Mekong giant catfish

This giant fish eats plants!

It eats algae, the green fuzzy stuff that grows on rocks under water. It finds its food on the river bottom.

For such a giant, the Mekong giant catfish has a gentle diet. Grown-ups feed on algae and detritus (bits of dead plants and other matter) on the bottom of the river. In the river they eat only plant food.

Baby giant catfish are different. Young fish have teeth, but they lose them by the time they are about 30 to 50 centimeters long. In captivity, giant catfish will take other kinds of food.

FishBase reports that adults feed on detritus and algae on the river bottom, and that in the river the species feeds only on vegetation, though it takes other food in captivity.

The diet matches its anatomy. Oral teeth and gill rakers are present in small juveniles but disappear at about 30 to 50 centimeters standard length, and the gill rakers of adults are rudimentary or absent. The jaw barbels of juveniles are also reduced to rudiments in subadults and adults.

Giant catfish are travelers.

They swim a long way up the river each year. Fast-flowing parts of the river are where they lay their eggs.

Mekong giant catfish are migratory: they travel along the river at different times of year. Fishers in northern Thailand once caught them each year as they swam upstream in the breeding season, and the fish were caught for their eggs, called roe.

The places where they breed have strong currents. That matters, because the giant catfish needs a river it can swim along freely, from its deep pools to its spawning grounds.

The species is migratory (potamodromous: moving within fresh water), but its migration routes are poorly known. Catch records suggest movement from deep pools of the lower Mekong upstream into northeastern Cambodia and beyond. Previously known spawning sites lie between Loei and Nong Khai provinces and in Ubon Ratchathani, Thailand; the Kok River near Chiang Saen is a possible site that still needs confirmation.

The IUCN notes that spawning areas have relatively strong currents, which keep excessive sediment from settling, and that the loss of migratory routes through dam building may reduce numbers. Fish were historically targeted during the spawning season in Thailand and Laos for their roe.

There are very few giant catfish left.

People caught too many of them. Now it is against the law to catch them in some countries, and people are working to keep the river healthy.

The Mekong giant catfish is critically endangered. In the early 1900s it was common, but by the 1970s fishers were reporting that it was disappearing. Scientists estimate its numbers fell by more than 80 percent between 1990 and 2011.

The main danger has been overfishing. Dams that block migration routes and damage to its habitat are big threats too. Today it is protected in Cambodia, Laos and Thailand, and international trade in it has been banned under CITES since 1975.

The IUCN assessed P. gigas as Critically Endangered (A4) on 13 April 2011, an assessment marked as needing updating; earlier assessments listed it as Vulnerable (1986 to 1994), Endangered (1996) and Critically Endangered (2003). From catch records, the assessors estimated a decline of more than 80 percent since 1990: Mekong-wide catches fell from about 40 to 50 fish a year in the 1990s to about 5 to 8 by 2003.

Overfishing is the main past threat; mainstream dams were judged the major future threat, along with habitat loss and genetic mixing with cultured stocks. The species has been on CITES Appendix I since 1975. It is illegal to capture, sell or transport it in Cambodia, Thailand regulates fishing with a yearly quota of fewer than 20 catches (and in 2010 the quota was zero), and Laos protects it by law, though it is still fished there. Between 2000 and 2003 about 10,000 captive-bred fish were released into the Mekong; releases now go only to Thai reservoirs, where there is no evidence of self-sustaining populations.