On heavily dammed Mekong, tracking study tries to find where the fish are going (2024)

Key migratory fish species travel hundreds of kilometers along the Mekong River, including through several active and planned hydropower dam projects in Laos, a recent

Researchers outfitted hundreds of fish with tracking tags known as passive integrated transponders (PIT) before releasing them in the Laotian capital, Vientiane. Aimed at developing a better, data-driven understanding of fish migrations in the Mekong, the resulting study, published this year, also found empirical evidence that at least some fish were able to make use of the fish ladder at the controversial Xayaburi Dam in northern Laos, a pathway previously considered untested.

But of the hundreds of fish tagged, only five made it up the ladder.

The study called for continued research to better understand “the migration patterns of fish in the face of upcoming developments” and cautioned that government plans to build yet more dams on the mainstream of the Mekong may turn this already treacherous fish migration into an impossible odyssey.

“It is highly likely that [key migratory species] will be blocked from continuing long distance migrations, and subsequently completing their life-cycles, if planned [hydropower projects] are constructed without suitable fish passage measures,” the study says.

Over more than a month in 2022, fishers in Vientiane captured hundreds of fish for the study. More than 230 individual fish were tagged and released, most of them gold tinfoil barb (Hypsibarbus malcolmi) and sharp-nosed catfish (Pangasius conchophilus).

The two species are considered economically important because they’re among the top contributors to the total catch weight across Lower Mekong Basin countries, according to a study by the Mekong River Commission (MRC), an intergovernmental organization tasked with promoting cooperative development of the waterway.

Despite what the study referred to as the “obvious risk to mainstem Mekong fish migrations from hydropower development,” the authors noted past research that underlines how “limited” and “deficient” this type of empirical data collection on fish migrations remains in the Lower Mekong Basin.

The report advocated for the integration of PIT tagging systems in the fish passages throughout the lower basin’s dams to create a standardized monitoring method for gold tinfoil barb and other key fish species.

This would also allow for standardized data collection across several hundred kilometers of the Mekong, instead of individual data points from the fish passages of specific projects.

The researchers, who didn’t respond to Mongabay’s requests for comment, used their PIT tag data to map a nearly 360-kilometer (220-mile) fish migration that passed one current and three proposed hydropower dams in Laos.

“More studies like these need to be done. Tagging is now proving to be an effective way of understanding how fish move through the Mekong system,” Brian Eyler, author of the book Last Days of the Mighty Mekong and co-lead of the Mekong Dam Monitor, wrote in an email to Mongabay. “But fish ladders aren’t a panacea, rather they are a limited solution which could preserve species but do little to help the tens of millions of people who rely on robust fish catches from the Mekong for their livelihoods.”

Fifteen months after the release in April 2022, five of the 77 tagged gold tinfoil barbs were detected at the top of the fish ladder at Xayaburi Dam.

During their migration from Vientiane to Xayaburi, these five barbs passed the planned Pak Chom, Sanakham and Pak Lay hydropower projects. If these barbs continued north after topping the Xayaburi dam’s fish ladder, they would soon pass the construction site of another hydropower project, in Luang Prabang.

“The team established that some fish can indeed climb the fish ladder which is good but limited news. What needs to be demonstrated is the range of fish species that can successfully climb that fish ladder and the mass of fish that make it through,” Eyler said. “Also, importantly, fish need to move both up and down through dam structures in order for their population to survive. The study does not demonstrate that fish are able to move in both directions through the dam. So, more evidence is needed to demonstrate the success of the Xayaburi fish ladder.”

“Now consider three dams below Xayaburi,” Eyler continued, hypothesizing that not all of the five tagged fish would have made it to the top of the ladder if the other dams had been active. Dams have a cumulative effect on fish migrations, he says, “even if one or each dam individually has a relatively high success rate.”

The MRC has issued reports on design guidance for mainstream dams and fish passages in recent years. The river commission told Mongabay that “it’s critical to thoroughly assess the potential impacts of dam construction on fish migrations and incorporate measures to mitigate any adverse effects.”

The commission is a conduit for dialogue among its signatory countries of Laos, Thailand, Cambodia and Vietnam. The final decisions on dams and their designs fall to the national governments.

While the MRC notes that other factors, like climate change, affect the water flow of the Lower Mekong Basin — and thus fish migration pathways — it said “the effect of climate change on fisheries in the LMB is relatively minor compared to the impacts caused by human activities, such as building dams and other water infrastructure.”

“While hydropower plays a role in economic development, balancing it with the need to preserve the Mekong River’s ecological integrity and sustainable livelihoods is essential,” the river commission told Mongabay.

Banner image: A fisherman feeds his net into the Mekong River on the Thai-Lao border by Chiang Saen. Image by Anton L. Delgado.

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Citations:

Robinson, W., Baumgartner, L. J., Homsombath, K., Ning, N., Phommachanh, K., Phommavong, T., … Vorasane, P. (2024). PIT tagging systems are suitable for assessing cumulative impacts of Mekong River hydropower plants on (upstream) fish migrations in Lao PDR. Fisheries Research, 274, 106995. doi:10.1016/j.fishres.2024.106995

Vu, A. V., Baumgartner, L. J., Mallen‐Cooper, M., Doran, G. S., Limburg, K. E., Gillanders, B. M., … Cowx, I. G. (2022). Diverse migration tactics of fishes within the large tropical Mekong River system. Fisheries Management and Ecology, 29(5), 708-723. doi:10.1111/fme.12566