Bycatch reveals patterns of occurrence and post-release mortality of the critically endangered Mekong giant catfish (Pangasianodon gigas): Implications for conservation and threat mitigation

The Mekong giant catfish ( Pangasianodon gigas ) is a large, migratory species endemic to the Mekong Basin. Owing to declining abundance and a shrinking distribution, it is listed as Critically Endangered on the IUCN Red List. Bycatch is a major contributor to the decline of P. gigas and many other threatened freshwater fishes; yet monitoring is difficult, and data are often lacking. We investigated bycatch of P. gigas in the Cambodian Mekong River system. Between October 1999 and January 2025, we documented (i) 132 wild P. gigas caught as bycatch, with high survival (>83%) upon release; (ii) seasonal peaks during the October–January migration; (iii) declining body weight over time, indicating shifts in demographic structure; and (iv) spatial and temporal catch patterns showing broad distribution across the Tonle Sap and Mekong main channels, confirming continued use of historical migration routes. Incidental capture occurs at all life stages, in many locations, particularly between October and January each year, and bycatch is a major source of mortality for the species, alongside other threats. This research underscores the underappreciated importance of bycatch in freshwater ecosystems and highlights the value of integrating fisher participation and knowledge into conservation strategies to balance species protection with sustainable livelihoods. • Mekong giant catfish (Pangasianodon gigas) is a critically endangered flagship species endemic to the Mekong River, whose population trends, distribution, and habitat remain little known • Giant catfish bycatch data were recorded during the last 25 years and show a broad distribution of the species across the Tonle Sap and Mekong main channels, with bycatch peaks during migration season (October–January) • Captures became more abundant in the last years, indicating a possible recovery of the species, but this is mitigated by a decline in the body size of the captured specimens. • Collecting bycatch data is an efficient way to follow population trends and to involve fishermen with conservation actions, with limited detrimental impact on the species (83% of the specimens captured were released to the water after capture)