The Challenge

The Siamese Crocodile (Crocodylus siamensis) is a critically endangered medium‐size crocodile native to Southeast Asia. Now extinct from much of its natural range, and with fewer than 1000 breeding adults remaining in the wild, conservation efforts in the Cardamom Mountains of Cambodia have focussed on the release of captive‐reared juveniles and sub‐adults into river reaches known to support adult C. siamensis populations. However, the biology of wild C. siamensis is not well understood and the ecology of ecosystems at release locations is virtually unknown. Human residents of Cambodia obtain approximately 60–80% of their total animal protein from the inland fisheries, and along with poaching, competition for food resources is thought to be a significant driver of C. siamensis population declines.

 

Our Response

Fish are thought to comprise a major component of the diet of C. siamensis and our aim in this project was to characterize fish communities within three potential C. siamensis release locations, focusing on community composition, density, size class structure and food web dynamics. Our survey sites varied in both C. siamensis density and human fishing pressure, and we sought to interpret our results in light of these drivers.

 

Science and Innovation

We used a combination of next generation DNA sequencing approaches and classical ecological techniques to address this knowledge gaps in our understanding of C. siamensis ecology. We developed techniques that (a) provided genetic documentation of the fish taxa that were potential prey at crocodile release sites, (b) determined the fish population structure and (c) derived basic trophic structure information of the fish community.

Results

Genomic interrogation of fishes of the Cardamom Mountains distinguished a total of 13 unique fish species and our results contribute to genetic databases and have added to the documented taxon list for the region. We record the presence of two previously unconfirmed fish genera in the region. We provide the first estimates of fish density, biomass and size class distribution for three rivers in the Cardamom Mountains. The three potential C. siamensis release reaches that we sampled showed clear differences in fish community composition, structural and trophic dynamics. Fish density and biomass were highest in reaches containing highest density remnant C. siamensis populations and lowest in areas with high human fishing pressure. Survey reaches with food webs that were more reliant on in-stream basal food resources supported higher densities and biomass of fish. Our results have important implications for future C. siamensis conservation efforts in Cambodia and contribute valuable ecological information on a relatively unexplored part of Asia.

Contact person

• Dr Paul McInerney

Additional Information

• Paul’s article in ‘The Conversation‘
• Research article ‘Release of critically endangered crocodiles…‘ (2019)