visitors to the area and they especially like to visit Koh Bon. Koh Tachai sees both mantas and whale sharks (and schooling fish species), but in fewer numbers than Richelieu Rock and Koh Bon. For those planning a Thailand scuba diving trip focused on big animals, this region is the priority. Liveaboards from Phuket and Khao Lak run regular trips here between October and early May. The best chance to encounter whale sharks is from February to April, when plankton blooms are at their peak.
Dive Site Descriptions
Koh Bon
Koh Bon is part of the Similan Island national park and lies approximately 1 hour north of the main Similan chain. It is worth special mention as one of the best dive sites in Thailand for manta rays. The island's western side features a submerged ridge that drops from 15 metres to over 35 metres. This ridge acts as a cleaning station. Manta rays visit regularly, often at the start of the incoming tide. They glide slowly over the ridge while cleaner fish pick parasites from their gills. You kneel on the sand or hook into the rock and watch them circle. Some have wingspans over 4 metres. The experience is humbling.
Koh Bon also delivers other large animals. Napoleon wrasse patrol the reef edge, leopard sharks rest on the sandy patches near the base of the ridge. Eagle rays pass through, blacktip reef sharks are common. The coral cover is healthy, with hard corals dominating the shallower sections and soft corals on the deeper slopes. The macro life is also notable. Nudibranchs, pipefish, and shrimp hide in the cracks and crevices. This combination of big and small makes night diving here exceptional. Octopus, cuttlefish, and hunting lionfish emerge after dark.
Koh Bon Pinnacle
Also known as 'Fishermen's Nightmare' , this is a separate range of pinnacles a few hundred metres to the north west of Koh Bon island itself. The shallowest pinnacle starts at 18m and is subject to strong currents so this is a site for experienced divers only. You need to be capable of making a negative buoyancy entry, descending immediately without lingering on the surface. The current can sweep you away from the site if you hesitate.
Across the top of the colourful coral-covered peaks, there is a good chance of encountering Napoleon wrasse, hunting trevallies and manta rays. The pinnacles rise from depths exceeding 40 metres. The steep walls are covered in sea fans, whip corals, and barrel sponges. Down in the deeper areas, you can look for sleeping leopard sharks wedged under overhangs. Blotched fantail rays rest on the sandy patches between the pinnacles. Schools of barracuda and trevally circle the deeper sections.
Richelieu Rock
his horseshoe-shaped site consists of a huge central pinnacle surrounded by smaller jutting rocks. Every inch has been colonised by something: soft corals, anemones, barrel sponges and sea fans. The colours are astonishing. Purple, pink, orange, yellow. The rock rises from a depth of over 35 metres to within 2-3 metres of the surface at low tide. The shape of the horseshoe creates sheltered bays where smaller fish gather. The outer walls are swept by current, attracting larger animals.
Whale sharks are the headline act. They visit Richelieu Rock more reliably than any other site in Thailand. Sightings are most common from February to April, when plankton blooms are at their peak, as the whale sharks come to feed. They are not always here. You may see one on a single dive or you may see them on multiple dives. Each encounter is different. Manta rays visit the cleaning stations on the eastern side of the pinnacle. Huge schools of barracuda circle the deeper sections. Chevron barracuda form silvery tornadoes that block out the light., giant groupers lurk near the base, dogtooth tuna tear through bait balls. The list goes on.
Then there is the macro life. Harlequin shrimp hide in the sea fans. Ghost pipefish drift among the crinoids, seahorses cling to the soft corals. Nudibranchs in dozens of species crawl across the rocks. Mantis shrimp peer from their burrows. A dive at Richelieu Rock can keep a photographer busy for a week. The site is large enough to be dived 4 or 5 times without repeating the same route.
Koh Tachai
Koh Tachai is often visited en route to Richelieu Rock from the Similans. It is an island with several nearby dive sites. Koh Tachai Plateau is the best known of these. As the name suggests, it is a submerged plateau of hard corals and boulders. The plateau starts at around 12 metres and slopes gently to 25 metres. The topography features swim-throughs and channels everywhere, created by huge granite boulders stacked on top of each other.
The larger fish here include manta rays, which visit the cleaning stations on the plateau's edge. Huge schools of chevron barracuda swirl above the boulders and large numbers of tuna pass through. Jacks and trevally hunt the shallower sections. If the whale sharks are not at Richelieu Rock, they may well be here. Sightings are less frequent than at Richelieu, but still possible.
The southern tip of Koh Tachai features a deeper wall that drops to over 35 metres. This is where the current runs strongest. Grey reef sharks patrol the edge and eagle rays glide past. The coral cover is excellent, with large gorgonian fans and barrel sponges. The northern side is more sheltered, with smaller boulders and sandy patches. Leopard sharks rest here, with blue-spotted stingrays half-buried in the sand.
How to Dive Richelieu Rock
Diving at Richelieu Rock and Koh Tachai and Koh Bon islands is best done by liveaboard and most of the dive cruise operators include these sites in their itineraries. Whether you wish to dive here as part of a long dive liveaboard taking in the Similans, and maybe even Burma, or as a short trip (minimum of 4 nights) you will find the itinerary that suits you.
It is also possible to dive here in season on day trip boats from Khao Lak, although the travel times are longer (40 km away). It's also possible to dive here on day trips from Phuket day trips (Koh Bon only, 85 km away). But if you are short on time or don't fancy sleeping on a boat for a few nights, then these might be the right choices for you.
Diving in Richelieu Rock, Koh Bon and Koh Tachai is limited to the months from October until early May, when the conditions are usually excellent and all the liveaboards visit regularly. February to April is the best time to encounter whale sharks in this region. It is also the time when manta rays are most frequently seen although there is a pretty good chance of seeing them at any time of the year here. The months of March and April might see a drop in visibility (from around 20-30m) due to a higher density of plankton in the water.
The water temperature is usually around 30°C at the surface and 27/28°C at depth, but occasionally it can drop down to 25°C due to thermoclines. These are like 'blankets' of cold water and are a natural phenomenon that happen most often in November and early December as a result of the transition of wet season into dry season.
February and March are the driest months and, with regular flat seas, are probably the best time to visit Richelieu and the rest of the area. From May to early October, the area is closed due to rough sea surface conditions. Visit The Traveler website for more details on the climate at the nearby Similan Islands.
Where are Richelieu Rock, Koh Bon/Tachai How Do I Get There?
