Fishes that migrate between fresh and marine waters are known as diadromous species. They are among the most vulnerable species to river infrastructure development. These fishes need to move between fresh water and the sea to complete their lifecycles, so barriers to migration can block access to critical habitats. The Lower Mekong Basin is undergoing an unprecedented boom in river development, with many dams and irrigation schemes being installed. General patterns of fish migration are known in the basin. But there’s been relatively little information on diadromous fish migration so the impacts of river development on their populations are probably underestimated.

This has now changed, thanks to a study led by biologist Vu Vi An of the Fisheries, Ecology and Aquatic Resources Division of the Research Institute for Aquaculture No 2 in Ho Chi Minh City, part of the Vietnamese Ministry of Agriculture and Rural Development. An and his colleagues reviewed information for over one thousand Mekong fish species including from the Mekong River Commission’s fish database. They found that diadromy is likely to be a more common life history trait in the Lower Mekong than previously suspected.

Sixty-one of the Mekong fish species assessed showed diadromous traits. Of these, it seemed that 9 were anadromous (growing at sea and breeding in freshwater), 8 were catadromous (growing in freshwater and breeding at sea) and 44 were amphidromous (migrating between freshwater and the sea or vice versa at some stage of their life cycle but not for breeding).

Gobies (Gobiidae) accounted for 20 of the species. Seven other families each accounted for 1 to 6 species. These were bully sleepers (Eleotridae), anchovies (Engraulidae), sea catfishes (Ariidae), herrings (Clupeidae), mullets (Mugilidae), shark catfishes (Pangasiidae) and lates perches (Latidae). Most of the 61 species were found in the Mekong Delta in Vietnam and Cambodia (within 700 km of the sea). But 8 were found in Laos and 6 in Thailand. The 61 fishes were mainly recorded in the Mekong mainstream but also found in tributaries.

Only one species – Krempf’s catfish (Pangasius krempfi), known as “trey bong lau” in Khmer – has been confirmed as anadromous in the Mekong (by microchemistry). Other anadromous species were “implied” – based on local knowledge or confirmed in other rivers. Freshwater and marine amphidromy could not be distinguished so these were all grouped together. Freshwater amphidromous adult fishes grow and spawn in freshwater with juveniles growing at sea. Marine amphidromous fishes do the opposite – the adults grow and spawn at sea with juveniles growing in freshwater.

In addition to the 61 diadromous fishes, the study found 119 euryhaline species that can tolerate a wide range of salinity. These non-diadromous “wanderers” move occasionally between freshwater and brackish or marine environments, probably for feeding. But migrations by freshwater or marine wanderers are not regular. Nor do they mainly involve most of their populations. Other fishes in the Mekong are classified as potamodromous – migrating exclusively in freshwater.

The study noted that the Mekong River Commission’s Regional Fish Abundance and Diversity Monitoring Programme had recorded 25 of the 61 species between 2007 and 2012. These accounted for about 3 percent of the catch of 11 tonnes during the five-year period, of which 2.8 percent was from the Mekong Delta in Viet Nam. In the delta, catches of anadromous species were higher in the dry season. But in countries upstream, catches were bigger in the wet season. In all countries, catadromous and amphidromous catches were likely to be higher in the dry season.

Some diadromous fishes are of extremely high value. The giant mottled eel (Anguilla marmorata), a catadromous species that can reach 70 cm, can fetch as much as $32 per kg. The eel, known as “trey chhlok” in Khmer, spawns in the ocean and has also been found in the Khone Falls area on the Cambodia-Lao border. The anadromous Krempf’s catfish has a first-sale price of $10 to $17 per kg. This catfish can reach around 80 cm. Krempf’s catfish inhabits the Mekong estuary and adjacent sea in Vietnam and swims upstream at least 720 km to spawn in the Khone Falls area in Laos. But they have also been reported in catches during the early wet season as far upstream as upper Laos, some 1,500 km from the sea.

These two species are particularly preferred for consumption even at much higher prices. Local fishers often sell high-value fishes to traders who deliver the fish directly to consumers and restaurants to earn more income. So diadromous species make significant contributions to livelihoods and income generation across the Lower Mekong.

The study notes that river infrastructure like dams, weirs and pumps and floodplain developments for agriculture have had significant impacts on the abundance of diadromous fishes in sub-tropical or tropical rivers. These include the hilsa shad (Tenualosa ilisha), a highly prized anadromous species across the Indian sub-continent. In the Pearl River in China, the anadromous Chinese sturgeon (Acipenser sinensis) is now critically endangered. In Australia, several diadromous fish species have declined and are now likely extinct above river barriers.

Findings of the study are considered the “first step” to investigate the diadromous status of many fish species in the Mekong. But considerable work remains, especially for species tentatively assigned as diadromous that require scientific tools like microchemistry, tagging and telemetry to validate their diadromy.

“We found diadromy appears to be more common than previously assumed in the Mekong River,” the authors conclude. “Some of the diadromous fish are economically important and are distributed among all Mekong countries. Many of these species are known to migrate relatively long distances, connecting with the Mekong estuary and the sea to complete their life cycles. River development will impact them. They need to be considered in holistic ecosystem planning to pre- vent declines, whilst also meeting food and energy demands in the region.”

Source: Agency Kampuchea Press

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