Occurrence of the threatened dugong in Indonesia

The dugong, Dugong dugon, known in Dutch as the doejong, is a large herbivorous marine mammal that lives in shallow coastal waters with seagrass, mainly in Australia, Southeast Asia and the Persian Gulf. Worldwide, numbers are declining and the species is classified as vulnerable. Indonesia lies at the centre of the dugong’s distribution range, but reliable data on numbers, occurrence and population structure are scarce.

To gain better insight into this, researchers from Wageningen University & Research (WUR) and Indonesian partners collected all publicly accessible information on dugong observations in Indonesia for the period 2010 to 2022. This included data from scientific publications, reports, news items and social media. In total, 1033 observations and records were compiled and analyzed.

Concentration around small islands

The analysis shows that the dugong is not evenly distributed across Indonesia. Most reports are concentrated around small island groups, particularly in the Banda Sea and in the western part of the Java Sea. These areas are close to known dugong populations in neighbouring countries, suggesting the importance of Indonesia for connectivity between regional populations.

“The picture that emerges from this study is that dugongs occur in a limited number of areas within Indonesia”, says Dolf Debrot, marine ecologist at WUR and co-author of the study. “This also makes clear where protection and management can have the greatest impact.”

Notably, the number of observations was not related to the amount of seagrass, the dugong’s main food source. For example, there were coastal areas with abundant and healthy seagrass, such as along the west coast of Sumatra, where hardly any dugongs have been reported.

Bycatch and signals from the age structure

Seventeen percent of all recorded registrations concerned dead animals. Where the cause of death was known, bycatch in fishing nets by far proved to be the most important factor. Many deaths were especially recorded in parts of the western Java Sea.

In addition, the data show that the population structure was not smooth. This may indicate one or more periods of reduced reproduction or recent immigration of adult animals from neighbouring countries, possibly prompted by food shortages. The researchers emphasize that these data do not allow firm conclusions, but they do give cause for concern. “These are signals that raise questions”, says Debrot. “In combination with the high mortality due to bycatch, they show how vulnerable these populations are.”

Valuable information, but not a replacement for quantitative monitoring

The study is the first integrated assessment of the dugong for the entire Indonesian archipelago. Previous studies were usually limited to smaller areas. According to the study, reports from citizens and other publicly accessible sources provide valuable information in areas without systematic monitoring.

At the same time, the authors emphasize that these data are not a substitute for standardized quantitative counts and long-term research. Better population estimates and more insight into reproduction and distribution are critically needed in order to protect the dugong in Indonesia more effectively.

More information

• The article, published in Marine Mammal Science: Using Gray Literature and Social Media Records to Help Identify Population Hotspots and Conservation Priorities for the Vulnerable Dugong, Dugong dugon (Müller, 1776) in Indonesia.

Text: Dolfi Debrot and Cecile Leuverink, Wageningen University & Research
Images: Faishal Umar,YAPEKA (lead photo: Dugong in the Sangihe Islands, North Sulawesi)