Mekong: Giant salmon carp, thought extinct since 2005, rediscovered

A study recently published in the journal Biological Conservation documents the rediscovery of the giant salmon carp (Aaptosyax grypus) in the Mekong river system. The species had been considered possibly extinct since 2005 because no confirmed records were available for a period of 15 years. Between 2020 and 2023, however, three adult specimens were caught in Cambodia and clearly identified.

The giant salmon carp, a large predatory fish, was first scientifically described in 1991. With a length of up to 130 centimeters and a weight of 30 kilograms, it is one of the most impressive freshwater species in the region. Characteristic features include a notch in the upper jaw and an adipose eyelid that covers much of the eye.

How the rediscovery succeeded

Historical observations were limited to a few sightings in the Mekong river system. From 2005 onward, there were no confirmed sightings at all, and the International Union for Conservation of Nature (IUCN) has listed the species since 2011 as “critically endangered,” with a declining population trend.

The rediscovery of the fish species was made possible through cooperation between scientists and local fishers. All three individuals caught between 2020 and 2023 were documented in Cambodia’s Stung Treng Province. In October 2020, one specimen was recorded in the lower Sesan Reservoir, a tributary of the Mekong. Two further catches followed in June 2022, also in the Sesan Reservoir, and in October 2023 in the Mekong River near Siem Bok.

The catches from 2022 and 2023 were examined and preserved by researchers at the Inland Fisheries Research and Development Institute (IFReDI), while the 2020 specimen was identified from photographs. These records extend the known range of the species, which had previously been centered mainly in Laos and Thailand, to the Cambodian Mekong and its tributaries. They also confirm that the giant salmon carp still exists despite decades of uncertainty and has survived in parts of its habitat.

Reasons for the decline of the giant salmon carp

According to the IUCN, the population of the salmon carp has declined by more than 90 percent over the past ten years. This decline is the result of a combination of human impacts and a lack of conservation measures.

One key factor is overfishing. Because of its large size and rapid growth, the species was a sought-after food fish in the region. In particular, the use of modern fishing methods such as gill nets has greatly reduced the population. Catching adult animals not only reduces the number of breeding individuals but also impairs the long-term stability of the population.

Habitat destruction caused by dam construction along the Mekong has interrupted the migration routes that are crucial for the species’ reproduction. Barriers hinder fish migrations that were once possible over great distances and have severely restricted the range of the species. Migrations that once extended for kilometers upstream are now often blocked by insurmountable barriers.

In addition, water pollution from industrial wastewater and agricultural chemicals has significantly worsened the living conditions for the giant salmon carp. Juvenile fish in particular are sensitive to such stressors, which reduces their survival and reproductive success.

The lack of conservation measures has intensified these threats. Monitoring programs and targeted initiatives were missing, so the decline was not recognized early enough and countered effectively. For a long time, the giant salmon carp was neither studied sufficiently from a scientific perspective nor included in conservation strategies—the fact that the species had been considered possibly extinct for a period of 15 years contributed to this situation.

The combination of these factors has placed the giant salmon carp in an extremely critical situation. The rediscovered specimens show that it has not yet been wiped out and provide grounds for hope that targeted protection measures could stabilize and possibly restore the population.

Conservation measures for the giant salmon carp

The scientists behind the study recommend several measures to ensure the long-term protection of the giant salmon carp. First, they propose the use of environmental DNA to systematically record the species’ distribution and habitat use. This could be complemented by telemetry studies and mark-based methods such as “capture-mark-recapture” to analyze movement patterns and population sizes.

The involvement of local communities plays a central role. Through citizen science, fishers and residents can actively participate in documenting sightings and habitats. Incentives for the live release of caught specimens could encourage participation and strengthen awareness of conservation.

In addition, the researchers emphasize the importance of protected areas in core habitats such as deep pools and rapids. These should be developed in cooperation with local communities so that the needs of the population are taken into account. Since the Mekong flows through several countries, transnational cooperation between Cambodia, Laos, and Thailand is essential.

Finally, a long-term monitoring program is recommended in order to understand the species’ population dynamics and ecological role. These measures could protect not only the giant salmon carp, but also promote biodiversity in the Mekong.

Significance for the Mekong and beyond

The giant salmon carp is of far-reaching importance for the ecology of the Mekong river system. As a large predatory fish, it plays a central role in the food chain and contributes to regulating ecological balance. Its presence and status are also regarded as an indicator of the general health of the river system. The absence of this species over a longer period would not only have meant the loss of a unique species, but would also have pointed to profound disruption in the ecosystem.

The protection measures required to conserve the giant salmon carp could have effects far beyond this one species through the so-called “umbrella effect.” Measures such as protecting core habitats, controlling overfishing, and improving water quality would also benefit other threatened species in the Mekong. These include, among others, the Mekong giant catfish (Pangasianodon gigas) and the giant freshwater stingray (Urogymnus polylepis), which face similar threats.

The rediscovery of the giant salmon carp shows that it is not yet too late to save threatened species. It underscores the urgency of coordinated action. Failing to act now could mean not only the extinction of this fish species, but also the loss of a significant part of the aquatic biodiversity of the Mekong river system. Success in protecting the giant salmon carp could serve as a model for similar efforts worldwide and help raise awareness of the value and fragility of freshwater ecosystems.

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Source

• Bunyeth Chan, Zeb S. Hogan, Michael I. Grant, Samol Chhuoy, Chheana Chhut, Kong Heng, Sébastien Brosse, Rediscovery and future approaches to conservation of the elusive giant salmon carp Aaptosyax grypus, a Critically Endangered megafish in the Mekong, Biological Conservation, Volume 299, 2024, 110814, ISSN 0006-3207, https://doi.org/10.1016/j.biocon.2024.110814.