Largest freshwater fish (species)
Giant freshwater stingray, Urogymnus polylepis, Mekong giant catfish, Pangasianodon gigas
300 kilogram(s)
Cambodia ()

The largest species of fish to spend its whole life in fresh/brackish water is currently shared between two primary contenders. The Mekong giant catfish (Pangasianodon gigas), principally of the Mekong River basin of south-east Asia, can grow up to 3 m (9 ft 10 in) long and weigh 293 kg (646 lb), based on a 2.7-m (8-ft 11-in) specimen collected in the Mekong River in Thailand in June 2005. Another Mekong megafish was caught in a Cambodian stretch of the Mekong River on 13 June 2022: a female giant freshwater stingray (Urogymnus polylepis) that weighed a slightly heftier 300 kg (661 lb) and had dimensions of 3.98 m (13 ft) long – including tail – and 2.2 m (7 ft 2.6 in) disc width.

The discovery of the 300-kg giant freshwater stingray followed a month after a 180-kg (397-lb) specimen was found in the same stretch of the Mekong River in Cambodia’s Stung Treng province. Both stingrays were assessed by international scientists from the US-Cambodian “Wonders of the Mekong” project, funded by the United States Agency for International Development.

A related species of shark catfish, the paroon shark or dog-eating catfish (Pangasius sanitwongsei) – found in Thailand’s Chao Phraya River basin as well as the Mekong River – is reputed to reach similar sizes to its cousin the Mekong giant, though average adult length is 2 m (6 ft 6 in).

The arapaima (Arapaima gigas) of South America is reported to attain a greater length of up to 4.5 m (14 ft 9 in), but weighs “only” in the region of 200 kg (440 lb).

A former contender for this title, the Chinese paddlefish (Psephurus gladius), once native to the Yangtze River Basin, grew to a confirmed length of 3 m (9 ft 10 in) and also weighed up to 300 kg (661 lb) – although much larger, uncorroborated specimens have been reported, one as long as 7 m (23 ft) and weighing 450 kg (990 lb)! However, in January 2020, this species was officially declared extinct by the Chinese Academy of Fishery Sciences and the IUCN, who consider that it likely died out sometime between 2005 and 2010, owing to overfishing and habitat loss.

In the 19th century, the European catfish or wels (Silurus glanis) was reputed to attain lengths of 4.6 m (15 ft) and weights of 336 kg (720 lb) were reported for Russian specimens – but today anything over 1.8 m (6 ft) and 90 kg (200 lb) is considered large. A particularly large specimen of wels was caught in the River Po in Italy on 19 February 2015 by twin brothers Dario and Dino Ferrari; it weighed 127 kg (279 lb) and measured 2.7 m (8 ft 10 in) long. According to Dr Zeb Hogan, an expert in giant fish and presenter of TV series Monster Fish, the largest wels are now most common in Italy, France and Spain (outside of its native range), where it is an invasive species.

Certain species of sturgeon (family Acipenseridae) may grow to even larger sizes than all of these - one often cited exceptional example is a female beluga sturgeon (Huso huso) taken in 1827 in the Volga estuary that was reportedly 1,571 kg (3,463 lb) and 7.2 m (23 ft 7 in) long. However, sturgeon are not exclusively freshwater fish, spending at least some of their lives at sea as part of their migratory life cycle, making them the largest anadromous fish.