The first IUCN Red List of Ecosystems assessment of oyster reefs in Asia finds that more than 99% of these ecosystems have been lost in the Pearl River Estuary and Hong Kong.

Oyster reefs once formed extensive biogenic structures along the shores of Hong Kong and the Pearl River Estuary, the largest estuarine delta in southern China and the most highly developed in Asia.

Historical and archaeological records describe reefs with several metres of vertical relief as early as 906 CE, built by diverse co-occurring species of Magallana, Saccostrea, Dendostrea and Ostrea. However, intensive harvesting for food and lime production, followed by rapid population growth and coastal reclamation, led to the loss of almost all of this structure.

In the first such assessment of oyster reefs in Asia, a team of explorers, academics, and citizen scientists sampled Hong Kong oyster reefs and followed the IUCN Red List of Ecosystems risk assessment protocol. The result was that Hong Kong’s oyster reef ecosystem was assessed as Critically Endangered. Using historical ecology techniques, it was estimated that the ecosystem covered 3,029 km² in 1750. Recent survey data indicated that only 1.34 km² of functional reef remains today, a loss of 99.96%.

Reclamation and shoreline modification remain the leading pressures on the ecosystem. Artificial shoreline in the assessment area rose from ~46% in the late 1980s to >79% by 2021. Water pollution and marine heatwaves are expected to intensify, alongside increasing acidification and deoxygenation of nearshore waters, creating significant future challenges for restoration. Currently there is no legislative protection for oyster habitats in the Pearl River Estuary or Hong Kong.

The authors have also identified conditions that favour recovery. In spite of the lack of functional habitat, oyster recruitment at Hong Kong sites reaches hundreds of thousands of individuals per square metre and water quality has improved in some regions. This work uses IUCN standards and strengthens the case for protecting remaining reef patches, and for permitting pathways that would allow restoration to be attempted at ecosystem scales. It also adds to other IUCN Red List of Ecosystems assessments of oyster reef ecosystems around the world, highlighting the severe declines these ecosystems have experienced while also pointing to opportunities for their protection and recovery.

The published findings can be read here: https://www.sciencedirect.com/science/article/pii/S2351989426003227?via%3Dihub

 

Written by: Dr. Adam A. Janikowski

Style and format: Emy Miyazawa

Photos: Adam Francis