Taxonomy & naming
Hoplias aimara was described by the French zoologist Achille Valenciennes in 1847. It belongs to the family Erythrinidae — the trahiras and wolf fishes — within the order Characiformes, a small Neotropical family of elongate, predatory, air-breathing characins that includes the genera Hoplias, Erythrinus and Hoplerythrinus. The genus Hoplias contains several large-bodied, superficially similar species collectively known in the trade as trahiras or wolf fish, and identification between them can be difficult without close examination; Hoplias aimara is distinguished by its size and the counter-current, fast-water habitat it favours within major river channels.
The common name "aimara" is used across much of its range in the Guianas and Amazon basin, while "wolf fish" and "trahira" are broader trade names applied to the genus as a whole. As with other Hoplias, care should be taken not to confuse the vernacular "wolf fish" with the unrelated marine wolffish (Anarhichas) of the North Atlantic. The name Hoplias macrophthalmus is treated as a synonym of this species in Eschmeyer's Catalog of Fishes.
Morphology
Hoplias aimara has the classic trahira build: a thick, rounded, torpedo-shaped body, a large flattened head and a wide mouth armed with heavy, conical, interlocking teeth built for gripping and crushing prey rather than slicing. FishBase gives a maximum size of 39.5 in standard length for males/unsexed fish, with some reports reaching 51 in and weights to roughly 40 kg, making it one of the largest species in its family. Maturity is reached at a comparatively modest 8.5 in, well before the species approaches its adult bulk.
Colouration is typically dark olive to blackish-brown over the back and flanks, often with a coppery or greenish sheen, fading to a paler belly — cryptic patterning suited to an ambush predator lying against submerged structure in fast water. The dorsal and caudal fins are set well back on the powerful body, and the overall impression, especially in large adults, is of a heavily built, unmistakably formidable predator.
Habitat
Hoplias aimara is found across a wide swath of northern South America: Amazon tributaries including the Trombetas, Jari, Tapajós, Xingu and Tocantins, the Orinoco basin, and the coastal rivers of Guyana, Suriname and French Guiana. Within these systems it favours the counter-current zones of principal rivers and rapids — fast-flowing, well-oxygenated water rather than still backwaters — where it holds position near cover and ambushes passing prey. As an air-breathing (benthopelagic) species it can tolerate lower dissolved-oxygen conditions than many of its potential prey, an advantage typical of the Erythrinidae.
The species is potamodromous, undertaking within-river movements, and reproductive activity is linked to the onset of the rainy season from December to March. Specific water-chemistry data from the cache is limited; prospective keepers should assume the warm, moderately soft-to-neutral conditions typical of Amazonian and Guiana Shield river systems rather than extreme blackwater.
Feeding
Hoplias aimara is an unambiguous piscivore and ambush predator. FishBase records its diet as feeding mainly on fish, supplemented by terrestrial invertebrates, and hobbyist sources describe a highly carnivorous, protein-hungry fish with a large, fearsome mouth capable of swallowing any fish that fits — including sizeable armoured catfish. It is described as primarily nocturnal, most active at dusk and through the night, consistent with a gape-ambush strategy in fast-flowing water.
In the aquarium this translates directly: the aimara needs whole meaty foods such as fish and prawn rather than dry or flake foods, and any tankmate must be too large to be swallowed whole — which, given this species' adult size and gape, effectively rules out all but the largest, toughest co-habitants, and most keepers house it alone.
Mating
Detailed courtship behaviour for Hoplias aimara specifically is not well documented in the cache. FishBase notes reproductive activity concentrated from December to March, coinciding with the onset of the rainy season and rising water levels — a common cue for reproduction among Neotropical river fish whose young benefit from expanding floodplain habitat. Fecundity is reported as high, ranging from roughly 6,000 to 60,000 eggs per female, indicative of a broadcast- or nest-based strategy producing large numbers of offspring rather than a small, intensively guarded clutch.
External sexual dimorphism is not documented in the available sources; data is sparse, and no reliable colour or morphological difference between sexes should be assumed without further confirmation.
Breeding
Several Hoplias and related erythrinids are substrate nest-spawners in which the male excavates and guards a pit or plant-associated nest and defends the fry after hatching, but species-specific confirmation of this behaviour for Hoplias aimara is not present in the available sources beyond the fecundity and seasonal-timing data from FishBase. Given the fish's adult size, solitary and highly aggressive nature, and the demands of a suitably vast aquarium, captive breeding of H. aimara is not established in the hobby.
Data is sparse for this species specifically; prospective breeders should treat any captive reproduction as effectively undocumented rather than assume the nest-guarding pattern reported for smaller congeners applies without confirmation.
In the aquarium
Hoplias aimara is not a beginner or community fish. Adults reaching a metre or more and up to roughly 40 kg require an aquarium on a scale most home hobbyists cannot provide — this is a fish more often kept by specialist predator-fish keepers or in large public-aquarium-style systems than in a typical home tank, and prospective keepers must plan for its full adult size from the outset, not the size it arrives at. It is a solitary predator: a single specimen will kill and eat essentially any tankmate it can engulf, including other large fish and armoured catfish, so it should be kept alone.
Strong filtration and high water flow suit a fish adapted to fast counter-current river zones, and robust, secure aquascaping (heavy rockwork, thick driftwood) should account for the animal's size and strength. The mouth, teeth and jaw strength are genuinely dangerous — hobbyist sources describe jaws capable of breaking apart prey — so all handling should be done with extreme caution, using a container rather than a net or hand wherever possible. This is a fish for experienced keepers only, and, as with any large predatory fish, it should never be released into open water.
Conservation
The IUCN Red List assesses Hoplias aimara as Least Concern (2020), reflecting its extensive distribution across major South American river basins spanning the Amazon, Orinoco and Guiana coastal drainages, and the absence of documented widespread population declines. The assessment notes localized pressure from subsistence and commercial overfishing in parts of its range, along with habitat degradation from deforestation, damming and agricultural expansion, but these have not translated into a broader conservation concern at the species level.
As a large, slow-maturing apex predator, H. aimara could in principle be more vulnerable to localized overfishing than smaller, faster-reproducing species, making ongoing monitoring worthwhile even though its current status is not concerning.