Usually when the University of Nevada, Reno fish biologist—and host of the Nat Geo Wild TV show “Monster Fish”—goes out on the river, he’s in search of aquatic giants. This time he has his sights set on much smaller quarry: the billions—or more likely trillions—of microscopic fish that are swept down the Mekong River during the monsoon season. Some of them are in steep decline, and Hogan wants to help scientists find out why.
What could be one of the most mysterious and least-studied mass movement of river fish in the world begins hundreds of miles upstream, in the deep pools and rugged river stretches of Laos and northern Cambodia, where soon after the rains set in, all kinds of different fish begin to spawn. As the Mekong transforms into a rushing, sediment-filled monster, it sends massive numbers of larvae and baby fish with it downstream.
Moving unseen in the dark and mud, the tiny fish disperse with the floodwaters that spread over large parts of Cambodia and Vietnam. Over time, a fraction of them will survive to grow into full-size edible fish, powering what is the largest inland fishery in the world with a quarter of global freshwater catches.
“This dispersal of baby fish underpins one of the most vibrant ecosystems on Earth,” says Hogan, who leads a USAID research project called Wonders of the Mekong. “Without this natural phenomenon, this incredibly productive harvest of the Mekong River would not be possible.”
Amid worrying signs that the numbers of certain types of fish, including striped catfish and Mekong giant catfish, are declining dramatically, Hogan is studying and documenting the dispersal together with Thach Phanara, head of laboratories at Cambodia’s Inland Fisheries Research and Development Institute, who has been collecting samples of the tiny fish since 2000.
This morning, they set off from Caong Caaoy, a small fishing settlement on the edge of Phnom Penh whose residents live in floating houses that have been pushed up against a concrete wall by the rising river. From here it is a short skip out on the Mekong to where a net has been laid to catch the baby fish.
The site is close to where the Tonle Sap River connects with the Mekong, and where a strange hydrological event occurs. For most of the year, the Tonle Sap runs south. But during the summer rains, floodwaters from the Mekong push up the Tonle Sap, reversing its course, something only a handful of rivers worldwide do. Some of the tiny fish travel up the Tonle Sap and into Tonle Sap Lake, Southeast Asia’s largest, while others continue down the Mekong.
“This is the best place to collect the fish before they go in different directions,” says Phanara.
To catch them, the researchers use a bongo net, a cone-shaped net with a square-meter-wide opening that is only allowed for scientific purposes. It has a mesh so fine that it catches everything that floats into it.
With the help of a fisherman, Phanara pulls in the net, which has been sitting 20 feet down at the bottom of the turbid river. Collected in a cannister tied to the end of the net is a primordial soup of liquid life. But it is not until the contents are emptied onto a large tray that the researchers can see what it contains: hundreds of minuscule fish—some juveniles a centimeter or two long, others little more than a speck.
It may seem like a good haul, especially since the net has only been in the water for half an hour. But it is nothing compared to the numbers collected during peaks in the flood pulse, when a catch may contain as many as a million baby fish. Such peaks usually occur—for reasons scientists are not sure about—for a day or two a few times a season. But this summer, no peak has been observed.
“I’m very curious to find out why that is,” says Phanara.
Bug Eyes and Long Whiskers
The diversity of fish being caught is still impressive. “There must be at least 20 different kinds of fish here,” says Hogan, as Phanara confidently identifies species after species of fish that to the untrained eye look like little more than goo.
In all, more than 330 fish species have been caught in Phanara’s bongo net at this single site, an astonishing number, even considering that the Mekong River basin has the second-highest biodiversity of any river in the world, after the Amazon.
Samples are taken to a laboratory for further identification and analysis, a painstaking process that can take half a day for one specimen. Under the microscope, the strange features of many of the tiny fish—with all manners of mouths, bug eyes, and long whiskers—come into sharper view.
Of particular concern to the researchers is the striped catfish, a large-bodied, migratory fish that spawns somewhere in the upper ranges of Cambodia and was once abundant throughout the lower Mekong River basin. Decades of overharvesting of the fish, including eggs and fry, may have caused catches in some areas to plummet by as much as 99 percent. In 2011, the striped catfish was listed as endangered on the IUCN Red List of Threatened Species.
In the catch this morning, the researchers find only a single river catfish specimen, smaller than a pea.
Other catfish species are even rarer. So far this year, not a single specimen has been found of the critically endangered Mekong giant catfish, the record-holder for world’s largest freshwater fish at 646 pounds (293 kilos).
“The decline of Mekong giant catfish young may mean that spawning adults are now so rare that they have trouble finding each other to spawn,” says Hogan. “Or it could signal a disruption of environmental cues or migration pathways necessary for reproduction. Either way, without juvenile fish, you can’t get adults, let alone record breakers.”
While it is extremely difficult to determine the exact numbers of tiny fish dispersing downstream—with little work having been done on the subject—a downward trend likely started several decades ago with fish fry being targeted directly.
In the early 1980s, Vietnamese fishers operating in Cambodia began collecting fry of the striped catfish, known locally as trey pra, using bag nets of mosquito netting. The practice was outlawed in Cambodia in 1994, but continued illegally for years. A study done in the late 1990s of the fry fishery in the Mekong River showed that that annual catch of trey pra fry declined by more than 85 percent between 1994 and 1997, from 355 tons to 43 tons.
Although fishers no longer target the fry, with extremely fine-mesh nets banned, the volume of larvae and baby fish moving downstream appears to have continued to decline. Since he began collecting samples 18 years ago, Phanara believes—based in part on data provided by the Mekong River Commission for the last couple of years—that his bongo net catch has gone down by 60 to 70 percent, and even more when it comes to the catfish species.
“It’s clear that there is a very big problem,” he says.
Hogan believes several factors may be at play, including overharvesting of broodstock, habitat fragmentation caused in part by new dams blocking passage for migrating fish, and possibly climate change, which could be disrupting the dispersal peaks.
“We know that only a small percentage of the baby fish will survive, so it’s very troubling if the density is declining as much as we fear it is,” says Hogan. “It threatens the existence of the river catfish fishery that was once one of the most productive in Cambodia and the food that was a Cambodian staple.”
National Fish Day
As part of the project, Hogan and his team also collect live larvae and juveniles of endangered fish for conservation purposes, stocking them in ponds at a research station outside Phnom Penh. There, the young fish are studied for growth and reared to be released back into protected watersheds.
The first such release took place on Cambodia’s National Fish Day on July 1, when hundreds of people gathered near the temples of Angkor Wat to observe the release of a wide variety of fish back into the rivers.
There are now plans to make the release of the endangered catfish an annual event, and next year Hogan hopes to be able to put one special fish back into the wild: a Mekong giant catfish reared from the one baby specimen that the researchers found of the species during last year’s dispersal down the Mekong.
So far, the fish, which has been named “Wonder,” has grown to be a foot long; if it survives to adulthood, it could grow another 8 feet.