The itoui belongs to the family Apteronotidae, the knifefishes, and can reach 85 cm. It is rare in catches, and little is known about its biology. Anglers using rods almost never encounter it. It occurs in rivers of the Guianas and in the Amazon basin, in fast-flowing water over sandy bottoms. It has been recorded together with piaus and with the cichlid Crenicichla multispinosa. Scattered catches suggest a solitary habit. It has commercial and aquarium use.
Essential profile. Identification, where it occurs and rules, with data from FishBase and the cited regulations. Season, time of day, technique and bait are added once we have a good source for the itoui; in the meantime, see the similar species.
how to recognize it
identification
A knifefish with a very elongated body, flattened from side to side, with a long anal fin that runs along the belly almost to the tail, and no dorsal fin, like the rest of its family, which produce weak electric discharges. Reaches 85 cm, far more than the macana knifefish (Apteronotus bonapartii), from the same family, which stays at 38 cm. It lives in fast current over sandy bottoms.
how itoui fishing looks
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Each card shows the day's index (0 to 100) and, below it, a strip with the index for each hour, from 0h to 23h. from poor to excellent.
Essential profile: the index uses a general fish model (weather, sun, moon and tide), without a season or time of day specific to this species.
Guianas and Amazon basin, with records in Brazil, in fast current over sandy bottoms.
Habitat: Fast current over sandy bottoms, in rivers of the Amazon basin and the Guianas.
native to the basins
Amazon
what the law says
rules for the itoui
Summary of the regulations we found, with the source for each. Rules change; before keeping or transporting fish, check with the state environmental agency. Conservation: Not on the Brazilian national list (Ministerial Ordinance GM/MMA No. 1.667/2026).
We found no species-specific rule beyond the region's general rules.
piracema closures in the basins where the species lives
Spawning closure (piracema) in Mato Grosso (all basins)
01/10 to 31/01MT
Allowed: All fishing modalities are prohibited in rivers, including catch and release (Law 9.096/2009, art. 19-A). Exceptions: subsistence fishing from shore (3 kg or 1 fish, no transport or sale) and scientific fishing. Law 13.159/2025 now allows licensed sport fishing in UHE/PCH hydropower reservoirs, natural lakes and border rivers, subject to a scientific study and a license from the environmental agency. We have not confirmed whether these licenses are already being issued.
Allowed: Recreational anglers with handline or rod, line and hook are outside the Annex II prohibitions, with a bag limit of 5 kg + 1 fish, unless a stricter state rule applies.
catch and release Releasing the fish alive, quickly and with a wet hand. During a closed season it isn't always allowed: what the regulation says applies.
A rare fish: if you catch one, carefully remove the hook while it is in the water and release it promptly. It poses no danger to anglers.
calculator
how much it weighs without taking it out of the water
Length-weight relationship W = a·Lb (a = 0.00347, b = 3.06; FishBase (relação peso-comprimento bayesiana, estimativa)). It's an average: a fat or lean fish can vary by 10% to 20%.
estimated weight
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sources and references
Essential profile: size, length-weight, depth, countries and conservation come from FishBase and the cited regulations, and the texts were written from those sources. Season, time of day, sensitivities, technique and baits and lures are not included yet, because we did not find a good source for this species.
