New studies are providing fisheries scientists with the first glimpses of life in some of the Mekong River’s deepest pools, a barely traced underwater topography that is believed to be home to seven species of giant fish.
Using hydro-acoustic technology, which works on the same principles as the echo-sounding sonar used on submarines, scientists conducted a survey of 14 pools in a 60-km stretch of the river in Stung Treng province.
The survey, undertaken by the Inland Fisheries Research and Development Institute in Phnom Penh, along with Laotian and Norwegian colleagues, revealed that fish are most abundant in depths of about 30 meters, according to a new report by the Mekong River Commission.
Below this depth, numbers of river inhabitants plummet until, surprisingly, the numbers start to rise again at depths greater than 60 meters, scientists said.
“The reason why some fish congregate in the deepest pools is not known,” wrote Chris Barlow, a fisheries program manager with the MRC, in an e-mail on Wednesday.
Pulsing sound waves travel differently through fish than through water because a fish’s air-filled swim bladder is of a different density. With the reflected sound technology, scientists can detect shoals of fish, but they cannot determine what type of fish inhabit the deepest recesses of the river, which is home to the dwindling Irrawaddy dolphin.
“Unfortunately, the equipment cannot be used to identify the species of fish, although there is some evidence that the fish living there are fairly large,” Barlow wrote.
“Some of the deepest pools are beyond the range of most legal fishing gears. It is therefore difficult to know what fish live there.”
What is known is that the deep pools provide important dry-season refuges for large species, such as the critically endangered giant Mekong catfish.
Scientists with the MRC—an organization currently based in Laos and established to promote sustainable development in the Mekong River Basin, which is one of the greatest freshwater biodiversities in the world—said the echo-sounding survey was one of the first trials of this type of equipment in a tropical river.
Until recently, the MRC said that much of what fisheries scientists knew about the ecology and fauna of the Mekong’s deep pools came from monitoring catches and interviewing fishermen.
“A lot more [detailed] work is needed to understand the ecology of the pools,” Barlow said.
(Additional reporting by Prak Chan Thul)
