Acoustic Monitoring Reveals Scale of Destructive Bomb-Fishing off Sulawesi
Artisanal fishing is often thought of as a sustainable pursuit - low-profile, low-impact, and of enormous social benefit for coastal populations across the global south. But small-boat fisheries are not necessarily harmless: the artisanal method of dynamite fishing (also "blast" or "bomb fishing") can physically break up a living reef, leaving a debris field that lasts for decades. Bomb fishing is more popular than previously believed in some hotspot areas, according to a team of researchers from University College London, who recently recorded an average of one blast an hour in just one patch of Indonesian reef habitat.
Bomb blasts generate powerful underwater shock waves, which can be readily detected for long distances. Military-grade hydrophone technology can pick up the sound of a large blast from hundreds or thousands of nautical miles away under the right conditions. Simple, inexpensive devices can detect bomb fishing at a sdistance of up to roughly nine nautical miles in a reef habitat, a team led by UCL doctoral student Ben Williams found.
Williams' specialty is in designing underwater acoustic monitoring devices, along with AI systems for analyzing acoustic signals. Working with colleagues from the UK and Indonesia, Williams decided to "listen" for blasting in the waters of the Spermonde Archipelago, a reef hotspot off the western coast of Makassar in the Java Sea. The area is thick with coral reefs, and it has a fishing-dependent local population. It is known as a problem region for bomb fishing, but the exact extent was unclear. Williams and his colleagues emplaced a single hydrophone on a reef in the archipelago and periodically came back to retrieve recordings. Over the course of 16 months, they recorded about five months (3,650 hours) worth of sound.
Since it would be impractical to have a person listen through that much audio, the team used machine learning to create a tool that could detect the signal of an explosion. Automated analysis trimmed the task down to hours rather than months. On average, the hydrophone picked up 23 blasts every 24 hours, just shy of one blast per hour. Activity peaked in the mornings and trailed off on Fridays, the local day of prayer.
Bomb fishing is quite destructive, and the team estimates that the recorded events translate into a loss of about 30 acres of reef per year, just in one 250-square-nautical-mile patch of water. The same practice can be found in reef habitats around Southeast Asia and the rest of the world - but since bomb fishing has such a powerful long-range acoustic signature, a solution might be at hand, the team suggested.
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"Now it’s possible to automatically detect bombs with low-cost equipment, it opens the door for strides forward in monitoring and enforcement. This technology could really help in the fight to end bomb-fishing around the world," coauthor Tim Lamont told Divernet.
Top image: Fakhrizal Setiawan / CC BY SA 4.0
