Monsters of the Pampas

Author: Ivan Gonzalez and Stan Sung

While collecting in the waters of Uruguay, our fearless explorers come across monster-like fish with sharp teeth, large bodies, and ferocious attitudes.

Lago Merin Adventures

The expansive Lago Merin looked like an ocean. We could hardly see the adjacent bank way off on the hazy horizon. The lake bottom appeared to be a wide, sandy stretch where only silvery characins roam in the open water. Our boat grounded on a sand bank with a slow, barely traceable slump and we grinded to a halt. With the motor shut off, we could only hear the lake gently lapping at the side of our craft. Rather than spend the night stranded, we decided to jump into the shoulder-deep water and push the boat to mobility. The fine sand felt like a soft carpet underfoot as we guided the hull off its sandy perch. Once free, Tom Ganley, Jim Herman, Nathan Okawa, John Niemans, Bill Cain, Felipe Cantera, Alvaro Mobilio, and I continued our search for exotic fishes in the windswept lands of Uruguay.

The Bizarre and Formidable

Much has been written about the beautiful killifish and Gymnogeophagus cichlids from Uruguay, but the monster fishes currently taking the aquarium world by storm also dominate this region. Eels, catfish, characins, and cichlids of grand sizes all vie for the position of top predator.

What exactly constitutes a “monster fish”? One characteristic is tank-busting size; most of these fish represented grow to a length of over a foot—very large by aquarium standards. There’s also the makeup of their teeth; being near the top of the predator hierarchy, the dentition must be intact and effective. Another characteristic of aquatic monsters may be bizarre morphology; some of the Synbranchus eels and loricariid catfishes look like miniature sea monsters with their heavily armored bodies, strange mouths, or serpentine forms.

Gargantuan Brasiliensis from the East

This was my fifth trip to Uruguay, where our dear friend Felipe Cantera has led us on all of our ensuing adventures. We began our collecting foray in the east of the country at the aforementioned Lago Merin. The collections were dismal in the lake proper, so once we freed our boat we decided to try our luck in some of the overgrown rivers feeding into the lake. Felipe, Bill, and I dragged the seine across a wide, brown river, just shy of a tangle of water hyacinth.

We could tell from the tugs on the seine that we had some good-size fishes inside of our net. Once the seine was on shore, we opened it like a large Christmas present to find jumbo-sized Geophagus brasiliensis madly flopping around. These foot-long, robust beasts were far larger then any captive specimen that I have seen. Some pleasing, gold-shaded Hoplias species were the next to be captured. We searched the mud-laden net for desired species we wanted to keep. While retrieving a very small 2-inch Hoplias from the seine, the little fish bit down on my index finger and made a surprisingly deep gash in it. The blood flowed freely from the fierce little fish’s bite. I could only imagine what terrible damage a large 3-foot Hoplias could do to an intruding hand!

The Golden Tigers of the Rio Uruguay

While motoring along the winding river, my thoughts drifted back to a fishing foray on my previous trip to Uruguay. Salminus brasiliensis is a characin of monstrous size and superb coloration. We trolled for S. brasiliensis (locally known as “dorado”), which are sometimes called the tigers of the river. These fierce fish fight hard once angled and leap far from the water. It is fun to throw chunks of fish or shrimp to captive dorado and watch them strike hard at the tasty morsels. These are extremely antagonistic aquarium subjects that are constantly attacking and biting tankmates.

On that collecting trip along the Rio Uruguay, I shared a boat with Jeff Rapps of “Tangled up in Cichlids,” John teamed up with Nathan in another boat, and Ken and Tom were together in a third boat. Jeff and I turned out to be the only boat that did not catch any of the amazing dorado. Somewhat disappointed, we decided to check out the tide pools of a rocky island in the middle of the river. While Jeff kept entertained by small pleco in the pools, I threw a cast net into the river rushing. Another jumbo characin Prochilodus lineatus, a heavy specimen at a solid 20 inches long, made it into my net. Although Jeff and I were not able to experience the excitement of landing a ferocious dorado that day, we were able to enjoy some that evening for dinner!

A Bucket Full of Eels

Big clouds began to appear overhead as we made our way to the Rio Santa Lucia in Paso Pache, Canelones. Tom and I stayed by a pond that had become isolated from the river due to low water levels. We caught some “mini monsters” of the characin group. The gaping-jawed Charax stenopterus were in superb breeding garb of gold and pink. Many small specimens of the toothy barracudas Oligosarcus jenynsii were also captured. Ironically, small specimens of both species were on Tom’s most requested list as bait to hook Crenicichla and Hoplias species!

Ivan, Felipe, and the others made their way to the main river channel, where they pulled the seine over a field of river rock. Besides multicolored Gymnogeophagus, they also came back with some medium-size swamp eels Synbranchus marmoratus. The ones collected at Paso Pache were dark brown in color. We have also collected specimens from various sites that were solid gray or light brown, or sometimes sprinkled with tiny black dots. Some were attractive with black marbling. Although there is only one species of eel described from Uruguay, it seems highly doubtful that all of these eels are S. marmoratus. Among this bucket of eels were also some eel cats Heptapterus mustelinus. These medium to small catfish develop a nice yellow coloration and make pleasing aquarium subjects.

Monster Fishes Collected in Uruguay


One of the most common fish found in the country is Hoplias malabaricus, known as “tararira” or wolfish. It inhabits almost all of the watercourses, from very small streams to the largest rivers. They belong to the family of characins called erithrynidae, which is characterized, among many things, by the absence of an adipose fin (the small fin just behind the dorsal fin and before the caudal fin).

H. malabaricus can be found in the entire country. Their cousin, H. lacerdae, a larger species, is present mostly in the north of Uruguay. H. lacerdae is dark metallic green in coloration and possesses a more elongated shape than H. malabaricus. A true monster, H. lacerdae can reach a weight of 26 pounds and 36 inches in length. Taxonomy work remains to be done on the Hoplias of Uruguay.

Hoplias are incredibly predatory fishes that spend most of the day hidden in the shallows in dense vegetation waiting for prey to pass by. Once they target the prey, they lunge at it with a swift move of the tail. The unfortunate fish are captured with a single bite of extremely sharp, canine-like teeth. Moreover, though they are not fast predators, on some occasions they will pursue their prey until they catch them.

Tarariras remain inactive during winter. They maintain low metabolic rates and do not feed during this time. In the beginning of spring, when the water temperature rises, they return to the awesome predators that they are. Adult tarariras feed mostly on other fishes. Anything from small characins like Buenos Aires tetras to large barracudas such as Oligosarcus species are consumed. They can eat fishes almost as big as themselves! In addition to fish, they also feed upon frogs, and large Hoplias will even take baby birds that have fallen from their nests. Juveniles feed mainly on aquatic insects.

Spawning begins in the summertime, when both the male and female make a nest with vegetation in a shallow, quiet part of the river. They take care and guard the eggs until they hatch. A group of Uruguayan scientists have recently succeeded in reproducing tarariras in large outdoor tanks, and proved that tarariras begin to be reproductively active at the small size of around 5 inches.


The premier predatory characin of South America is without a doubt Salminus brasiliensis (previously S. maxilosus). Dorado (gold color) is also a perfect game fish that can reach 65 pounds in weight and about 36 inches in length. Its flesh is great-tasting and it is an esteemed food fish. This hard-fighting dream fish of anglers must be caught with a steel lead, as it will break any fishing line with its powerful jaws and teeth. Banded knifefish Gymnotus carapo and different types of lures are used to capture the big gold. In Uruguay they inhabit the Uruguay River Basin. The dorado is a migratory species that used to make great migrations to the upper Uruguay River Basin as a part of its reproductive cycle. This was before the Salto Grande Dam (a hydroelectric dam) was built on the Uruguay River. The S. brasiliensis population decreased, but thankfully the population downstream appears to be recovering a little bit now. The population, however, is not even close to the enormous numbers of S. brasiliensis seen before the dam impact. There have not been any studies pertaining to the population impact upstream at the time of writing.

Dorado are powerful swimmers that constantly swim against the strong river currents. They hunt by pursuing their prey of fish and crustaceans. The silvery juvenile dorado are sometimes collected along the shoreline. The adults are intense yellow in color with dark horizontal stripes and glowing red fins. These are prime show fish for a (very large) monster tank. Be warned: they are incredibly aggressive to both conspecifics as well as other tankmates in captivity.


Another predator characin is Rhaphiodon vulpinus. Like the dorado, they also dwell in areas of strong current. R. vulpinus is known locally as “chafalote” or “machete.” They have highly compressed bodies, a small caudal fin, and enormous canine jaws. Perfectly hydrodynamic in shape, they are the fastest predator characin that can reach over 3 feet in length. Like the dorado they hunt their prey by pursuing them. Sometimes the machete will leap far out of the water when hunting, and it’s is a spectacular sight to see! Little is known about the biology of this most amazing predator from the Rio Uruguay.

Barracuda-like Characins

The “barracuda” characins (Oligosarcus hepsetus, O. oligolepis, O. jenynsii) are common in almost all streams and rivers of the country. Oligosarcus are medium-size predator characins, ranging in size from 5 to 15 inches in length. They have a mouthful of wickedly sharp canine teeth, which are used to feed on other small fishes and crustaceans. Although they are active predators, they are also a common prey for the tararira Hoplias sp.

The barracuda shape appears to be a very successful design in the evolution of fish, as there are other characins not closely related to Oligosarcus, such as the Acestrorhynchidae (Acestrorhynchus pantaneiro in Uruguay) with practically the same basic body traits. They both feed the same way by hunting actively. A. pantaneiro is found only in the Uruguay River and is the red-finned version of Oligosarcus (yellow fins) with a black humeral spot. A. pantaneiro is a migratory species (as opposed to the non-migratory Oligosarcus barracudas), they will move to the upper reaches of rivers in the spring where the river has more water, to feed and spawn.

Barracuda-like Characinae Tetras

Charax tetras belong to a subfamily, inside the characidae family called Characinae. Two of the four genera of this subfamily present in Uruguay have a barracuda shape, the species are Galeocharax humeralis and Cynopotamus argenteus. Both species inhabit the Uruguay River basin. C. argenteus grow up to 14 inches and are efficient predators, just like all the barracuda-shaped species. When young they have a fantastic black and white pattern in their dorsal and caudal fin. They are very aggressive predators and should not be kept with other fishes smaller then 4 inches in length.

Charax Tetras

The Charax tetras Charax stenopterus are distributed all over the country. They are 4 to 6 inches in length and inhabit littoral zones with dense, submerged vegetation (usually in Cabomba- and Myriophyllum-like plants). In nature they feed on snails and small crustaceans living on the plants. At breeding time they are attractive in a metallic yellow or golden coloration. In quiescent suit they are light brown or rather transparent with little color. Charax are placid fish that usually stay in a 45-degree, head-down position inside dense vegetation. These “mini-monsters” are easily collected with a quick pass of the hand-net through plants growing along the shore.


Several piranhas are present in the Uruguay River Basin. Most commonly found are members of the genera Serrasalmus and Pygocentrus. They all live in schools feeding on any fish, or most anything else, that moves towards them. They have the most sharp and fearsome teeth of any fish in the river, capable of cleanly cutting their prey with a single bite. Local fishermen call them “palometas,” and they are of minor importance as a game fish or food fish. They can spell disaster for the fish in shallow areas where they can be incredibly abundant. Piranhas would be happy to quickly consume every fish caught in a gill net, only leaving carcasses for the fishermen.

Serrasalmus in Uruguay are pale yellow in color most of the time, but during spawning time they get an intense yellow-to-orange color that makes them attractive aquarium subjects.

Prochilodus lineatus

Locally known as “sabalo,” Prochilodus lineatus belongs to the family Prochilodontidae. Despite their monstrous size (36 inches), they are not predatory fish, but rather feed on organic waste found in the mud. They have a protrusible mouth similar to that of a giant kissing gourami. They use this odd, disc-shaped mouth for sucking mud from the bottom and for extracting organic material from it.

Sabalo are the most important fish in commercial fisheries of the Rio Uruguay. This is despite a recent Argentinean study that revealed that P. lineatus store heavy metals brought on by contamination in their muscles. In Uruguay this species is still being commercialized for human consumption and for making pellets for cattle food. They swim in large schools and would make a stunning display in a giant show aquarium.

Iheringichthys labrosus

Iheringichthys labrosus is a spotted catfish common in the Rio Uruguay and Rio Negro basins. It is known as “trompudito” (large mouthed) in reference to the large, down-turned lips which it uses to feed mostly on benthonic invertebrates. It is probably the most common catfish in the Rio Uruguay and Rio Negro. Maximum size for I. labrosus must be around 15 inches in length.

This is a bottom-dwelling catfish that inhabits sandy, rocky, or mud bottoms in areas with moderate water current. They possess large pectoral and dorsal spines that make it very difficult to extract them out of nets. Care must always be taken when handling them, as the spines are very sharp and can cause very intense pain if pierced by one. Aside from being difficult to handle, they make wonderful aquarium subjects, as they display an attractive spotted pattern and are quite peaceable in temperament.

Pimelodus maculatus

Pimelodus maculatus is another spotted catfish common in the Rio Uruguay and Rio Negro basins. They are known as “bagre amarillo” (yellow catfish) in reference to a yellowish variety of this species. It is an important fish in Rio Uruguay and Rio Negro commercial fisheries, and it is an esteemed food fish. P. maculatus is a bottom dwelling catfish that inhabits sandy and rocky bottoms in moderate water current.

The striking spotted pattern makes it a beautiful aquarium fish, although a big tank is required, as adults can reach a tankbusting 24 inches in length!

Luciopimelodus pati

One of the largest Silurids in Uruguay is the formidable pati Luciopimelodus pati. This member of the family Pimelodidae inhabits the Uruguay River and will grow up to more than 3 feet in length and about 90 pounds. Juvenile pati have tiger stripes that fade as they grow. Adults are dark gray to light blue in color. Large pati live in the main river channel, in deep and turbid waters with a moderate to fast current.

Local fishermen catch pati using a technique they call “crazy jar.” In areas where the current is fast and the river is turbid, fishermen drop long fishing lines with a weight and an enormous hook baited with meat or fish, and a bottle full of air attached at the other end. Many bottles are tossed from their boats where they float and flow freely. They watch these bottles closely, and when a bottle bobs, it is quickly grasped with a large hand hook, and the pati is landed from the deep, turbid water. I guess you could call this fishing “monster style”!

Pseudopimelodus mangurus

Pseudopimelodus mangurus, known as “manguruyÚ de las piedras” is another member of the Pimelodidae family present in the Uruguay River. The manguruyu reaches 30 inches in length. This attractive catfish varies between dark orange to brown depending on the color of the substrate in which it is found. They are sprinkled with black spots. This pattern makes for a perfect camouflage on its nocturnal jaunts. The manguruyu makes a great aquarium fish, however keep in mind that it is nocturnal and posses a big mouth. It may eat fish sleeping on the bottom of the tank during the night. In nature it is usually caught by both hook and line with worms or fish as bait, or with gill nets during the night.

Bumblebee Cats

Heptapterus mustelinus is a bumblebee catfish common in the entire country. Its common name is “bagre anguila,” which means eel catfish, in reference to its long, serpentine body.

This catfish lives hidden among small rocks in zones with fast-flowing water. They are frequently found coexisting in the same habitat with Ancistrus sp., various plecos, and other fast-current catfishes such as Microglanis sp. and trichomycterid catfish.

The eel cat feeds on small invertebrates and reaches a maximum size of about one foot. H. mustelinus will hide most of the day in aquaria, becoming active at night and very frantic at feeding time.

Driftwood Cats

Trachelyopterus teaguei, called “torito” locally, is a driftwood catfish endemic to the Uruguay River Basin. A member of the Auchenipteridae family, T. teaguei is nocturnal and feeds mostly on insects, snails, and crustaceans. This species is never collected during the day, but as soon as night falls it becomes one of the river’s most common fish. They can be caught with hooks, gillnets, or even cast nets. When an artificial light such as a camping lamp is placed by the water, attracting insects that fall into the water, hundreds of driftwood catfishes can be seen feeding on the insects. This is truly a remarkable sight, especially considering the fact that not a single one can be seen in the daytime.

Like most auchenipterid cats, the male has a genital papilla, located anteriorly on the anal fin, which is used for internal fertilization. This is a characteristic of the woodcat family. Male T. teaguei possess extremely large and hard pectoral and dorsal fin rays covered with barbs. It is thought that the elongation and development of these fins happens only during the spawning season, much like with other members of the family Auchenipteridae, such as Ageneiosus inermis. A. inermis and A. militaris are piscivorous auchenipterid catfishes. These nocturnal driftwood cats grow to around a foot long and are mid-water swimmers. These two species make interesting aquarium rarities, but they also have to be handled with extreme care, as they have, like many auchenipterid cats, the ability to clamp their pectoral fins very fast. A finger could easily be trapped between the body of the fish and their saw-like pectoral spine. This vice grip has a locking system that will not release until the fin is broken!

Rhinodoras dorbignyi

A common nocturnal catfish from the Rio Uruguay is Rhinodoras dorbignyi, known in Uruguay as “marieta.” This species has amazing camouflage colors, and as a member of the Doradidae family it has a series of bony plates (scutes) on its sides. They also possess powerful saw-like pectoral fin rays that make them difficult to handle. Care must be taken, as they can close and lock their pectoral fins down on the handler’s fingers much like the preceding species, causing much pain. To make matters worse, it is very difficult to release trapped fingers from their vice-like clamp.

These are bottom-dwelling catfish that feed mostly on benthonic invertebrates. Maximum size is around 15 inches in length. Rhinodoras make great aquarium inhabitants, being passive and friendly. They may hide in the daytime and become active at night. Just remember to keep your fingers away from the pectoral fins!

Hoplo Cats

Hoplosternum littorale are members of the family Callichthyidae. Other members of Callichthyidae are various Corydoras species, and Callichthys callichthys can also be found in Uruguay. C. callichthys is similar in appearance to H. littorale, but C. callichthys inhabit the southeast of the country, while H. littorale inhabit the north of the country. H. littorale lives among dense vegetation and is found in anoxic (oxygen-deprived) habitats. They are not in great rivers, but rather in small streams and ponds. It is the largest species of hoplo cat, reaching a size of 8 inches. It has a steel blue to deep gray color, depending on the habitat and season. They feed on small invertebrates and plants.

During the spawning season, the male’s pectoral fins curve up to 45 degrees to form weapons for defensive purposes. Both male and female make a bubblenest where they lay their eggs. They exhibit parental care and take care of the nest and eggs. These wonderful aquarium fishes have been spawned in captivity.

A Variety of Plecos

Many pleco species are found in Uruguay, but surely the most beautiful ones are Hypostomus luteomaculatus and Hypostomus alatus, both spotted species that inhabit the Uruguay River and Rio Negro Basins. The main difference between them is that H. alatus has circular spots everywhere, while H. luteomaculatus has lines on its dorsal fin in addition to the spots. The spots on both species can be dark pink, orange, or white, depending on the substrate in which they live. The maximum size is approximately 15 inches.

Many of our plecos are fantastic algae eaters, so not only are we treated by their good looks, but also by some of the wonderful domestic duties they perform. Another beautiful and unique pleco is the light blue Hypostomus commersoni, which inhabits the entire country and can reach immense proportions, up to 3 feet. These giant specimens are a uniform sky blue and really appear to be manufactured out of plastic. They are truly great show fish that are always a thrill to collect in the wild. Like many others, these plecos have the ability to breathe air, which gives them the capacity to remain alive for a long time outside the water. They have been known to “walk” to find another watercourse when their habitat dries out completely during the dry season. These amazing fish also dig large caves (up to 3 feet deep) along mud banks in which they spawn. Other genera of plecos not present in Uruguay can even remain alive during the entire dry season hidden in caves above water levels.

Pseudohemiodon devincenzii and Paraloricaria vetula are other common and endemic Uruguay River Basin lurkers. Both of them reach jumbo sizes of over 15 inches.

Pseudohemiodon devincenzii is mostly a detritivore, not an algae eater like its Hypostomus relatives. There is sexual dimorphism in P. devincenzii. The male’s lower lips are larger, because when spawning he carries the eggs attached to his lips until the fry are born. During breeding season, the male can be caught carrying an enormous bag of eggs on his belly, attached to his lower lips.

Paraloricaria vetula is a carrion-eater (carroÑera) species. They have many tiny, needle-like teeth that they use to rasp at dead animal carcasses. These monsters feed on the dead meat of carcasses, somewhat like aquatic vultures. They can also eat dead fish trapped in gill nets, and sometimes not much is left of the snared fish due to these carrion eaters. Paraloricaria and Pseudohemiodon, as well as all plecos, can be caught with gill nets during the night.

Synbranchus marmoratus

A very strange and modified fish indeed is Synbranchus marmoratus, locally known as “anguila” (eel). They can be found in all bodies of water, from small ponds and wetlands to the largest rivers. These swamp eels grow to a length of over 5 feet. Anguilas do not have pectoral or ventral fins, and the dorsal fin is vestigial. It is a facultative air breather, which makes it extremely successful for living in all habitats, even those deprived of oxygen. They dig tubular burrows on the bottom that lead to the exterior with one opening, and this way they can remain alive during an entire dry season in the damp mud, breathing air.

Swamp eels are protogynous hermaphrodites, which means that most are born female and when they reach a certain size they become males. The young females may breed many times and will eventually become males at a size of around 30 inches in length. There also some that are born males, but they account for only approximately 20 percent of the entire male population (80 percent of the males were females when young and change sex at the size of 30 to 35 inches).

Crenicichla spp.

Along with Crenicichla scottii, one of Uruguay’s biggest cichlids is Crenicichla vittata. Both are Uruguay River predators. These great pike cichlids can reach sizes of 20 inches and feed on fish and crustaceans. Their reproductive behavior is the same as in many other cichlids, as they take great care of their eggs and fry. During spawning, both males and females get intense orange colors on their bellies. The coloration of the caudal eyespot is enhanced for confusing prey and predators, for both defense and feeding functions.

Geophagus brasiliensis

Geophagus brasiliensis inhabits the Laguna Merin Basin in the East of the country. They are seldom collected in Uruguay but can be found in some parts of the Olimar River. It is a big (up to 15 inches), laterally compressed cichlid. During spawning time (in the beginning of summer), they become very territorial and aggressive fishes. They also become highly colorful at this time, which makes them spectacular aquarium subjects. In the wild they feed mostly on small fishes and invertebrates, however they will take any food offered in the aquarium.

So whether your vice—or weapon of choice—is a monstrous catfish, a toothy characin, or a beautiful and aggressive cichlid, they can all be found in the flowing rivers of the pampas.


Note from Stan Sung: I would like to extend my sincere thanks to my good friend Felipe Cantera. Without his assistance and knowledge, I would never have explored the habitats of these wonderful beasts of South America. I am proud to have worked on this article with my good friend Ivan, someone who will without a doubt be very influential in South American ichthyology in the coming years.

Note from Ivan Gonzalez: I would like to thank Franco Texeira De Mello for the wonderful photographs he lent for this article and for the very valued information he shared with me. Thank you also to my friend Stan Sung for this opportunity and for being such a great friend!