# Saving the Mahachai Betta: Genetic Erosion and Conservation Priorities Under Urbanization Pressure

## TL;DR

The Mahachai Betta, a rare fish in Thailand, is losing genetic diversity due to urbanization, and conservation efforts are urgently needed to prevent its extinction.

## Contribution

This study provides the first integrated genetic and landscape-based assessment of the Mahachai Betta under urbanization pressure.

## Key findings

- Mahachai Betta populations show low genetic diversity and high genetic subdivision due to habitat fragmentation.
- Computer simulations predict severe genetic erosion within the next 12.5–37.5 years if no action is taken.
- Samut Sakhon populations reflect strong environmental heterogeneity, indicating localized adaptation risks.

## Abstract

The Mahachai Betta is a rare fighting fish that lives only in brackish canals and ponds around Bangkok and nearby provinces. These habitats are being rapidly lost because of urban growth, pollution, and changes in water quality. To understand how this affects the species, we studied ten populations and analyzed their genetic diversity. The results showed that many groups of Mahachai Betta have very low genetic variation and are becoming increasingly isolated from each other. Computer simulations further suggest that the loss of diversity will continue rapidly in the coming decades if no action is taken. This means that the fish will become less able to adapt to environmental changes and more at risk of extinction. Our findings highlight the urgent need to protect and connect the remaining habitats and to involve local communities in conservation. By safeguarding both the environment and the genetic diversity of this unique fish, we can improve its chances of survival for future generations.

Background/Objectives: Mahachai Betta (Betta mahachaiensis) is a bubble-nesting fighting fish endemic to brackish habitats in Bangkok, Samut Sakhon, and Samut Prakan, where rapid urbanization and industrial growth threaten persistence. We evaluated genetic structure and diversity across 10 populations (81 individuals) to inform conservation planning. Methods: This study combined microsatellite genotyping (13 loci) with ecological niche modeling to assess genetic variability, population connectivity, and landscape–environmental drivers of differentiation. Results: Habitat loss and fragmentation were associated with reduced gene flow and decreased genetic diversity. Mean allelic richness was 2.65 and expected heterozygosity ranged from 0.20 to 0.46, with FST values up to 0.400. Forward simulations predicted severe erosion of diversity within the next 12.5–37.5 years. Populations showed clear genetic subdivision, most pronounced in Samut Prakan and Samut Sakhon, with two Samut Sakhon populations (SKN3 and SKN7) reflecting strong environmental heterogeneity. Conclusions: Improving habitat connectivity and intensifying local community engagement are priority actions to enhance the resilience and long-term persistence of Mahachai Betta. This study provides the first integrated genetic and landscape-based assessment of the species, highlighting its rapid genetic erosion under urbanization and offering a foundation for targeted, evidence-based conservation strategies.