What the diving is like
The Lembeh Strait is a channel about 16 km long and one to two kilometres wide, separating Lembeh island from the North Sulawesi mainland at the port of Bitung. Its sand is volcanic, derived from the Tangkoko volcanoes and the old lava fields on the mainland coast, and it lies on gentle slopes that run down from both shores. On the muck sites the bottom is fine black sand or silt with very little natural relief, so animals gather around whatever breaks the surface: weed, sponges, sea pens, whip corals, coconut shells, bottles and rope, much of it carried into the water from the villages and the port.
The strait is open to the sea at both ends. During the southeast monsoon, coastal upwelling lifts cooler water rich in nutrients into the shallows, and the combination of plankton in the water and soft sediment that is easy to bury into, hide under or build in supports a dense community of small, cryptic animals.
About a third of the named sites are black sand. The others are coral slopes, mixed rubble and coral, a handful of small walls, two white sand sites and one wreck, so a week in the strait can include reef as well as sand. The sand slopes are usually dived between about 5 m and 25 m. Current inside the channel is usually mild, and it strengthens on the incoming tide.
The first dive resort in the strait was built in the early 1990s. Its guides, trained by the divemaster Larry Smith, did much to establish muck diving as a style of diving later in that decade.
Critters
The strait is known above all for frogfish and cephalopods. Eleven frogfish species have been recorded, from the painted and clown frogfish that are seen most often to the hairy frogfish, the most sought-after. The octopuses include the mimic octopus, the wunderpus, the coconut octopus and blue-ringed octopuses, and the flamboyant cuttlefish forages openly on the sand by day.
Nudibranchs and sea slugs run to several hundred species. The seahorses and pipefish include pygmy seahorses on gorgonians and several ghost pipefishes, and the scorpionfish group includes rhinopias.
Among the muck fish, three yellow-nosed shrimpgobies share burrows with snapping shrimps, one of them the Lembeh shrimpgoby, known only from the strait. Mandarinfish spawn at dusk among coral rubble, and the Banggai cardinalfish, native to the Banggai islands far to the south, has an introduced population that is well established here. Crustaceans and the worms and other invertebrates, including the bobbit worm, are night-dive subjects, and blackwater dives add larval fish and other open-ocean animals.
Conditions through the year
Water in the strait stays between about 23 °C and 29 °C. It is warmest, at 28 °C to 29 °C at the surface, from about October to March, and coolest in July and August, when the upwelling can bring the shallows down to 25 °C or 26 °C. Water at depth tends to be a degree or two cooler than at the surface.
Visibility is usually between 5 m and 20 m. The northwest monsoon, from December to April, often brings clearer water to the strait despite the rain, while the plankton of the upwelling months tends to reduce it. Close to the bottom, visibility on a silt slope depends as much on the fin technique of the group as on the season.
The shape of a day
Diving is from small boats. No site is far from another, so the rides between them take minutes and most surface intervals are spent on shore. Operators commonly run three or four dives a day, the last of them often a night dive, and groups are kept small because a large part of each dive is spent searching one patch of sand or one sponge. Progress along a slope is slow, and a guide may stay with one animal for several minutes while a photographer works on it.
Night dives are usually shallow, most of them within 15 m. Some operators also run blackwater dives, drifting at night over deep water with lights to photograph larval and open-ocean animals; the northwest monsoon is usually considered the better period for these.
Getting there
The airport for the strait is Sam Ratulangi at Manado. The usual international route is through Singapore, which has a direct service to Manado on several days a week, and there are seasonal direct services from several cities in China. Other routes connect through Jakarta, Makassar, Surabaya or Denpasar onto a domestic flight, where the baggage allowance is often lower than on the international ticket, which matters for anyone carrying camera equipment.
From the airport the transfer to the strait takes about an hour by road to Bitung, with a short boat crossing to resorts on the island shore.
Combining it with other trips
Most trips to the strait are combined with reef diving elsewhere in North Sulawesi. Bunaken National Park, off Manado, has steep reef walls in clearer water. Bangka island, off the northern tip of the peninsula, is reached by an open-water crossing and has soft corals, sea fans and boulders, with more current than the strait. The outer, eastern coast of Lembeh island can be dived as a day trip, and lighter winds make the months from December to May the most reliable for it.
On land, the Tangkoko nature reserve north of Bitung is visited in the late afternoon, when spectral tarsiers leave their roosts, and it also has crested black macaques, bear cuscus and hornbills.
Manado has domestic connections through Denpasar and Makassar, so the strait can be combined with the black sand of Bali, or with Ambon, which is reached by connecting through Makassar.
When to come
The strait is dived all year, and two monsoons set the conditions. The southeast monsoon, from about June to October, brings southeasterly winds, coastal upwelling and the strongest flow of the Indonesian Throughflow. Productivity in the water rises, many of the rarer animals are reported more often in these months, and they are the busiest months of the year for visiting divers.
From December to April the northwest monsoon brings the wetter season. The surface water is warmer and the sea usually calmer, visibility often improves, and lighter winds make the outer coast of Lembeh and the offshore reefs more reliable. The months of May and November fall between the two monsoons and can resemble either.
Fees and what is not included
Divers in the strait pay a dive permit that was introduced to fund the removal of floating rubbish from the villages and the port. It is charged once a year for each diver and is usually collected by the operator on arrival. The amount has changed over time, so the current figure is best confirmed with the operator.
Bunaken National Park, off Manado, is administered separately and charges its own daily entry and diving fees, which apply on any day spent diving there.