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The endangered striped catfish, Pangasianodon hypophthalmus is a large-bodied, migratory catfish, and one of the most cultured catfishes in the world. This study examines the otoliths of drifting wild-caught and hatchery-raised larvae of striped catfish to validate daily ring counts, calculate fish age, and estimate spawning time and spawning location to improve our understanding of the species’ life history. We conducted daily sampling of larval/juvenile striped catfish in the Mekong mainstream in Phnom Penh in 2015 and at a fish hatchery in Cambodia. Daily water levels and river distances at and between four hydrological gauges over the Mekong River reach between Phnom Penh and Cambodia-Lao border were also collected. Standard length and otolith ring counts were examined in both hatchery-raised (150) and wild-caught (390) samples. This study revealed that (i) mass larval drift of striped catfish at the Mekong River in Phnom Penh took place from July 13th to July 23rd, 2015 corresponding with increasing flow, (ii) otolith rings are formed daily in hatchery-raised larvae, and can be used to determine the age of wild larvae/juveniles, (iii) otolith ring increments are strongly correlated with standard length in hatchery-raised larvae, but not in wild-caught larvae, (iv) mean standard length and age of striped catfish larvae collected in Phnom Penh are 14.65 mm (se = 0.07) and 27.6 days (se = 0.13), respectively, and (v) most striped catfish likely spawn in June and early July between the full and new moon, and the spawning grounds are most likely located along the Cambodian Mekong between Stung Treng and Kratie and, in 2015, travel time from spawning grounds to Phnom Penh was approximately 27 days. Upper Cambodian Mekong River is therefore likely very necessary for the persistence of all striped catfish populations and fisheries in the lower floodplains of Cambodia and Mekong delta. The existing and proposed dams in the upper Mekong River in Cambodia and beyond would drastically affect spawning sites and dry season refuge habitats and disconnect the upstream spawning from the downstream rearing habitats of the species.

Both hydropower dams and global warming pose threats to freshwater fish diversity. While the extent of global warming may be reduced by a shift towards energy generation by large dams in order to reduce fossil-fuel use, such dams profoundly modify riverine habitats. Furthermore, the threats posed by dams and global warming will interact: for example, dams constrain range adjustments by fishes that might compensate for warming temperatures. Evaluation of their combined or synergistic effects is thus essential for adequate assessment of the consequences of planned water-resource developments. We made projections of the responses of 363 fish species within the Indo-Burma global biodiversity hotspot to the separate and joint impacts of dams and global warming. The hotspot encompasses the Lower Mekong Basin, which is the world's largest freshwater capture fishery. Projections for 81 dam-building scenarios revealed progressive impacts upon projected species richness, habitable area, and the proportion of threatened species as generating capacity increased. Projections from 126 global-warming scenarios included a rise in species richness, a reduction in habitable area, and an increase in the proportion of threatened species; however, there was substantial variation in the extent of these changes among warming projections. Projections from scenarios that combined the effects of dams and global warming were derived either by simply adding the two threats, or by combining them in a synergistic manner that took account of the likelihood that habitat shifts under global warming would be constrained by river fragmentation. Impacts on fish diversity under the synergistic projections were 10-20% higher than those attributable to additive scenarios, and were exacerbated as generating capacity increased-particularly if CO2 emissions remained high. The impacts of dams, especially those on river mainstreams, are likely to be greater, more predictable and more immediately pressing for fishes than the consequences of global warming. Limits upon dam construction should therefore be a priority action for conserving fish biodiversity in the Indo-Burma hotspot. This would minimize synergistic impacts attributable to dams plus global warming, and help ensure the continued provision of ecosystem services represented by the Lower Mekong fishery.