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Cambodia: Grey-headed Fish Eagle Project, Part 1
Ruth Tingay — in Asia-Pacific    Share

It’s January 1st and it’s an unusual start to the New Year for me. Instead of being out partying last night, I was at home, packing. It’s a familiar task and one I always look forward to as it signals the end of a long period of pre-fieldwork planning and preparation. The funding proposals had been written, submitted, and accepted; the research permit from the host country’s government applied for and received; this year’s field team selected and briefed; the field transport and accommodation booked; the fieldwork schedule planned; the budget checked and revised; immunisations updated; medical insurance updated; emergency evacuation procedure planned; flights researched, booked and confirmed; visa procedures confirmed; specialist sampling equipment procured; export and import permit restrictions for shipping biological samples from one country to another read and (grudgingly) understood; currency exchanged; passport found.

Grey-headed Fish Eagle

Grey-headed Fish Eagle

This evening I’m heading back to Cambodia to continue my long-term research on the poorly-known grey-headed fish eagle (GHFE). I set up the study back in 2005 after finding a previously undocumented, high-density GHFE breeding population in the swamp forest around South East Asia’s largest freshwater body, the Tonle Sap Lake. This lake is part of an ecosystem like no other. Unlike the permanently flooded swamp forests of Indonesia, the forest around the Tonle Sap Lake is subject to seasonal flooding from the Mekong River. When the mighty Mekong fills with Himalayan melt-water and rain from the monsoon in June and July, it gushes downstream through China, Myanmar, Laos and Thailand and into the Tonle Sap Lake with such force that the lake expands to over four times its size, from 2,500 km2 in the dry season to a whopping 12,000 km2 during the wet season. This immense influx of water floods the forest surrounding the lake, creating up to a 10 m rise in the lake’s water level. Short vegetation and smaller trees become fully submerged, and only the top halves of the larger trees are visible above the water’s surface. This pulsing flux of seasonal water has created a nutrient-rich environment that is recognised as one of the most productive inland fisheries in the world. It shouldn’t be a surprise that the piscivorous GHFE has decided to set up shop here!

Tonle Sap lake is seasonaly inundated with water from the Mekong River

Prior to this GHFE study, very little had been published on the species; not even on its elementary biology and ecology. It was known to have a wide distribution, ranging from north east India, down the Thai-Malay peninsula, and into Indonesia, but it was also suspected to be in decline. The reasons for that weren’t clear, and if they were to be addressed, then some basic knowledge on what this eagle needs to survive would be essential. Seven years later, although we have learned a great deal about the GHFEs at the Tonle Sap, so far our findings have raised more questions than answers. Nevertheless, we have learned enough to say that this population, of between 60-80 GHFE breeding pairs, is regionally significant in South East Asia, and in all likelihood it is probably a globally significant population, as researchers in other parts of the species’ range report only small numbers of breeding pairs from their study areas.

A fish eagle nest with two eggs and the remains of a water snake

Most importantly, we have learned enough to have identified several potential conservation threats to this special population. One of these is the annual mass removal of at least 6.9 million watersnakes from the lake. The snakes are an important resource for the local human population, who collect them for food and also for the wildlife trade; there is an insatiable demand in Vietnam and China for these species, with some commanding an irresistible price tag for local people living around the lake. Unfortunately, the snakes are also an important food resource for the Tonle Sap GHFEs, and the un-regulated removal of so many on such a regular basis is unlikely to be sustainable.

Tonle Sap lakeandthe Mekong River

The second potential conservation threat is perhaps not so visibly obvious at the lake itself, but is no less insidious; that is, the proposed construction of at least 19 hydropower dams in the upstream reaches of the Mekong River. These dams not only have the potential to alter significantly the downstream flood regime (and thus productivity) of the Tonle Sap Lake, but these types of structures have also been implicated in other parts of the world in the accumulation of methylated mercury in the river sediment, that is then flushed downstream. Piscivorous raptors are known to be especially susceptible to mercury contamination (as the mercury accumulates up through the food chain) and there is a particular concern about mercury exposure in our GHFE population. In all the previous field seasons, I have probably seen no more than a total of five juvenile GHFEs at the Tonle Sap, which is unusually low for such a large breeding population. One of the known side effects of mercury poisoning is reproductive difficulties. So, either our GHFEs are producing plenty of offspring and those young birds are moving away to another area somewhere else around the lake until they’re old enough to breed, or, our GHFEs are already contaminated and their reproductive output is affected. One of the main aims of this year’s study is to measure the level of mercury in our fish eagles and their prey (fish and watersnakes) and evaluate their exposure to this otherwise invisible threat. A specialist contaminants biologist has been recruited to the team this year, all the way from Alaska, specifically for this job – more on that later.

Just time for one last decent cup of tea then it’ll be time to head off into the night to begin this year’s adventure in the great Cambodian swamp.

The great Cambodian swamp

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