Simulated impacts of climate change on current farming locations of striped catfish (Pangasianodon hypophthalmus; Sauvage) in the Mekong Delta, Vietnam

Natal origin and migration pathways of Mekong catfish (Pangasius krempfi) using strontium isotopes and trace element concentrations in environmental water and otoliths

To improve our knowledge of the migration pathway of a highly threatened fish species along the Mekong River, strontium isotope ratios (87Sr/86Sr) and 18 trace element concentrations were measured in the water and in the otoliths of an anadromous catfish, Pangasius krempfi, to infer its natal origin and potential migration pathways. Water was sampled at 18 locations along the mainstream, tributaries and distributaries of the Mekong River. To check for accuracy and precision, measurements of the 87Sr/86Sr ratios and trace element concentrations were then compared in two laboratories that use different analytical methods. Differences in trace element concentrations between locations were not significant and could not, therefore, be used to discriminate between migration pathways. However, the Mekong mainstream, tributaries and distributaries could all be discriminated using Sr isotopes. The 87Sr/86Sr profiles recorded in P. krempfi otoliths showed that there were three contingents with obligate freshwater hatching and variable spawning sites along the Mekong mainstream, from Phnom Penh (Cambodia) to Nong Khai (Thailand) or further. After hatching, the fish migrated more or less rapidly to the Mekong Delta and then settled for most of their lifetime in brackish water. Spawning habitats and migration routes may be threatened by habitat shifts and the increasing number of hydropower dams along the river, especially the contingents born above Khone Falls (Laos). The conservation of P. krempfi, as well as other migratory fish in the Mekong River, requires agreements, common actions and management by all countries along the Mekong River. This study highlighted the importance of using both Sr/Ca and 87Sr/86Sr ratios to understand life history of anadromous fishes as the 87Sr/86Sr ratio in the water was shown to be less effective than the Sr/Ca ratio in identifying movements between different saline areas.

Comparative Transcriptome Analysis During the Seven Developmental Stages of Channel Catfish (Ictalurus punctatus) and Tra Catfish (Pangasianodon hypophthalmus) Provides Novel Insights for Terrestrial Adaptation

Tra catfish (Pangasianodon hypophthalmus), also known as striped catfish, is a facultative air-breather that uses its swim bladder as an air-breathing organ (ABO). A related species in the same order (Siluriformes), channel catfish (Ictalurus punctatus), does not possess an ABO and thus cannot breathe in the air. Tra and channel catfish serve as great comparative models for investigating possible genetic underpinnings of aquatic to land transitions, as well as for understanding genes that are crucial for the development of the swim bladder and the function of air-breathing in tra catfish. In this study, hypoxia challenge and microtomy experiments collectively revealed critical time points for the development of the air-breathing function and swim bladder in tra catfish. Seven developmental stages in tra catfish were selected for RNA-seq analysis based on their transition to a stage that could live at 0 ppm oxygen. More than 587 million sequencing clean reads were generated, and a total of 21,448 unique genes were detected. A comparative genomic analysis between channel catfish and tra catfish revealed 76 genes that were present in tra catfish, but absent from channel catfish. In order to further narrow down the list of these candidate genes, gene expression analysis was performed for these tra catfish-specific genes. Fourteen genes were inferred to be important for air-breathing. Of these, HRG, GRP, and CX3CL1 were identified to be the most likely genes related to air-breathing ability in tra catfish. This study provides a foundational data resource for functional genomic studies in air-breathing function in tra catfish and sheds light on the adaptation of aquatic organisms to the terrestrial environment.

Economic Efficiency of Extensive and Intensive Shrimp Production under Conditions of Disease and Natural Disaster Risks in Khánh Hòa and Trà Vinh Provinces, Vietnam

Uncertainty about efficiency and sustainability of shrimp production due to diseases and climatic events may prevent Vietnam from attaining US $10 million target from shrimp exports by 2025. We surveyed 120 and 159 shrimp farmers from Khánh Hòa and Trà Vinh provinces, respectively, to obtain information on their input use, production levels and the effects of diseases and climate change events on their farm profitability. Stochastic production frontier analysis (SFA) discovered that the number of workers, crops and seed costs positively influenced farmers’ profits, while cost of chemicals and labour negatively affected the profit of Khánh Hòa farmers. The number of workers and chemical costs positively affected profits in Trà Vinh, while cost of labour and energy, the number of crops and average stocking density negatively influenced farmer profit in Trà Vinh. Number of years of schooling, experience and average size of ponds positively influenced economic efficiency, while the number of ponds and climatic change events negatively influenced efficiency in Khánh Hòa province. Age and alternative power source positively affected economic efficiency, while disease prevalence affected efficiency of Trà Vinh. All farms practicing intensive or extensive shrimp production achieved 90% efficiency. The government should encourage the wise use of resources, high-quality seeds and a sensitive balance between intensive and extensive culture to sustainably attain its national production and export goals.

Exploring Freshwater Regimes and Impact Factors in the Coastal Estuaries of the Vietnamese Mekong Delta

Freshwater resources make an essential contribution to the livelihoods of millions of local people in the coastal estuaries of the Vietnamese Mekong Delta (VMD). However, coastal freshwaters currently face numerous threats, not least (i) changing tidal dynamics due to sea level rise and (ii) changes in river regimes due to dam construction upstream. This research explores the evolution of freshwater regimes in these coastal estuaries. Using process diagrams, freshwater distributions are mapped and analyzed. Application of statistical methods provides insight into freshwater flow cycles and variations in water regimes upstream at various measurement points within the estuaries. A previously calibrated and validated hydraulic model is used to simulate drought-year scenarios and spatial changes in freshwaters over time. Findings indicate decreasing river discharges in the flood season, but increasing discharges in the dry season, due to the impacts of hydropower dams. In addition, the driest months are shifting earlier. From this data, we derive rules of thumb regarding freshwater distributions in the coastal estuaries of the VMD. These relate to (i) the boundary beyond which freshwater is always found; (ii) the boundary where freshwater appears daily; (iii) the start of the freshwater season; (iii) the boundary where freshwater appears until February and until April; (iv) the end of the flood season; and (v) the number of days without freshwater per year. The trends discerned will help local freshwater users and decision makers formulate forward-looking, flexible strategies for freshwater exploitation, while also providing avenues for further research.

Global Aquaculture Productivity, Environmental Sustainability, and Climate Change Adaptability

To meet the demand for food from a growing global population, aquaculture production is under great pressure to increase as capture fisheries have stagnated. However, aquaculture has raised a range of environmental concerns, and further increases in aquaculture production will face widespread environmental challenges. The effects of climate change will pose a further threat to global aquaculture production. Aquaculture is often at risk from a combination of climatic variables, including cyclone, drought, flood, global warming, ocean acidification, rainfall variation, salinity, and sea level rise. For aquaculture growth to be sustainable its environmental impacts must reduce significantly. Adaptation to climate change is also needed to produce more fish without environmental impacts. Some adaptation strategies including integrated aquaculture, recirculating aquaculture systems (RAS), and the expansion of seafood farming could increase aquaculture productivity, environmental sustainability, and climate change adaptability.

Managing flood risks in the Mekong Delta: How to address emerging challenges under climate change and socioeconomic developments

Climate change and accelerating socioeconomic developments increasingly challenge flood-risk management in the Vietnamese Mekong River Delta-a typical large, economically dynamic and highly vulnerable delta. This study identifies and addresses the emerging challenges for flood-risk management. Furthermore, we identify and analyse response solutions, focusing on meaningful configurations of the individual solutions and how they can be tailored to specific challenges using expert surveys, content analysis techniques and statistical inferences. Our findings show that the challenges for flood-risk management are diverse, but critical challenges predominantly arise from the current governance and institutional settings. The top-three challenges include weak collaboration, conflicting management objectives and low responsiveness to new issues. We identified 114 reported solutions and developed six flood management strategies that are tailored to specific challenges. We conclude that the current technology-centric flood management approach is insufficient given the rapid socioecological changes. This approach therefore should be adapted towards a more balanced management configuration where technical and infrastructural measures are combined with institutional and governance resolutions. Insights from this study contribute to the emerging repertoire of contemporary flood management solutions, especially through their configurations and tailoring to specific challenges.

Chronic hyperosmotic stress inhibits renal Toll-Like Receptors expression in striped catfish (Pangasianodon hypophthalmus, Sauvage) exposed or not to bacterial infection

Toll-like Receptors (TLRs) are the first innate receptors in recognizing pathogen-associated molecular patterns. In fish, upregulation of toll-like receptors during infection has been largely demonstrated while the effects of abiotic stressors on their expression remain poorly investigated. In this study, striped catfish were submitted during 20 days to three salinity profiles (freshwater, low saline water, saline water), followed by injection of a bacterial strain of Edwardsiella ictaluri. The expression of TLRs 1, 3, 4, 5, 7, 9, 19, 21, and 22 was measured in kidney at different time points in non infected and infected striped catfish. Infection induced overexpression of TLRs 1, 3, 4, 5, 7, 21 and 22. With elevated salinity, the expression of all TLRs, except TLR5, was severely decreased, particularly after bacterial infection. The TLRs responsiveness of striped catfish facing bacterial disease and salinity stress and possible consequences on striped catfish immune response's efficiency are discussed.

Gill remodelling and growth rate of striped catfish Pangasianodon hypophthalmus under impacts of hypoxia and temperature

Gill morphometric and gill plasticity of the air-breathing striped catfish (Pangasianodon hypophthalmus) exposed to different temperatures (present day 27°C and future 33°C) and different air saturation levels (92% and 35%) during 6weeks were investigated using vertical sections to estimate the respiratory lamellae surface areas, harmonic mean barrier thicknesses, and gill component volumes. Gill respiratory surface area (SA) and harmonic mean water - blood barrier thicknesses (HM) of the fish were strongly affected by both environmental temperature and oxygen level. Thus initial values for 27°C normoxic fish (12.4±0.8g) were 211.8±21.6mm²g^-1 and 1.67±0.12μm for SA and HM respectively. After 5weeks in same conditions or in the combinations of 33°C and/or PO₂ of 55mmHg, this initial surface area scaled allometrically with size for the 33°C hypoxic group, whereas branchial SA was almost eliminated in the 27°C normoxic group, with other groups intermediate. In addition, elevated temperature had an astounding effect on growth with the 33°C group growing nearly 8-fold faster than the 27°C fish.

Exploring the climate change concerns of striped catfish producers in the Mekong Delta, Vietnam

This study investigated the perceptions on and adaptations to climate change impacts of 235 pangasius farmers in the Mekong Delta, Vietnam. Data were collected using semi-structured household surveys in six provinces, from three regions along the Mekong river branches. A Chi-Square test was used to determine the association between variables, and a logit regression model was employed to identify factors correlated with farmer’s perception and adaptation. Less than half of respondents were concerned about climate change and sought suitable adaptation measures to alleviate its impacts. Improving information on climate change and introducing early warning systems could improve the adaptive capacity of pangasius farmers, in particularly for those farmers, who were not concerned yet. Farmers relied strongly on technical support from government agencies, but farmers in the coastal provinces did not express the need for training by these institutions. This contrasting result requires further assessment of the effectiveness of adaptation measures such as breeding salinity tolerant pangasius.