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Marine Ecology Enhancement Fund – Study on Value of Peri-urban and Small-scale Mangrove Forests in the Pearl River Estuary as Fish Habitats

18  April  2022

Professor Joe Lee, Director of Simon F. S. Li Marine Science Laboratory of The Chinese University of Hong Kong
Conventional field trip
Actual physcial mangrove model
Constructing 3D digital models from mangrove photos using photogrammetry
The research team installs actual physical mangrove model in the laboratory's experiment tanks to mimic mangrove habitat
Mangrove Jack
Glass Perchlet
Underwater cameras capture various behaviours of fish

Mangrove forests are an important ecosystem in both Hong Kong and the Pearl River Estuary, with their complex structure playing a vital role in it. The Marine Ecology Enhancement Fund (MEEF) established by Airport Authority Hong Kong (AAHK) funded a research team from The Chinese University of Hong Kong (CUHK) to study the ecological value of mangrove forests as habitats for juvenile fish. The team adopted a first-of-its-kind research approach, by combining the use of photogrammetry and 3D printing.

Professor Joe Lee, Director of Simon F.S. Li Marine Science Laboratory of CUHK, recalled that in the past, mangrove field trips and research was time-consuming and involved heavy investment in manpower and resources, yet ineffective. “Mangrove forests are usually remote and difficult to access, let alone conducting experiments, especially when you need to carry large equipment for research. Moreover, the fragmentation of mangrove forests in the Pearl River Estuary coupled with the declining number of large predators, caused natural predation events in this area to become rare phenomena. We have to try our luck to encounter the species of fish that is the target of our study to appear right near that tree, the chance is rather slim. Thus, it is difficult and ineffective to conduct this study with traditional field trips,” said Professor Lee.

Apart from field trips, researchers in the past chopped off roots of mangroves and brought them back to the research centre for studying, which caused permanent damage to mangroves. PVC and iron pipes, as well as wooden strips were also used in some researches to mimic mangrove structure. However, Professor Lee opined that this was a relatively crude method, and the findings provided little explanation of the dynamics of fish in the wild.

The research team from CUHK hence came up with a new research approach combining photogrammetry and 3D printing technology to tackle challenges in studying mangrove forests. Six mangroves with different structural complexity and intensity were selected from Ting Kok, Lai Chi Wo, and Shui Hau. Researchers took over 3,000 photos of the mangroves at the site and reconstructed the photos into 3D digital models with computer software. Three 3D models of the actual mangroves in 1:1 ratio were replicated by 3D printing and installed in the laboratory’s experiment tanks to mimic mangrove habitat.

The team selected Mangrove Red Snapper as predators, and Glass Perchlet and Grey Mullet as prey. They were placed in experiment tanks and underwater cameras recorded activities of the fishes. Numerous experiments were conducted to study association between predatory behaviours as well as survival rate of fish, and the complex habitat created by mangroves.

“Photogrammetry and 3D printing allow us to study fish ecology of mangrove forests under a simulated and controllable environment. We are able to transfer mangrove in the wild to an experiment tank with 3D models of up to 95% accuracy, so that we can carefully observe the correlation between mangrove structure and predator-prey relationship, and at the same time, adjust critical factors such as fish species, habitat complexity and water level in the experiments if necessary.

In the 30-month research, it was concluded that even small-scale mangrove forests possessed important ecological value. Meanwhile, the more complex the mangrove structure, the higher survival rate of juvenile fish. Researchers observed from footages that the structure of mangroves would impede the predators’ pursuit of the juvenile fish while the latter would use the structure to hide. When the water level is low, individual juvenile fish on their own could use mangrove roots to hide from predators, hence the fish are scattered. Yet, when the water level rose above and away from mangrove roots, the juvenile fish swam in schools with quick movements to increase survival rate. Nonetheless, the mangrove structure not only benefits juvenile fish. Researchers observed that predators would ambush their prey using the mangrove structure. These scenarios were difficult to observe in conventional field trips.

Professor Nora Tam, Chairperson of the MEEF Management Committee, said, “The project adopted innovative technology to study the ecological value of mangrove forests. The committee supports this project as mangrove forests play a vital role in marine ecology.”

Professor Tam was also satisfied with the project result. “The footage obtained from the experiments simply demonstrates the symbiotic relationship of mangrove forests and fish, which helps promote future education and conservation projects. The brand new research method advocated in the project can also encourage more mangrove forest-related research to be conducted by scientists to further study their role in the ecosystem, that helps formulate relevant mangrove forests management measures,” added Professor Tam.

Peter Lee, General Manager, Sustainability of AAHK, said, “The project result facilitated technical skill exchanges in research amongst scholars, environmental groups, as well as stakeholders, while providing useful information for the sector to discuss conservation strategies in depth.”

Since the establishment of MEEF, around HK$ 30 million has been granted to 25 projects. The fund will continue to welcome project applications to enhance marine environment in the western waters of Hong Kong and the Pearl River Estuary to benefit marine ecology.

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