Diversity in migration patterns of selected Mekong River fishes

Inland fisheries in tropical rivers play an important role for food security and livelihoods for local people, but are threatened by river development such as hydropower and irrigated agriculture. The capture fisheries depend heavily on migratory species. However, knowledge on migrations of Mekong fishes has been generated largely by local fisher interviews, and the basic ecology of very few species is known. My thesis sought to apply innovative approaches to help increase this understanding for better management and conservation outcomes. I initially reviewed available information for all Mekong fish species and found that 61 diadromous and putatively diadromous fish species in the Lower Mekong Basin (migrating regularly between fresh and marine waters). Of which, nine species are anadromous (growing at sea, then migrating to fresh water for reproduction); eight species are catadromous (growing in fresh water then migrating to the sea for reproduction); and 44 species are amphidromous (adult growth and reproduction in fresh water, with juvenile growth in the estuary/sea or vice versa). These classifications were simply based on knowledge from a range of sources that are largely anecdotal (FishBase, expert knowledge and fisher interviews), but were not based on strong scientific evidence. To validate these classifications, I collected specimens of some of these species from the wild to determine their migratory needs using more robust approaches. Otolith microchemistry was used to reconstruct life-histories of Mekong fish species in my study. This approach requires understanding the relationships between elemental concentrations in ambient water and fish otoliths. Therefore, I examined the spatial and temporal variation of water chemistry over 2,000 km along the length of the main Mekong River channel including variation among arms in the complex Delta region. I then reconstructed the life-histories of 10 Mekong fish species (across 245 individuals) using an otolith chemistry approach to examine variation of life histories among and within species. My results showed that life histories of Mekong fish species are highly different among species such as anadromy (Pangasius mekongensis and Pangasius krempfi), catadromy (Anguilla marmorata and Pisodonophis boro), potamodromy (Pangasius elongatus and Pangasius bocourti), and estuarine species (Lates calcarifer). Other three species (Plotosus canius; Polynemus melanochir; and Hilsa kelee) unlikely fit existing migration categories. In fact, these species had many individuals displaying multiple migratory strategies. Most species did not conform to a single migration strategy as previous thought, but they exhibited multiple migration patterns within the same species. Two Mekong fish species (Plotosus canius and Polynemus melanochir) showed highly flexible migration patterns. Both migratory individuals (with multiple migration patterns) and resident individuals (either in both fresh or marine waters) co-exist in the same species. Fish body condition, body length, and migration distance are important causes of facultative migration. My findings show that migration strategies are diverse and more common than previously assumed, with greater implications for management in the Mekong River. Management, conservation, and other applications such as fishway design should consider all the flexibility of fish migrations for better outcomes.