Zeb Hogan and I recently visited Thailand and Cambodia to study fish reserves in freshwater systems. “What we’re learning is that, despite being poorly studied, freshwater fish reserves are widespread and may be one of the most effective tools we have to sustain fisheries and aquatic biodiversity,” says Hogan.

This story that I wrote was published by National Geographic on April 25, 2019. You can read it on Nat Geo here.

Ngao River, Thailand — It may run through a region riven by conflict, but western Thailand’s Ngao River, near the troubled border with Myanmar, is a peaceful haven to the more than 50 species of fish living in it. Below Ban Luiy, one of the many villages dotting the hillsides of this meandering river valley, scores of blue mahseer, a type of carp, congregate in crystal clear waters while a group of children splash nearby.

Fishing is not allowed here, or, more specifically, not in the stretches of river set up as protection zones. Ban Luiy was the first village to establish such a reserve 25 years ago, protecting about 1,000 yards of the more than 40-mile-long Ngao River as it runs past the settlement before eventually joining Myanmar’s Salween River, the largest undammed river in Asia. Over time, other villages followed suit, setting up their own fish sanctuaries of varying sizes. Today, there are 52 independently operated fish reserves on the Ngao River and its tributaries.

Five years ago, Aaron Koning, an aquatic ecologist with Cornell University’s Atkinson Center for a Sustainable Future, began studying the effects of those reserves on fish populations. Although river reserves are widespread in Southeast Asia, their impact has rarely been studied in Thailand or elsewhere in the region. Would the reserves, covering only short stretches of the river, really help protect large, migrating fish that may use an entire river system, he wondered.

What he found is that fish, across all species, are in fact very good at taking advantage of the protection that reserves can afford, and, in doing so, they may be able to substantially boost their numbers. A survey of nearly two dozen of the reserves in the Ngao River showed the density of fish to be several times higher on average inside than outside them, and for the river’s large-bodied species—such as the long-whiskered catfish, which can grow six feet long—even more so.

“The fish seem to know where they are likely to be caught,” said Koning. “If you stand near the boundary of a reserve and cast a shadow on fish lingering just outside it, the fish will inevitably go back into the reserve.”

Koning also found that, as space becomes more crowded inside the reserve, smaller fish, which in the Ngao include many typical aquarium species, including glass fish, loaches and danios, will move out of the sanctuaries to avoid being eaten by the larger fish. “Overall, the number of fish in the river today is likely higher than 15 years ago because of the reserves,” he says.

While the benefits of marine protected areas to fish conservation have long been documented, the effects of such reserves in freshwater systems have largely been ignored. Now, researchers are finding that fish in rivers and lakes may benefit from protected zones at least as much as their marine counterparts, and that the reserves can also boost inland fisheries by improving catches, as they’ve been shown to do in marine settings.

Those findings are of particular interest in Southeast Asia, which has some of the world’s richest aquatic biodiversity and strong human dependence on freshwater resources. For example, the Mekong River, which runs through six Asian countries, contains almost 1,000 different species of fish. Its fisheries are essential to the food security of the 60 million people of the Lower Mekong Basin. But many populations of fish in the Mekong and elsewhere in Southeast Asia have plummeted due to a variety of threats, not least overfishing.

“In at-risk regions like the Mekong, effective conservation strategies are hard to come by yet urgently needed to bolster both commercially important and endangered fish stocks,” says Zeb Hogan, a National Geographic explorer and TV host and fish biologist at the University of Nevada, Reno, who heads a USAID research project called Wonders of the Mekong.

Hogan recently traveled through the region to study fish reserves in freshwater systems. “What we’re learning is that, despite being poorly studied, freshwater fish reserves, in their many incarnations, are widespread and may be one of the most effective tools we have to sustain fisheries and aquatic biodiversity,” he says.

The bigger, the better

Marine protected areas have a history dating back more than 100 years. Increased awareness of the threats to the oceans, and the need to better manage biodiversity and fisheries, led to many marine reserves being set up around the world starting in the 1970s. Nearly five percent of the seas worldwide are today protected, with almost half of that space made up of no-take reserves where fishing is not allowed at all.

Far less attention has been given to freshwater protected areas, even though rivers and lakes contain several times more vertebrate species per unit area than land and ocean environments, and are often more degraded. In many places, rivers are part of terrestrial national parks, rather than specifically afforded their own protection.

While most marine fishing is highly commercialized and targeted at specific species, freshwater fisheries in Southeast Asia are often family-based, with fishers using all types of gear and catching many different kinds of fish. As a consequence, freshwater reserves are often informal and far smaller than their marine counterparts. Some, like those on the Ngao River, are managed by local communities without any outside support, while others are run with government authorization.

In land-locked but river-rich Laos, a rewriting of the country’s fishing laws in the 1990s helped spur the establishment of riverine reserves there. According to a survey by the California-based conservation group Fishbio, the country now has more than 1,300 community-based fish sanctuaries recognized by the government. “Having a legal foundation for these reserves has been very important,” says Fishbio’s communications director Erin Loury, who is also a fisheries biologist.

How effective the reserves are for fish conservation in Laos is not entirely clear, however, since no studies of them have yet been conducted there. “We think the regulations are helping, but there is definitely room to look deeper into what makes these protected zones successful,” says Loury.

In the Ngao River, Koning has found that the reserves’ conservation benefits will depend on a range of factors, perhaps the most important being size. “We see that the larger the reserves are, the stronger the benefits to all fish,” he says. Reserves placed closer to each other see more positive effects than others, as do those located nearer village centers, with villagers being able to monitor those reserves more closely, which especially helps larger fish of greater catch value.

The age of the reserves also plays an important role, but perhaps in unexpected ways. “Once a reserve has been established for several years, we actually start to see less of a difference in fish density between the reserve and the surrounding fished areas,” Koning says. “It’s evidence of a potential spill-over effect, which occurs when reserves become crowded over time and fish start to leave.” With increased numbers of fish protected inside reserves, this spillover can provide local fishermen with more large fish to catch right outside their doorsteps, he adds.

Koning’s findings also suggest that reserves provide the most protection for fish in the dry season when water levels are low and fish are at their most vulnerable to being caught. It is something that appears to hold true not only for relatively small rivers like the Ngao, but also very large freshwater systems, as Hogan found out during a recent visit to the Tonle Sap lake in Cambodia.

Fish frenzy

The Tonle Sap is Southeast Asia’s biggest lake and home to the largest inland fishery in the world. Connected to the Mekong through its eponymous river, it operates on a remarkable flood-pulse system. After the annual monsoon rains begin, around June, the lake expands up to six times, to a size substantially larger than the state of Connecticut, before shrinking back down during the dry season.

At least 500,000 tons of fish are drawn from the Tonle Sap each year—more than from all of North America’s rivers and lakes combined—feeding millions of Cambodians. But the intense fishing pressure in the lake has nearly wiped out populations of many medium and large fish, such as the critically endangered Mekong giant catfish, the current record-holder for largest freshwater fish on Earth.

On a blazing hot Saturday morning last month, at the height of the dry season and with water level low, Hogan joined a delegation of Cambodian fisheries officials to visit some of the sanctuaries set up around Tonle Sap. About 15 percent of Tonle Sap’s size during the dry season has been set aside as fish reserves with varying sizes and shapes. Inside them, no fishing is allowed, except for scientific study.

On this day, an experimental net had been set in one of the reserves in the middle of the lake to gauge the fish density there. Passing a wooden structure demarcating the boundary of the reserve, the delegation arrived just as fishermen began to pull up the massive net, which turned out to be chock full of catfish and carp. As the sweat-soaked fishermen strained to pull the net in, the trapped fish churned the water into a frenzied froth.

When a four-foot-long wallago, a type of catfish, suddenly jumped out of the lake and into Hogan’s boat, nearly knocking him over, the head of the Cambodian Fisheries Administration, Eng Chea San, decided to end the trial, worried that it would unintentionally stress or kill the fish.

Later, Hogan said the density of the fish in the reserve “totally blew me away.” That large-bodied fish were being caught showed, he said, “that these sanctuaries have the potential to protect big fish, many of which have disappeared from other areas of the Tonle Sap.”

The reserves can also boost fisheries, according to new research, helping the more than 1.2 million people in the Tonle Sap area who depend on fishing for their livelihoods. A recent study in the journal Ecological Modeling concluded that protecting up to 50 percent of Tonle Sap lake would have big benefits to those fisheries by creating a “blue halo” effect, with biomass built up inside the protected areas that then spills out into the adjacent fished areas.

“It may be counterintuitive that the best way to increase harvest is to restrict fishing, but at very high levels of fishing effort, reducing effort or the area fished will improve both harvest and profit,” says the study’s lead author, Lee Hannah, a senior scientist for climate change biology at Conservation International.

He points out, however, that closing areas to fishing may mean lower catches initially, which can present a huge challenge for subsistence fishers. “Families and communities dependent on fish for survival are in a poverty trap, they can’t afford to fish less now to get more later,” says Hannah, who says governments, NGOs, and donors must help bridge poor families through to better times.

Community rangers

Then there is the issue of enforcement. Keeping illegal fishers out of sanctuaries in a lake as large as the Tonle Sap is almost impossible, officials there say.

Hok Men An, who heads a local law enforcement unit, says poachers use silencers on their boat motors to carry out clandestine incursions into the sanctuaries, mostly at night. When the water is low, the poachers may even be able to wade into a reserve on foot, drop a net for a couple of hours, and then sneak out, he says. In his estimation, officials are only able to stop up to 70 percent of the poaching.

For poachers, the financial incentives are clear: a catch of a 200-pound giant barb, for example, could be worth as much as $10,000. But the punishments for getting caught are also severe, including prison terms of up to five years. In recent years, close to 100 people a year have been convicted and sent to prison for illegal fishing in the Tonle Sap sanctuaries.

On the Ngao River in Thailand, the job of protecting the reserves falls on the villagers who manage them. That, says Koning, can make the job easier. “Instead of having five park rangers patrolling 40 miles of river, you’ve got 2,000 people who are here every day,” he says. “There is a feeling that a certain river section belongs to them, and there is a sense of pride involved in taking care of the fish there.”

Koning is now working on a project to tag blue mahseer and a species of tor, another type of carp, to learn more about the movements of the fish and how they use the conservation areas. Last year, he also started a survey of 75 households to track fish catches in different parts of the river.

For Pannee Phoemchatchai, a 38-year-old woman in Ban Luiy, managing the fish reserve is a divine responsibility. “God created the fish and told humans to look after them,” she says, adding that when she’s feeling down, she can always go down to the river and look at the beautiful fish. “It eases my mind,” she says.