The weird world of megafish

By Jack Flanagan

By definition, a Megafish is greater than 6ft in length – larger than the average human. They’re mostly found in freshwater systems and are benthic i.e. they live on the bottom of the riverbed. Some are gentle giants, feeding on algae and plankton, others are voracious feeders who grab animals from the banks and pull them into the water.

These fish have what’s known as “indeterminate growth”, growing larger as long as they live without disease or predation. So the larger fishes also tend to be the oldest ones.

The largest freshwater fish ever captured was the Mekong Catfish from Thailand. Typically reaching around 200kg, and 7/8 feet, this fish hadn’t stopped growing throughout its life. A research expedition on the Mekong river captured a 300kg, 10 foot beast – the “size of a grizzling bear”. Unfortunately, Thai Fisheries officials stripped the fish of its eggs as part of a breeding programme, hoping to release it, but the fish died in captivity and was sold as food to local villagers.

Mekong giant catfish

Mekong giant catfish

The Mekong Catfish is only one of many (the titanic Freshwater Stingray also lives in the Mekong river). The Arapaima of Brazil is an Amazon fish which mostly eats small fish and crustaceans – but occasionally grabs animals that stray too close to its habitat. The Arapaima is an air-breather: a complex set of tubes feed oxygen into its blood, because the lakes it live in contain next to no dissolved oxygen.

The Freshwater Stingray is not typically aggressive, but the tail is covered in a toxin for defense.

The Arapaima lives in the lakes and rivers of the Brazilian Amazon.

Unsurprisingly, because the fish are so large, they are important as food. In the case of the Mekong catfish: as populations boomed in Thailand in the 1960s the fish numbers dwindled until none were being caught along the river. Steadily that number has risen – but the Mekong Catfish is still looking at an uncertain future. Other fish, such as the Chinese sturgeon and the “Mongolian Terror Trout” (dubbed by the National Geographic) have also seen declining populations.

The Hucho Taimen - also known as the Mongolian Terror Trout

The Hucho Taimen – also known as the Mongolian Terror Trout

The question of why these populations are declining is important. Like the Panda or Polar Bear, they are recognisable and enigmatic flags of the welfare of their habitats. If the Mekong river is being polluted by Bangkok, the catfish tells that story.

Zeb Hogan is the man behind the Megafish project, taking a global census of the Megafish population around the world. The project, which spans almost every continent, is the subject of the National Geographic documentary Quest For The Megafish. The project is ongoing, but has yet to publish any findings.

Little is said about marine giant fish, probably because of their large habitat and often wide or unpredictable migrations. While sharks typically are the largest marine fish, the Sturgeon family can achieve incredible sizes, including one which weighed half a ton.

The largest-growing fish alive today is the Whale Shark (largest capture is 12.65m), which is herbivorous. The largest fish to ever live was the Megalodon, said to grow up to 30m.

Something about giant fish swimming the murky depths of rivers and oceans is not just a little creepy: a bit It Came from Beneath the SeaBut almost all these animals are strictly herbivorous, and none attack humans (well, the Megalodon might’ve). They are simple massive, beautiful animals. And endangered, too.

Awed visitors of a captive Whale Shark in the Okinawa Churaumi aquarium in Japan.