The solution, according to Touch Seang Tana, was to try to radically change the economy of the river to make dolphins more valuable alive than dead.
Alternate means of livelihood would be introduced to villages along the river to take advantage of a booming tourism sector that has already benefited other parts of the country.
"My idea is ... to try to get poor fishermen to change over to tourism," Touch Seang Tana said. "I give them tour boats" to bring visitors to see the dolphins.
Reducing villagers' dependence on fishing is hoped to also see a drop in the use of gillnets.
Cheap and easy to use, gillnets are as efficient a killer of dolphins as they are of fish, said Touch Seang Tana, a marine scientist by training who blames this now-illegal fishing method for 99 percent of dolphin deaths.
But getting local fishermen to risk their livelihoods for an animal that is of little value commercially or as food has been hard, he explained.
"I tried to invite them to meetings to explain our actions, even tried to pay them money
... . But they didn't come, they sent their pregnant wives," he said.
"We decided then that we'll confiscate (fishing gear) and then they came. They came with knives, they wanted to kill the river guards," he added.
"They want to kill all of the dolphins because they are keeping people from making a living."
Since the introduction of the river guards, however, illegal net fishing, along with the use of explosives or electrical charges to catch fish - practices that also inadvertently kill dolphins - have dropped, said guard Em Pheap.
"Before it was a big problem, but now people are understanding more about this," he said.
The WWF's Zanre said that while the number of dying dolphin calves is still dangerously high, "adult dolphin mortalities have declined" as a result of conservation.
Even before the Discovery Trail becomes a reality, its successes are evident in Kampi, where dolphins have become the local industry.
Aside from the boats lining up for sightseers, nearly every house along the narrow tree-lined road shadowing the river hawks tiny dolphin carvings.
"Everyone earns the money, even the children, from dolphins. You can see their livelihood has changed - you can see televisions in houses, some people even have motorcycles," Touch Seang Tana said.
"The dolphin is so important. I tell them 'The dolphin is everything for you' and now they can see that," he added.
"Its future is their future."
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