Tonle Sap Visit Finds Little Evidence of Once-Abundant Species

© Conservation International/photo by Koulang Chey

Editor's note: CI marine biologist Les Kaufman spends most of his trips to the “field” near, on or in the ocean. But this week in Cambodia’s Tonle Sap region, he’s in for something different. Les is part of a team studying the interactions between one of the world’s largest freshwater fisheries and the people who depend on it. Read the first post from his trip.

It was with great expectations that we set off down a seemingly interminable canal on our first mission out into the “Great Lake” — Tonle Sap. The 15 of us were distributed among three surprisingly stable long boats made for crossing vast, shallow bodies of water.

The channel we were traveling requires constant dredging to keep it open across the miles that separate the current lake levels from the stranded high-water mark — currently dry. On the way out we passed a big hydraulic shovel, now asleep, which would later come to life with Sisyphean digging by the time we found ourselves homeward bound at eventide.

Soon the channel gave way to low scrub and grassland on either side. The scrub was dominated by Mimosa pigra, a thorny shrub native to Central and South America. A highly invasive species, it has carpeted the seasonal lake bed, blocked access to fish and driven some lake residents to seek their livelihoods elsewhere.


The scrub zone soon gave way to a widening wetland woodland of Barringtonia acutangula, a sort of freshwater mangrove and one of four tree species that make up the backbone of Tonle Sap’s seasonally flooded gallery forests. Now in their dry season incarnation, the low trees loomed above rooted aquatic plants and floating water hyacinth, yet another green invader.

At last we found ourselves out in the open lake, where water slid seamlessly into hazy sky. We stopped to take samples. Adrian, a graduate student at the University of Washington, busily set about gathering physical measurements and lake bottom mud as I idly mucked around with my hand in the water. Much of the lake is incredibly shallow; even far from land, in the largest lake in Southeast Asia, we were able to grab benthic samples of shallow-water organisms with our hands.

One mud-mucking unearthed four types of bivalves and a snail. One looked astonishingly marine in character, with a round, highly sculptured shell. Another was smooth and elongate, with one blunt end and the other gracefully drawn out. The shells have stories to tell: these clam species likely draw differently from the lake’s bounty prior to passing it on to the fishes that eat them, and then the people who eat the fishes.

Our next step was to hook up with some fishermen on the open lake who were heading back fully loaded with fresh catch. We came alongside a fishing boat no bigger than our own. Phanara, our colleague from Cambodia Fisheries Research, immediately jumped down into the hold to commence sorting through the catch for species that were on the program’s wanted list.

Meanwhile, I examined the mound of little fishes: blue gouramis, a half-dozen species of small silver carps, bullet pufferfish, barbs, catfish and gobies that latched on to my fingers as I ran them through the pile. Under close scrutiny, what seemed at first to be just a silvery mess of minnows turned into an astonishingly varied community of quite different fishes.

Given that we had shade, ample water, and each other’s company it was pleasant out on the open lake, enjoying a delicious floating lunch of snakehead fish. With some regret, we put away our sampling gear and cameras and made off for the floating village of Prek Toal, and the core biodiversity area that surrounds it.

Up the river we went, crossing an especially species-rich area of exposed (un)flooded forest and lake floor. Riverside tree crowns were heavy with snakebirds (also known as darters or anhingas). Lily-trotters and Mekong wagtails danced about the water hyacinth, while storks and egrets flapped from one fishing hole to the next. Having getting an excellent look around, we headed back out into the open lake, and back up our access canal to find our vehicles waiting for us.

Our group had many revelations on this trip — perhaps some things we’d heard before, but which made a much stronger impact upon seeing them in person. The small fishes we saw were fascinating and diverse; however, of greater importance was what was missing.

Large fishes are very, very scarce due to overexploitation. Two dog-sized giant barbs (perhaps 1.5 meters, or 5 feet, in length) arrived preserved at the fisheries research lab a few days ago; these were found dead, and brought immediately back to the lab. Even these were smaller than usual; maximum length used to be around 2.5 meters (8 feet) or more.

Snakes are another cause for concern. Tonle Sap hosts seven or eight species of rear-fanged (mildly venomous) water snakes. For the last several years, up to 7 million of these per year have been collected to feed crocodiles on farms, increasingly necessary as the price of fish has gone up. I saw not a single snake this time (a few of my colleagues did see some dead ones drying in a village), and was told that this is because they are now gone. Quite frankly, this is astonishing.

Another major realization is that the folks living in Tonle Sap’s floating villages are far from destitute. This may not be a lifestyle attractive to a western urbanite, but by local standards the lake offers many ways to make a reasonable living — or, at least, it had done so in the past.

I have the feeling that we’re only beginning to scratch the surface of a lake crawling with interesting mysteries, challenges and change.

Les Kaufman is a marine conservation fellow with CI and a biology professor with Boston University. Read his previous post, and stay tuned for more here on Human Nature.