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Mekong Giant Catfish in the Lower Songkhram River Basin, Thailand

Lower Songkhram River Basin
August 2006

David J.H. Blake, MWBP Technical Advisor

Pangasianodon gigas or Mekong Giant Catfish has a long history of being caught as part of the rainy season fishery in the Lower Songkhram River Basin. Older fishers can recount many tales of catching huge P. gigas and there are written records of annual catches (number and size of individual fish) stretching back to 1951 at Ban Tha Bor village, Sri Songkhram District, Nakhon Phanom.

From 1957 to 1966, 30 individual P. gigas were caught, which decreased steadily to just six fish between 1977 and 1986, but since then the numbers have recovered somewhat, reaching 16 and 14 fish between 1987 – 1996 and 1997 – 2005 respectively. These fish were mostly caught in targeted large-meshed fish nets, which are strung across known P. gigas migratory corridors along floodplain channels. The largest recorded specimen, caught in 1952/1953, weighed 270kg. In 2005, only two specimens were apparently caught in Ban Tha Bor, weighing just 18 and 27kg.

Surprisingly, the fishery still persists to this day at Ban Tha Bor village, with fishermen still laying their nets hopeful of catching a pla beug from the murky waters on the edge of the paa boong paa thaam (seasonally inundated forest) in July and August, when the fish are known to ascend the river and go onto the floodplain. At one time, several tributary streams around Sri Songkhram District were known favoured migratory routes for P. gigas, but since the construction of dams and weirs across these streams and large-scale land use changes, the numbers have declined dramatically.

The only place where villagers from Ban Tha Bor are still known to set nets is on the flooded Huay Sing stream, to the north of the village. Here a massive area of floodplain habitat is accessible to fish, but it is the presence of salt and mineral deposits (din euad) which villagers believe is the reason that the catfish return here each year. In the dry season, these deposits are visible as white salt crusts on the surface, but in the rainy season they disappear under water and become sites of a soft green type of weed (tao), which apparently becomes food for the pla beug. The fish stay for a few weeks feeding and then head off back downstream, during which time they may be snared by a net strung across their return route.

Another mystery about the Songkhram pla beug fishery is whether the fish caught are wild specimens or individuals which have been stocked into the river from a Department of Fishery (DoF) stocking programme (or a mixture of both). It is known that DoF have released many P. gigas, originating from stock in north Thailand, into the Nam Songkhram and Mekong Rivers during recent years, but it is not known where they have followed stocking or the recapture rate.

At the same time it is also clear that only a couple of decades ago there was still a wild population of P. gigas migrating into the Songkhram to feed, and possibly spawn. But trends in numbers suggest that this population may have died out in the late 1980s or possibly 1990s, to be replaced by introduced fish. It has also been reported that there has been some private stocking of P. gigas in a large lakes/reservoir, which floods in the rainy season, allowing the fish to escape into the wider riverine system. Hence, it would be fascinating to use genetic studies to trace the likely source of these Mekong Giant Catfish of the Songkhram River.