Fish caught on the river in Sumbawa Besar, a village on the Indonesian island of Sumbawa in the province of of West Nusa Tenggara. CIFOR/Aulia Erlangga

Sound land-use policies to conserve freshwater habitats in tropical forests can help maintain fish biodiversity, reduce the risk of food insecurity and bolster international restoration and biodiversity goals, scientists say.

Research indicates that forests play a key role in regulating the quality of aquatic habitats for fish, but concrete data that could help inform government and community management strategies are lacking, according to a new report published in the journal Bioscience.

“A better understanding of the relationship between forests and freshwater fish communities is urgently needed,” said Michaela Lo, lead author of the report, a scientific researcher with the Center for International Forestry Research (CIFOR) and a Ph.D. candidate at Britain’s University of Kent.

“The challenge is partly that it’s extremely difficult to measure this relationship because forests are static, remaining in one place, while bodies of water flow and the fish living in them can swim over a range of hundreds of kilometers.”

Because of this dynamic relationship, fish are vulnerable to habitat changes, and currently, the full consequences of major-land use changes are unknown, she added. “Most existing studies focus on the impact of land use change on terrestrial biodiversity, but these changes also have large implications for freshwater biodiversity, demonstrating the need for more research in this area.”

Although-forest and water ecosystems are integral to meeting international development and environmental targets under the U.N. Framework Convention on Climate Change, Convention on Biological Diversity and the Sustainable Development Goals, freshwater species populations have declined by more than 80 percent over the past 50 years.

Fish provide food security for tropical forest dwellers and rural communities, offering a significant source of dietary protein and micronutrients.

For example, they contribute more than half the protein intake for more than 400 million people in the poorest countries of Africa and Asia, said Lo, urging greater research investment into these two continents, where countries are experiencing high rates of deforestation and habitat degradation and where there is relatively little research on this topic.

To reach their findings, Lo and scientists at CIFOR, the Virginia Polytechnic Institute and State University (Virginia Tech) in the United States and the U.N. Food and Agriculture Organization conducted a systematic review of 61 studies produced between 1984 and 2019, whittling down an initial list of more than 16,000 articles.

They synthesized data on the relationship between forests and fish diversity, size, and populations.

“We found strong evidence that forests have a central function in maintaining the diversity of freshwater fish,” Lo said.

Riparian forests that run alongside streams and rivers are an important supply of woody debris and leaf litter, creating a range of habitat spaces conducive to fish diversity. These small niches also act as nursing grounds and refuges to hide away from predators.

More than half of the studies reviewed demonstrated that forests contributed to freshwater habitats by controlling sedimentation and siltation.

A build-up of silt and sediment is typical in freshwater systems without riparian forests or where deforestation has occurred, leading to more homogenous and less varied habitats with fewer bottom-feeding fish species and a less diverse fish population overall. Studies demonstrated that shrimp and fish quantities dropped when sedimentation caused by ecosystem degradation increased.

Although the number of articles and studies on the relationship between fish and forests has dramatically increased since 2015, there are still many unknowns about the conditions of freshwater habitats in Africa and Southeast Asia where high rates of deforestation have disrupted ecosystems.

“This geographical imbalance of studies also reflects the lack of existing policies supporting the effective management of forest-freshwater ecosystems in Asia and Africa,” Lo said.

“Forests are inextricably interlinked with fish in freshwater ecosystems, providing regulatory and provisioning functions that support a healthy aquatic habitat, water quality and food to sustain them,” Lo said, adding that a greater understanding of the sensitivities to change can help with constructing effective conservation management strategies.

This research was supported by European Commission DEVCO as part of the Governing Multifunctional Landscapes Project and by the USAID Office of Forestry and Biodiversity.