Bhutan may not compete with big fishing industries around the globe but we do have unique and relatively unexplored river aquatic ecosystems. High-end opportunities such as the caviar food industry and recreational fishing are a few of the potential opportunities which are being currently explored. We do not want our river and stream networks to be altered before realizing the true potential of their contribution to the Bhutanese economy and culture.

Potential threats to native fish populations in Bhutan’s major river basins include the construction of dams, artificial noise, heavy metals pollution, the introduction of non-native fishes, and micro-plastic contents. There is no clear understanding of trends in the population of native fish across rivers and stream networks in Bhutan. Similarly, the effects of potential threats and their interactive effects on overall aquatic ecosystem functioning are unclear. The article explores and proposes a way forward for improving our knowledge of the native fish populations beyond the species inventory explorations.

Minimizing alteration of river flow regime –  Major civil works such as the construction and development of hydropower stations along the narrow river valley such as in Bhutan significantly change the river ecosystem and the rest of the biogeochemistry of the area. Except for a few, the flow in almost all the major rivers across the country is altered. However, to date, we do not have any research data to aid our decision-making for sustainable management of those rivers and associated ecosystems. The so-called “fish ladder” at the hydropower dams is often the most trusted passage for fish migration, but its effectiveness is being argued, and suitability to the migratory fishes found in the region is questionable.

Mitigating and managing physical barriers to fish migration – Physical barriers to natural river flow affect the aquatic ecosystem by changing the nutrient cycle, altering the biological and physical properties of rivers and floodplains, and fragmenting the continuity of rivers. Consequently, these impacts block critical fish migration routes that connect the downstream floodplains and upstream tributaries. Research in similar mountainous regions like Bhutan reports significant changes upstream and downstream of the dams due to physical obstruction which is a direct result of impeding and delays in both migration and spawning.

Reducing nutrient load and temperature alterations along the river – The spawning of fish in the river augments the biodiversity in and around the river and also influences the nutrient cycle. The change in flow and reduced water quantity due to physical barriers such as dams are attributed to creating temperature differences before and after the dam. For example,  researchers recorded mean water temperature below the Xinanjiang and Danjiangkou dams decreased from 19.0oC to 13.5oC. The decrease in temperature led to delayed fish spawning by 20-60 days.

Establishing buffers along and around aquatic habitats – In the recent past, in addition to the major hydropower projects, the stress on aquatic ecosystems increased due to development of urban centers, and the construction of roads and bridges resulting in increased artificial noise near the critical sections of rivers. A rise in artificial noise levels underwater is known to negatively affect populations. For example, the very loud noise of relatively short exposure produced by pile driving and explosion can harm nearby fish. Similarly, moderate underwater noises of longer duration such as those produced by public transport, fishing, and recreational activities could also potentially impact a larger number of fish species. Such observation highlights the need to have a safe buffer zone between critical sections of the river or stream networks from major civil and construction projects.

Gathering data on hazardous pollutants like heavy metals – Anthropogenic activities along the river basin are the main sources for the accumulation of heavy metals in the freshwater ecosystems. The sources include the dumping of sewage effluents, hospital wastes, and religious activities and offerings into water bodies. In addition, heavy metals may enter the aquatic system through ore-bearing rocks, windblown dust, and forest fires. The effects of heavy metal pollution can be felt beyond the fish population – the accumulation of heavy metals in fish could kill predator bird species feeding on fish, while a reduction in predator bird species can see growth in the population of pest birds and rodents. The fact that there is limited data on heavy metal pollution in our river and stream networks shows that we do not understand the magnitude of such risk – both to ecosystems and public health. Currently, the fish caught for consumption from urban rivers in Bhutan may contain these pollutants. Further, all the pollution in our rivers accumulates in the bay of Bengal – the main source of favorite Bhutanese delicacy, the “dried fish”.

Reducing the threat of micro-plastic pollution – The National Environment Commission of Bhutan initiated numerous strategies to reduce plastic usage in the country in recent years, although its usage can still be ubiquitously seen. Plastic polymers have detrimental effects on aquatic ecosystems. The science on emerging pollutants such as micro-plastic pollution is recent with too many unknowns regarding its long-term risk to the ecosystem and public health. Research revealed a high amount of ingestion of microplastics by the fishes such as Atlantic silversides, European flounder, and others. Plastic filaments and films of a mean length of 2.2 mm were found to be ingested by mesopelagic fish species. There is no information on such threats to public health and the native fish population in Bhutan.

Improving our understanding of introduced species – The diversity and distribution of non-native fishes are also anticipated to be one of the major drivers affecting the native fish population, although the research and data remain deficient for Bhutan. An adequate understanding of the ecological risks associated with fish introductions is indispensable. Non-native fishes like brown trout are suspected to have negatively affected the native fishes due to their carnivorous nature when adult. The rate at which non-native freshwater fishes have been introduced worldwide was reported to be doubled in the span of the past 30 years, mainly for aquaculture (39%) and the improvement of wild stocks (17%). It is commonly cited as one of the key challenges to native fish diversity after habitat alteration. Non-native species affect the population of native fishes through predation, hybridization, competition for food, habitat modification, and the introduction of new diseases. The introduced species also compete for spawning ground and could lead to the extirpation of native species. The release of ‘tshethar’ fishes and escape of ornamental fishes from homes and recreational parks to the natural river system also needs urgent risk assessment and management for Bhutan.

The article does not advocate for banning or restricting development in every section of rivers or lakes. We are hopeful that the article promotes evidence-based decision-making through high-quality research, which in long run would identify key challenges and critical water ecosystems which need higher levels of protection with the hope to provide sustainable ecology and economic services to Bhutan and millions of population living downstream.

The article is published based on personal experiences and observations by a group of water researchers from Bhutan. The group can be contacted at waterresearchbhutan@gmail.com