Essequibo River

Most maps of South American cichlid diversity skip past the Guianas, but the Essequibo and its great northern tributary the Mazaruni hide some of the strangest small cichlids on the continent. The river drains the old, hard rock of the Guiana Shield in tea-black and clear water, falling off the highlands in cataracts — Kaieteur on the Potaro drops 823 ft in a single sheet — and pooling behind rapids that sort fishes by how fast the water moves. On a high, half-isolated plateau in the upper Mazaruni lives the dwarf genus Mazarunia, found nowhere else: three blackwater cichlids, one of them a true rapids specialist. This page is about that river fauna and the Mazaruni rather than the broader Essequibo basin treated elsewhere in the atlas.

Geography & hydrology

The Essequibo is Guyana's master river, and FEOW treats its whole catchment — plus a slice of southeastern Venezuela along the Guiana Shield border — as Ecoregion 310. The basin spans a low coastal plain, forested hills, and highlands that climb from sea level to nearly 2,1312 ft in the Pakaraima, Kanuku, and Acarai ranges. Five large rivers organize the system: the Essequibo itself, the Cuyuni, the Mazaruni, the Potaro, and the Rupununi, all eventually reaching the Atlantic through the Essequibo's wide, island-studded mouth (FEOW Ecoregion 310).

The water here comes off the Guiana Shield, one of the oldest exposed landmasses on Earth — Precambrian crystalline rock and the sandstone tablelands that cap it, including the table mountains of the Roraima massif on the Venezuela–Brazil–Guyana border. Rivers running off that basement tend to be either clear or stained black with dissolved plant tannins, and poor in dissolved minerals, because there is little soft sediment left to carry. The upland reaches are steep: moderate-gradient streams and rivers broken by rapids and cataracts, with the lowland coastal plain holding the slow water, lakes, and seasonally flooded swamp. The single most dramatic feature is Kaieteur Falls on the Potaro, where the river plunges 823 ft off a sandstone escarpment with an average discharge around 2175 ft³/s — one of the largest single-drop waterfalls in the world by combined height and volume (FEOW Ecoregion 310).

The Mazaruni, this page's focus, joins the Cuyuni and then the Essequibo near Bartica, but its upper basin is a world apart. Above a long gorge the river opens into a high interior basin ringed by mountains, draining the southern and western slopes of the Roraima massif. Tributaries such as the Kako, Kukui, and Kamarang rivers fall off those slopes as steep, rocky, rapid-strewn streams before reaching the slower, sand-bedded main channel near the village of Kamarang. That combination — an elevated, partly enclosed basin fed by mountain torrents and drained through a single constricted outlet — has kept the upper Mazaruni fish fauna isolated long enough to evolve in place, and it is the geographic reason the region carries so many species found nowhere else (López-Fernández et al., 2012; FEOW Ecoregion 310).

Water & habitat

Shield rivers come in two flavors and the Essequibo system has both. Clearwater reaches run transparent off the crystalline rock; blackwater reaches, including much of the upper Mazaruni, run the color of strong tea, stained by humic and fulvic acids leached from forest litter and white-sand soils. Both are soft and weakly buffered. What the upper Mazaruni's own surveyors measured in the field gives an unusually concrete picture: water temperatures of roughly 71–76 °F, dissolved oxygen from about 5.2 to 9.3 mg/L, acidic pH between 4.0 and 4.8, and conductivity so low it barely registered — under 10 µS/cm, sometimes effectively zero (López-Fernández et al., 2012).

That chemistry sits on top of a sharp habitat gradient set by current. In the main Mazaruni channel the bottom is sand or light mud, the flow sluggish — often 0–1 ft/s — with structure supplied by drowned grass, riparian plants leaning into the margin, and scattered driftwood. The steep tributaries off the Roraima slopes are the opposite: rocky beds, faster water reaching 2 ft/s, and the broken, oxygen-rich flow that rapids produce. Transparency tracks the substrate, running about 2–3 ft on a Secchi disk in clean tributaries. The fishes sort themselves along that gradient, and at night they shift again — the dwarf cichlids here move up into shallow, flooded banks thick with terrestrial vegetation after dark (López-Fernández et al., 2012).

Gold dredging has begun to blur the picture in the main channel. Mining throws up artificial 'tailings' beaches of dumped sand and pushes fine sediment into the water, so dredged sites read murkier — Secchi depths of 1 ft — than the 2–3 ft of unaltered banks nearby. Several of the Mazaruni cichlids turn up readily on those man-made beaches, which makes them easy to find but says nothing good about the long-term state of the channel. The contrast the surveyors kept drawing was between near-pristine tributaries of deep, clear black water with almost no suspended solids, and a main stem whose transparency is sliding as the dredges work it (López-Fernández et al., 2012).

The cichlid fauna

The headline cichlids of the upper Mazaruni are the three species of Mazarunia, a dwarf genus described from this river and found nowhere else. Sven Kullander erected the genus in 1990 on a single species, Mazarunia mazarunii, collected from the blackwater Kamarang River; in 2012 Hernán López-Fernández, Donald Taphorn, and Elford Liverpool expanded the genus to three, adding M. charadrica and M. pala. They are small fish — adults run only a few centimeters — and anatomically odd enough to stand apart from every other cichlid: the genus uniquely lacks the sixth infraorbital bone, among a long list of skeletal traits. Phylogenetically Mazarunia is the sister group of Guianacara, and both fall in a clade tied to the Guiana Shield (López-Fernández et al., 2012).

The three divide up the river by current. Mazarunia mazarunii and M. pala both favor the slower main channel, over sand or light mud near grass and wood, and the two seem to coexist there, though pala is the scarcer of the pair. M. charadrica is the rheophile — the rapids fish. Its name comes from the Greek charadra, a mountain torrent, chosen because it turns up most in the steep, rocky, rapid-rich tributaries pouring off the Roraima massif, though it will also share the sandy main-channel sites with its relatives. M. pala takes its name from the Greek for a gold nugget, partly for the golden spots under and behind its eye and partly as a quiet warning: it has only ever been found in the main channel, the same water the gold dredges are reworking. The three are a small natural experiment in how an isolated shield basin packs related fishes into adjacent niches (López-Fernández et al., 2012).

Beyond the endemic dwarfs, the atlas roster carries the larger river cichlids of the Essequibo system. Krobia guianensis is a hand-sized acara of Guianan coastal drainages, ranging from the Demerara east into Suriname and onto the Amapá coast, holding to slower freshwater margins. Crenicichla vaillanti is a pike cichlid known from the Essequibo basin, one of the elongate, ambush-hunting Crenicichla that haunt structure and current edges throughout the Guianas. And Cichla ocellaris — the lukanani, the peacock bass that anchors local fisheries — is native to the Essequibo and Marowijne drainages; unlike the rocky-shoal peacock recently split off from the basin's swift channels, C. ocellaris leans toward quieter, lentic water. Set against the Mazarunia endemics, these species show the two faces of the fauna: a handful of wide-ranging Guiana-Shield generalists, and a clutch of micro-endemic specialists locked into one high plateau (FishBase; Sabaj et al., 2020; López-Fernández et al., 2012).

Conservation

The thing that makes the upper Mazaruni precious also makes it fragile. A large share of its fishes live nowhere else, and they live in a small area: alongside Mazarunia the basin holds the endemic fish genera Derhamia, Paulasquama, and Skiotocharax, plus endemic species in wider-ranging genera, several still being described when the 2012 Mazarunia paper was written. Pack that many irreplaceable species into one modest basin and any single pressure carries outsized risk (López-Fernández et al., 2012; FEOW Ecoregion 310).

Gold and diamond mining is the pressure already at work. Across Guyana, artisanal and small-scale gold mining runs on mercury, used to bind gold from crushed ore, and the mercury ends up in rivers and then in fish. A WWF assessment of the Guianas reported that on the Potaro — the Essequibo tributary that carries Kaieteur Falls — 57% of carnivorous fish sampled exceeded the WHO guideline of 0.5 µg/g of mercury, while non-carnivorous species stayed below it, the signature of a contaminant climbing the food chain into the very predators the roster includes. Indigenous communities in mining districts who eat local fish carry measurable methylmercury exposure as a result. In the Mazaruni itself the surveyors watched the mechanism directly: dredging silts the channel, raises turbidity, buries shallow habitat under tailings, and degrades exactly the clear, structured water the endemic cichlids need. Work elsewhere on the Guiana Shield has shown that even small-scale gold mining erodes stream fish communities, so the trend in the Mazaruni is not a mystery (WWF, 2015; López-Fernández et al., 2012; Brosse et al., 2011).

The other looming pressure is a dam. Guyana has long planned the Amaila Falls Hydropower Project, a roughly 165 MW station on the Kuribrong (Amaila) River in the Potaro-Siparuni region, within the Potaro arm of the Essequibo system, with a reservoir on the order of 14 mi². The project has stalled and restarted repeatedly, and the government reopened bidding in the 2020s. Its environmental due diligence has had to weigh endemic fishes — the IDB funded supplemental surveys specifically to learn whether species known only from a narrow area occur more widely before the river is impounded, a tacit admission that a single dam here could flood an entire range. Damming a shield river replaces fast, oxygenated, rocky habitat with still water, blocks movement, and traps the sediment that mining is already adding. Add accelerating deforestation in the mining belts, and the conservation picture for the upper Essequibo and Mazaruni comes down to a simple arithmetic: many one-place species, several converging threats, and a basin small enough that losses would be permanent (Power-Technology; IDB GY-T1108; López-Fernández et al., 2012).

Where these cichlids live

Geography & hydrology

Water & habitat

The cichlid fauna

Conservation

Sources

Cichlids recorded here

Cichla · 1

Crenicichla · 1

Krobia · 1

Mazarunia · 3