Myanmar is a global biodiversity hotspot, home to over 570 freshwater fish species and many endemics. Yet its rivers face mounting pressure from pollution, land-use change and declining water quality – challenges that threaten both ecosystems and the communities depending on inland fisheries for food security. This study co-authored by Ecologic Institute's Dr. Benjamin Kupilas offers novel insights into how tropical fish communities respond to these stresses.

Assessing human impacts in tropical lowland rivers

The researcher team surveyed 12 stream sites in Myanmar's Bago region, spanning a gradient from relatively untouched forested reaches to heavily impacted stretches receiving untreated sewage. Using electro fishing, DNA barcoding, stable isotope analysis, and detailed environmental measurements, the team examined how degradation affects:

• Fish species richness and diversity
• Biomass and community composition
• Trophic structure and food-web organisation

Weak fish community responses to pollution

Although impacted sites showed clear signs of pollution (higher nitrogen levels and lower oxygen saturation) fish communities responded only weakly:

• Species richness and community composition remained largely similar across sites.
• Trophic niches also showed little change across the impact gradient.
• Fish biomass increased in degraded streams, likely driven by nutrient-stimulated primary production.

Only subtle trophic shifts emerged: fish at impacted sites exhibited lower minimum δ¹⁵N values and a slightly broader δ¹⁵N range, hinting at modified feeding pathways but not major disruption.

Fish alone may not be reliable bio-indicators in Myanmar's rivers

Unlike macroinvertebrates, which have shown clear sensitivity to pollution in the same river network, fish appear more tolerant, possibly due to higher mobility and physiological resilience to low oxygen. The strongest predictor of community similarity was catchment forest cover, underscoring the importance of land-use planning and watershed protection.