Shrinking fish size

There is plenty of fishing on the Tonle Sap, Southeast Asia’s largest lake. However, the make-up of the fish catch has changed in the last 20 years. Larger fish species have become less common, and the average size of individual fish caught is shrinking. Experts now are debating why. 

Is this caused by heavy fishing pressure, by upstream hydropower dams, or by drier conditions — perhaps related to climate change? 

A 2018 scientific paper by Peng Bun Ngor and colleagues documented the decline in the body weight of six common fish species of the Tonle Sap. Their paper, ‘Evidence of indiscriminate fishing effects in one of the world’s largest inland fisheries’, published on the Nature website, explains this is an outcome of ‘fishing down’ the food web. ‘Fishing down’ occurs when the fish catch increasingly consists of smaller varieties of fish. Ngor and other co-authors warned that “heavy indiscriminate fishing pressure” had resulted in changes to the composition of the Tonle Sap fishery.

In a follow-up article, published on the Nature website in early 2021, scientists Ashley Halls and Kent Hortle counter this claim. They reviewed water level and temperature data, combined with catch statistics, to show that seasonal flooding has a clear effect on the size of fish species available. They argue that the decreasing size of fish reflects changing hydrological conditions, likely caused by climate change and the operation of upstream hydropower dams. Their paper, ‘Flooding is a key driver of the Tonle Sap dai fishery in Cambodia’, states that dry years have an ongoing impact on larger, longer-lived species, affecting not only their body size but also their ability to reproduce and ultimately survive. 

They add that, “Indeed, fishing effort may have actually declined in Cambodia’s inland waters, following the removal of highly efficient industrial fishing operations (fishing lots) in Cambodia in 2012, and as former fishers take up alternative employment in booming regional economies.”

In response, Gaël Grenouillet and other authors of the 2018 paper, including the original lead author Peng Bun Ngor, reanalyzed their data. Their response highlights the finding that 50% of species still showed a declining trend that couldn’t be accounted for by changes in seasonal flooding. These declines, they say, are significantly more pronounced for large-bodied species. They state that there is no research data available on changes in fishing effort, and therefore it is not possible to say whether fishing pressure has really decreased since 2012, when the Government of Cambodia returned more than 500 fishing lots to local communities and small-scale fishers. 

However, Grenouillet and colleagues note that, “Fishers indeed borrow money to buy fishing gears, and felt compelled to catch as many fish as possible to repay their loans and meet the household needs….Our reading of the situation is that the fisheries remain under intense pressure despite attempts to implement improve fisheries governance.” Read their full response here.

There is no single answer to the question of what’s causing the decline in body weight and size of the Tonle Sap catches. But this exchange shows how scientific debate can help to identify key issues, as a foundation to developing well-targeted policies to protect biodiversity and livelihoods.