Value of small mangrove patches in the Pearl River estuary to commercial fisheries

Marine Ecology Enhancement Fund (MEEF), Hong Kong

1

Introduction

We strive to understand the role of small-scale mangrove patches as a nursery site for commercially important fish during the juvenile stage of their lives.

 

In particular, we are interested to study how complex mangrove root structures can protect juvenile fish by giving them visual cover from predators. To test this hypothesis, we are utilizing 3D printing technology on small patches of mangrove forests, allowing us to closely observe fish behaviour in the presence of a predator in the laboratory using underwater video cameras.

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From inside the mangrove forest at Ting Kok

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Boardwalk among the mangroves at Mai Po

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Inside the mangrove forest at Ting Kok

2

Mangrove Fish Use

Some species of fish, such as the predatory yellowfin seabream (Acanthopagrus latus) and grey mullet (Mugil cephalus), spend their juvenile stage in mangrove forests during high tide. They use the complex mangrove structures for protection as well as a source of food.

 

We have used underwater video cameras and several types of nets to investigate the types of fish that utilize mangrove patches during high tide. Here are four of many types of fish we have encountered in Ting Kok, Shui Hau, and Lai Chi Wo in Hong Kong:

GoPro cameras are used to observe the types of fish that come into the mangrove forest with the rising tide

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3D Scanning

To further understand how juvenile fish use mangrove structures for protection, we scanned and 3D-printed life-size mangrove patches (1x1x1m) with various degrees of complexity for mesocosm experiments. We used a method called “photogrammetry”, which allows for automated reconstruction of mangrove trees from pictures we take using our smartphones. 

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Making 3D life-size models of mangrove roots

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3D Printing

The scanned objects were then sent to a vendor for printing. We currently are in the possession of three different types of models: one high-complexity Kandelia obovata, one low-complexity Kandelia obovata, and one mixed model of Kandelia obovata and Aegiceras corniculatum. The models were also coated with a non-volatile paint in a colour resembling natural tree surfaces.

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3D-printed life-size models of mangrove roots

5

Acquiring Fish

We purchased our juvenile predator fish from local wet markets. The target species we look for are yellowfin seabream (Acanthopagrus latus), red mangrove snapper (Lutjanus argentomaculatus), and the Russell’s snapper (Lutjanus russelli). The prey fish we use were collected from the field using nets. Our target species is mainly the glassy perchlet (Ambassis gymnocephalus), but we are also on the lookout for Gerres sp., Terapon jarbua, and Mugil cephalus.

MEEF team members, J and Rinaldi, and Joe observing the snappers in the mesocosm

Two Lutjanus russelli with one Lutjanus argentomaculatus in the center

We collect our prey fish from the field using nets. Our target species is mainly the glassy perchlet (Ambassis gymnocephalus), but we are also on the lookout for Gerres sp., Terapon jarbua, and Mugil cephalus.

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Experiment

Each experiment consists of three stages that last one hour each day. On each stage, we place four cameras on top of our model and another four on the side of the tank to monitor the location of each fish: in the model or outside the model. 

Four GoPro cameras are set up on top of the model to observe fish behaviours among the mangrove roots

On day one, we transfer 20 prey fish into the mesocosm. We want to allow the prey fish to get used to the new environment before we include predators in the mesocosm.

 

The next day, we transfer two predator fish into a holding area in the tank. The two areas are separated by a porous piece of black plastic cloth, which allows for the diffusion of scent and chemical cues but not visual cues.

 

On the final day, we release the predator from the holding area, allowing the juvenile fish to be chased and preyed upon. From the videos we collect, we would then count prey mortality and study their behavior, e.g., how they use mangrove structure to hide or escape from the predators.

GoPro cameras are used to observe fish behaviours among the mangrove roots

We have so far conducted one trial experiment, which resulted in no predation event. We are working on getting more predators from the wet market, so we can continue to conduct more experiments. We hope that our research project can contribute to the future management of the globally threatened mangrove ecosystems, especially the relatively small patches typical of peri‐urban coasts.