Return of the titans: 1,000 giant fish poured into Tonle Sap Lake

Zeb Hogan, a US research biologist at the University of Nevada, Reno, said the move was “first step in an effort to restore populations of the Mekong’s largest freshwater fishes”, according to a March 4 press release from Wonders of the Mekong, a project funded by the US Agency for International Development (USAID).

“Fish reserves have been shown to be an effective tool to protect aquatic biodiversity and boost fish biomass. It’s one action, of many that are needed, to bring these species back from the brink of extinction,” said Hogan, who is also lead researcher for Wonders of the Mekong.

Among the species to be released are the critically endangered Mekong giant catfish (Pangasianodon gigas), a catch of which currently holds the record for the world’s largest freshwater fish at 293kg, although French marine biologist Daniel Pauly and other experts have claimed that it can weigh as much as 350kg.

Also represented is the endangered iridescent shark (Pangasianodon hypophthalmus), which is not a shark in spite of its name. The shark catfish species was once a regional staple food, but its numbers have drastically dropped over the years.

Then there’s the critically endangered giant barb (Catlocarpio siamensis), the world’s largest carp species. It was designated as the Kingdom’s national fish in March 2005 by Royal Decree No NS/RKT/0305/149 and is protected by Article 18, Chapter 2 of Royal Decree 33 on Management of the Fisheries Sector, adopted in March 1987.

“More than 1,000 individual fish will be released into a government-operated fish reserve, former fishing lot No 4 in the Tonle Sap Lake, a short drive and boat ride from the city of Siem Reap and the famous temples of Angkor Wat,” the release said.

Ranging in size from 0.3-1.6m, most of the fish are juveniles that have been raised by the Fisheries Administration (FiA), in partnership with Wonders of the Mekong. “By tagging the fish before their release, researchers will have a unique opportunity to study the animals’ survival, growth, and movement,” it said.

Ngor Peng Bun, fish ecologist and Dean of Faculty of Fisheries Science at the Royal University of Agriculture, said in the release: “The purpose of this event was to reintroduce captive-reared endangered fishes back to the wild and track their fate.

“We need to better understand the effectiveness of fish reserves as a refuge for threatened fish; this release today will inform future conservation practices, and help us understand whether such methods are effective in supporting the restoration of these species’ wild populations,” he said.

The release added that as Southeast Asia’s largest lake by surface area – but not by volume – and home to more than 300 species of fish, the Tonle Sap Lake “is a hotspot for freshwater biodiversity and a UNESCO Biosphere Reserve”.

“Historically, it has served as a crucial nursery ground for endangered giant fish and many other migratory fish populations in the Mekong River basin. In recent years, these fish have come under increasing threat from dam-building upstream, overfishing, and drought.

“To combat these threats to fish populations, a series of government-run fish sanctuaries and community conservation areas had been established in the Tonle Sap Lake, forming one of the largest networks of aquatic conservation zones in the world,” it said.

FiA director-general Poum Sotha said in the release: “The Cambodian government has taken action to establish fish sanctuaries, protect core areas of the Tonle Sap Biosphere Reserve, and formalise community fisheries and associated reserves – these actions made Cambodia an ideal place to initiate restoration of the Mekong’s endangered fish stocks.

“Today is part of a multi-year effort to test the efficacy of using Cambodia’s large network fish reserves for the reintroduction of captive-reared endangered fishes into the wild. The ultimate goal is to protect fish until they grow large enough to reproduce, to support fisheries and biodiversity,” he said.