Lake Bermin

Lake Bermin is a freshwater paradox: a crater lake you could walk around in twenty minutes, holding one of the densest cichlid radiations on Earth relative to its size. Set in the volcanic highlands of southwestern Cameroon, in the Rumpi Hills north of the better-known Lake Barombi Mbo, it covers only about a fifth of a square mile (roughly 0 mi²) — a few hundred meters across — and is shallow, on the order of 45–50 feet (about 43–49 ft) at its deepest. Yet packed into that single bowl are nine species of Coptodon (the West African tilapias formerly placed in Tilapia), every one of them endemic, every one found in this lake and nowhere else on the planet. The flock is thought to have evolved in place, from a single colonizing ancestor, in a basin so small and so uniform that it became a textbook test case for whether new species can arise without ever being geographically separated. Bermin is, in effect, a natural laboratory of evolution the size of a few football fields — and, for the same reason, one of the most fragile cichlid assemblages in the world.

Setting & origin

Lake Bermin lies in Cameroon, in the Southwest Region, within the cluster of crater lakes strung along the Cameroon Volcanic Line — a roughly 1,000-mile (1,600-km) chain of volcanism that runs southwest-to-northeast from the island of Bioko through Mount Cameroon and on into the interior highlands. Over roughly the last 25 million years this line has repeatedly blown out calderas and maars, and where these explosion craters filled with rainwater they became deep, steep-sided lakes isolated from the surrounding river network. About three dozen such crater lakes are now known from Cameroon; Bermin sits in the Rumpi Hills, just north of Lake Barombi Mbo and Lake Dissoni, in an ecoregion that the Freshwater Ecoregions of the World project calls the Western Equatorial Crater Lakes (Schliewen, Peck & Burgess, FEOW).

By the standards of the African Great Lakes, Bermin is almost vanishingly small. Authoritative compilations put its surface area at little more than 0 mi² — about a fifth of a square mile — making it one of the tiniest water bodies anywhere to host an endemic species flock (Hughes & Hughes 1992; Stiassny, Schliewen & Dominey 1992; FEOW). It is roughly circular, only a few hundred meters across (the original describers and later hobby accounts give a diameter of around 2,000 feet, about 2001 ft), and shallow for a crater lake, reaching only about 45–50 feet (43–49 ft) at its deepest point. Unlike many of its neighbors, it has no significant inflowing streams and only one small outlet, so its water is recharged chiefly by rain and seepage and the basin is essentially self-contained. That isolation — a closed bowl perched in the hills, cut off from the rivers below — is the single most important fact about the lake, because it is what trapped a founding cichlid population and let it diversify undisturbed. As a small, montane Cameroonian lake, Bermin sits wholly within one country, so there is no transboundary dimension to its governance; the pressures on it, taken up below, are entirely local.

Temperature, oxygen & mixing

Honesty first: Bermin is a remote, rarely visited lake, and its physical limnology is thinly documented — there is no long-term monitoring record of the kind that exists for Malawi or Tanganyika, and most published numbers are scattered, one-off field measurements. What can be said is framed by its small size and shallow depth. Surface water sits warm and fairly steady, around the mid-70s °F (roughly 75 °F) in the readings that reach the hobby and survey literature, consistent with a low-elevation tropical lake (Stiassny et al. 1992; Practical Fishkeeping field notes). Seasonal swing is modest and there is no published thermocline depth or bottom-temperature series specific to Bermin, so any surface-to-deep delta given here would be invented rather than measured.

The key physical point is what the lake's depth implies for mixing — and here a regional study does the heavy lifting. In his comparative survey of 39 Cameroonian lakes, Kling (1988) found a sharp threshold: lakes deeper than about 60 feet (59 ft) developed persistent thermoclines and permanently anoxic, oxygen-free bottom water, while no lake shallower than that formed a stable anoxic hypolimnion. At roughly 43–49 ft, Bermin falls on the shallow side of that line, meaning it is almost certainly well mixed rather than permanently stratified — wind and nightly cooling can stir it from top to bottom, keeping oxygen in contact with the whole water column rather than sealing a dead deep layer below a thermocline. That mixing regime is exactly what makes the lake habitable for fish at all depths, and it is why one species, Coptodon bythobates — the 'bloody deepwalker' — is notable precisely for living in the deepest, dimmest water (below about 26 feet / 26 ft) and reportedly carrying elevated blood haemoglobin to cope with the lower oxygen there (Practical Fishkeeping; Stiassny et al. 1992). Bermin's shallowness also spares it the catastrophic gas-burst risk of deeper Cameroonian crater lakes such as Nyos and Monoun, where carbon dioxide accumulates in stagnant bottom water and can erupt lethally; conservation assessors nonetheless flag a smaller version of that hazard for Bermin should deforestation increase wind and force a sudden overturn of any oxygen-poor deep layer (Moelants 2010, IUCN).

Water chemistry & clarity

Bermin's water chemistry is, again, sparsely measured, and the figures that exist should be read as a handful of spot readings rather than a well-characterized profile. The consistent picture is of soft, near-neutral to mildly alkaline water. Reported pH clusters around 7.5, and conductivity is low — on the order of 80 µS/cm — marking it as a soft, dilute lake far gentler than the ion-rich rift lakes of East Africa (Practical Fishkeeping field notes; cf. the soft-water, low-conductivity character documented across the Rumpi Hills crater lakes by Kling 1988, whose neighboring lakes ran surface conductivities from the low teens to a few tens of µS/cm). No published Secchi-disk transparency, nutrient budget or dissolved-ion analysis specific to Bermin is available in the public literature, so those facts are left blank here rather than borrowed from other lakes. Field descriptions note that the lake bottom is littered with leaf litter, submerged wood and plant detritus over volcanic rock, and that freshwater sponges grow on hard surfaces — a detail that matters enormously to the fish, as one cichlid has evolved to eat them. In short, the chemistry is benign and the lake is biologically productive for its size, but the limnological record is genuinely thin, and the responsible summary is that Bermin's water is soft, roughly neutral, and otherwise under-studied.

Habitats

For a lake that produced nine species, Bermin is strikingly uniform — and that uniformity is central to its scientific fame. There are no expanses of separate rocky reef and open sand of the kind that physically partition the cichlids of Lake Malawi; instead the whole small basin offers broadly similar conditions: shallow, soft-bottomed, fringed by forest, floored with volcanic rock, detritus and rotting leaves, and dotted with freshwater sponges. The main environmental gradient is simply depth, from the warm, well-lit shallows down to the dim deepest water around 45–50 feet (43–49 ft). Most of the cichlids share the shallow inshore zone, where they spawn on open substrate; only the deep-living Coptodon bythobates clearly partitions the lake by depth. Because the habitats are so monotonous, biologists have argued that Bermin's species could not have arisen by adapting to obviously different physical environments — there simply aren't enough distinct niches in a few hundred meters of shoreline — which is precisely what makes the radiation so interesting and what points investigators toward other engines of speciation, such as fine-grained dietary specialization, mate choice, and hybridization.

The cichlids

This is why Lake Bermin matters. In a basin barely half a square kilometer across live nine species of Coptodon — the West African tilapias long known under the catch-all genus Tilapia — and all nine are endemic, occurring in this one lake and nowhere else: C. bakossiorum, C. bemini, C. bythobates, C. flava, C. gutturosa, C. imbriferna, C. snyderae, C. spongotroktis and C. thysi. Eight of them were described in a single landmark monograph by Melanie Stiassny, Ulrich Schliewen and Wallace Dominey in 1992 ('A new species flock of cichlid fishes from Lake Bermin, Cameroon,' Ichthyological Exploration of Freshwaters), which formally recognized Bermin as a species flock; the ninth, C. bemini, had been named twenty years earlier by Thys van den Audenaerde (1972). Relative to lake area, it is one of the densest cichlid radiations known — a concentration of endemic species per square kilometer that rivals or exceeds anything in the far larger rift lakes.

The deeper finding came two years later. Using mitochondrial DNA, Schliewen, Tautz and Pääbo (1994, Nature) showed that the cichlid flocks of Bermin and of nearby Barombi Mbo are each monophyletic — every species in a lake descends from a single common ancestor that colonized that lake just once. Combined with the lakes' tiny size and ecological uniformity, that monophyly became one of the strongest empirical cases ever made for sympatric speciation: the idea that one species can split into many without any geographic barrier, the new forms diverging while sharing the same water. Bermin sits at the heart of that still-debated argument because it is so small that allopatric (geographically separated) speciation is hard to invoke — there is nowhere within the lake to be isolated.

How did nine species emerge from such monotony? Trophic specialization explains part of it. Most of the Coptodon are generalist grazers of algae, aufwuchs, detritus and small invertebrates, but at least two are specialists: C. imbriferna feeds heavily on phytoplankton, and C. spongotroktis has evolved to exploit the lake's freshwater sponges — eating organisms whose tissues are reinforced with glassy silica spicules, in effect feeding on broken glass for a meager return (Stiassny et al. 1992; FEOW; FishBase). The smallest member, C. snyderae, matures at about an inch (around 1 in) and is reckoned the smallest tilapiine fish known. But pure diet-driven adaptation seems insufficient given how similar most of the species' ecologies are, and later genomic work on the Cameroon crater-lake cichlids has pointed to a messier reality — repeated bouts of gene flow and hybridization, perhaps with the ancestral riverine stock recolonizing the lake more than once, generating new combinations from which reproductively isolated species eventually settled out. Bermin remains an active research frontier precisely because no single tidy mechanism fully accounts for it. The lake's only other fishes are a small cyprinid of the Barbus aboinensis group and a killifish of the Fundulopanchax mirabilis group (Stiassny et al. 1992) — the cichlids are the show.

People & pressures

Everything that makes Bermin extraordinary also makes it desperately vulnerable. A species flock confined to one tiny, isolated crater has no refuge: there is no second population, no neighboring lake, no upstream or downstream stretch of river to recolonize from. If the lake is damaged, the species are simply gone. That logic is reflected in the conservation listings — the Bermin Coptodon have been assessed by the IUCN as among the most threatened freshwater fish anywhere, with species such as C. snyderae, C. imbriferna and C. spongotroktis listed as Critically Endangered (assessed 2009, published 2010; Moelants), on criteria driven directly by the minuscule size of the single lake to which each is restricted. (Older hobby sources sometimes cite a 'Vulnerable' rating, but the current Red List places the assessed Bermin endemics in the Critically Endangered category.)

The threats are concrete and local. The IUCN assessments single out the spread of oil-palm plantations and slash-and-burn agriculture around the crater, which strip the surrounding forest and wash sediment and nutrients into a lake that has no through-flow to flush them out. Deforestation carries a second, subtler danger flagged by assessors: clearing the sheltering forest around the rim could expose the lake to stronger winds, and a sudden deep overturn could pull oxygen-poor bottom water up into the shallows where the fish live, with the potential to suffocate the whole flock at once — a milder echo of the gas-burst catastrophes seen at deeper Cameroonian crater lakes like Nyos. Bermin is remote and has been collected only rarely, which has limited both scientific monitoring and large-scale exploitation, but it also means there is little active management on the ground. The fishes do persist in the aquarium hobby, where several of the nine are maintained by conservation-minded keepers (the flock features in programs such as CARES), and captive populations offer a partial insurance policy. But the heart of the story is unchanged: nine unique species of cichlid, found nowhere else on Earth, riding out the present in a single warm bowl of water in the Cameroonian hills — a radiation as precarious as it is remarkable.

Sources

Last reviewed 2026-06-06.

How to cite

Aquarist Atlas (2026). Lake Bermin. Aquarist Atlas. https://www.aquaristatlas.com/water/lake-bermin/