Tenghoi's Fish Bomb Detector

Kevin Pang of WWF fixes the hydrophone cable bundle to the piles of the Marine Life Centre at Hoi Ha Wan.Kevin Pang of WWF fixes the hydrophone cable bundle to the piles of the Marine Life Centre at Hoi Ha Wan.In November 2006 Teng Hoi Conservation Organization and WWF Hong Kong agreed to place a blast detection station at the WWF's Marine Life Centre in Hoi Ha Wan.

The detector was planned to operate for a trial period of 4 months as the centre is scheduled to be refurbished starting in May 2007. As well as keeping a listening ear out for blasts in nearby Hong Kong waters, the operation of the detection station has provided an invaluable opportunity in checking the functionality of the detector and improving Teng Hoi's blast detection software prior to a full pilot project in Sabah, Malaysia. The detection station was dedicated to Professor Ridzwan of Universiti Malaysia Sabah, who is unfortunately ill at present.

The Design and Manufacturing Facility (DMSF) at Hong Kong University of Science and The framework is lowered into the water.The framework is lowered into the water.Technology (UST) donated three hydrophones and associated electronics as well as the underwater framework for mounting the hydrophones. The hydrophones cables were installed at the centre by lowering them through a hole in the Marine Life Centre platform and tying them to one of the centre's piles.

Coral Monitoring students from Li Po Chun United World College helped to erect the detector framework and lower it into the water below the Marine Life Centre on 18th November 2006. Four teams of students dived to correctly place the framework on the seabed and then fix the hydrophones into the mounts.


RESULTS

The vertical scale of the map is about 20 km.The vertical scale of the map is about 20 km.

The installation of the Fish Blast Detection Station at Hoi Ha Wan was successful in allowing Teng Hoi to check the detection system and upgrade its blast detection software.

Over the course of three months of continuous monitoring, eight blast events were recorded. The detection station uses an array of three hydrophones to determine the direction of the blast sound. However, because there is only one detection station, we cannot determine the location of the blast. We only know that it occurred somewhere along the direction line. See the map below for details.

The results from the detection station have indicated much more blasting activity than anyone suspected in Hong Kong. Discussions are now under way with the Hong Kong government about how we should follow it up.

Teng Hoi and WWF believe there should be a detection system deployed in Mirs Bay at Tung Ping Chau.


NOISY ENVIRONMENTs - the Snapping shrimp issue

Snapping shrimp create a lot of background noise underwater.Snapping shrimp create a lot of background noise underwater.The detector is designed to handle noisy environments whilst remaining sensitive to the sounds of distant blast events.

Sources of noise local to the detector include WWF's glass bottomed boat, nearby fishing boats and snapping shrimp. Snapping shrimp are only a few centimeters in length and are common in tropical and subtropical seas. They generate a lot of reef noise by using a specially adapted claw to create a clicking sound.

Remarkably, the shrimp's claw creates a small underwater explosion and when a shrimp clicks close to the detector, Teng Hoi's soft
ware must analyse the sound and determine if it was a nearby shrimp or a distant blast event. Typically the software detects 3-5000 shrimp clicks per day. By using a number of analytical techniques, the software has proven very reliable in distinguishing blast events from shrimp clicks. For the human ear, the difference seems quite obvious.

The shrimp click above was a big one isolated from the background sound. Take a listen to the background reef sound (1.6 MB) so you can appreciate just how many shrimp there are and how noisy the environment is in Hoi Ha Wan. The shrimp clicks combine to give an effect that sounds like radio static.



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