Up Close with South American Fishes of the Rio Guejar

Oliver Lucanus

The spectacular natural rock formation known as the Sierra de la Macarena is close to the Colombian Andes, but geographically it belongs to the highland mesas of the Guiana Shield. This two-billion-year-old geological formation—which has been called “the greenhouse of the world”—is home to a variety of ecosystems and a keystone to the Earth’s biodiversity. The expansive region spreads across six different countries, with only a small section extending into Colombia, where our adventure begins. 

A forbidden land that served as a frontier for Colombia’s long and violent civil war, the Sierra de la Macarena was considered an unsafe place to visit for decades. When we visited in 2009, the northern Macarena was so unsafe that we were allowed only minutes at the river, surrounded by soldiers. But times changed, and within five years the Rio Guejar had once again become a popular bathing spot for locals from the nearby town of San Juan de Arama, and a destination for visitors like us questing for the bountiful and beautiful South American fishes that live within.

Exploring the Rio Guejar

The northern Macarena mesa has broken apart and is now transected by the Rio Guejar. The river, part of the Orinoco River basin, originates nearby in the Andes and sections off a part of the mesa, now forming a shelf leaning eastward. The dense forest reaches down the slopes to the river. Farming and logging have been too dangerous in this sector for many years, and as a result the forest remains intact, with giant buttressed trees and a dense understory of ferns and mosses overgrowing loosely strewn boulders. A new species of whip-spider, Heterophrynus javieri, is found in this lush and beautiful place, clinging to the undersides of overhangs and boulders during the daylight hours. 

The river is shallow and very clear in the dry season. The water is unexpectedly cool, measuring around 72°F (22°C) in the dry season. Rains upstream can quickly turn the river into a muddy torrent, but for most of the late winter months it remains fast flowing but shallow enough to walk across. The river bed is filled with rocks that range from fist to football size, with a few larger round boulders blanketing a substrate of fine gray sand. Occasionally logs or larger pieces of driftwood become wedged between the larger boulders, creating pools with eddies and deeper sand. 

Were it not for the tropical surroundings above the surface, the river could be easily confused with a similar-sized river in the European Alps or the foothills of the Rocky Mountains. While faithfully recreating the tropical habitat of most of our aquarium fish can be a difficult task, the Rio Guejar is perhaps one of the easiest choices for a biotope setup, as it is so familiar-looking to those of us living in the northern hemisphere.

There is only a thin film of algae and biocover on the rocks, as without farms and cities upstream, there is little organic pollution in the river to fuel algae growth. There are also no aquatic plants at all in this sector, and few marginal plants, because the torrents of the rainy season sweep away saplings of the trees that manage to gain a foothold when the water is low.

Swimming with the South American Fishes of the Rio Guejar

Large schools of evenly sized characins Prochilodus mariae graze on the thin film of biocover in the shallows. The groups number up to 100 individuals, and they keep a nervous distance from people in the water. Like all fishes in the river, they are stalked by the large Salminus affinis, the apex predator in this habitat. Individuals of all sizes patrol the open water, attacking any fish that moves. Any sound, like the footsteps of people walking in the river, or a rock rolling over in the current, quickly brings the nearby Salminus trying to score an easy meal. They feed on insects at the surface, injured or careless fish in the water column, and even the plecos attempting to scour off the thin biofilm on the upper surfaces of the rocks. 

Where the current is strongest, bristlemouth plecos Chaetostoma joropo of all sizes scour the rocks. The Chaetostoma here reach up to 8 inches (20 cm) in size and are commonly seen in the river, quickly sliding off their feeding stations and hiding under the round boulders when approached. The smaller individuals frequent smaller boulders, where they can hide in narrow crevices, while the larger plecos prefer to stay in the vicinity of the bigger boulders or even along submerged trees. Large-size adult C. joropo can appear lime green to yellow with black spots, but like many South American freshwater fish species from these cooler rivers, they do not thrive in aquariums with much warmer water.

Both Parodon apolinari and P. cf. buckleyi, two members of a group of Andean fishes known as scrapetooths, occur in this habitat, with the more slender, faster-moving P. cf. buckleyi frequently seen in high current, and the deeper-bodied P. apolinari in the extreme shallows feeding on the biocover on smaller pebbles. These interesting and unusual fishes are rarely seen in the aquarium, partially because they do not transport well, but they make excellent community fishes. Parodon are in constant motion, feeding on each rock before moving on to raid the next boulder and strip off the short algae from the top.

Schools of the tetras Astyanax maximus and Bryconamericus spp. swim swiftly through the shallows, feeding on insects, seeds, and fruits at the surface, occasionally stopping to investigate the small particles stirred up by human footsteps in the river. These characins are more commonly seen in the smaller, shallow streams of the region, perhaps due to the presence of so many large Salminus in this section of the river.

Where water is just knee deep, the small characins Creagrutus maculosus and C. calai hunt microscopic food along the surface and substrate. Cichlids are few here, largely avoiding the cool water, but Bujurquina mariae are occasionally seen in the shallows, shepherding their young. Other cichlids occur in this region but have not colonized the main branch of the Rio Guejar. They include Apistogramma alacrina, Crenicichla anthurus, and Aequidens metae.

A Changing Region, for Better or Worse

The renewed stability in much of Colombia will bring change to regions such as these, which were no-go zones at the turn of the millennium and beyond. The question remains how much of the change will be for the better of the environment. With increased political stability, the palm-oil producers have begun to saw at the forests of the Eastern slopes of the Macarena. Before long, timber, cattle, and other farms may enter the range to undo the pristine habitats found hidden within the forest.

Keeping the Andean Fishes of Colombia

The number of aquarium South American freshwater fish species from cooler water habitats is not small, and includes not only the fishes from the Colombian and Peruvian Andes, but also many species found on Brazil’s Atlantic coast, and in Argentina and Paraguay. Unfortunately, these fishes’ special needs are often neglected, and many species—such as the Chaetostoma plecos—will not thrive in the warmer temperatures of a typical tropical fish tank. 

For the Andean fishes of Colombia, fast-moving water, with temperatures of less than 75°F (25°C) are required. Because these rivers look not unlike the alpine habitats seen in Europe and North America, it is easy to find materials to aquascape an aquarium that closely resembles their natural habitat. Many of us in the northern latitudes could find the materials for the hardscape within a short distance from our home, so this habitat would be easy to recreate and maintain. 

Perhaps this article will inspire some to set up aquariums with the needs of the cooler-water South American fishes in mind, and help better understand the special requirements of the fish caught in Andean rivers of Colombia and Peru—particularly now that they seem to be appearing in the hobby more and more frequently.

Check out this video of the author’s adventures exploring the Rio Guejar for an even more detailed look!

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