How is sound used to detect illegal fishing?

Illegal fishing is a global problem which includes prohibited and often unreported activities that endanger fish populations, such as species that are protected (threatened or endangered), overfished, fish from protected regions, and fish caught by methods destructive to marine habitats. This also contributes to food insecurity and unstable local economies. Up to 33% of the annual global fish harvest, approximately 11–20 billion kg of fish, are caught illegally (Kalaiselvi et al., 2022). In addition, fishing practices used in illegal fishing, including blasting and dredging, further disrupt the undersea environment and impact other marine life. Detecting illegal fishing can be assisted by the use of modern technologies including underwater acoustics.

Real-time fishing vessel information is available from Automatic Identification System (AIS), a global vessel-tracking system that provides real-time movement and location of ships in harbors and ports (Cheng et al., 2023). The AIS data is gathered by satellites and terrestrial receivers. It is analyzed by The Global Fishing Watch (GFW) organization. It is important to note that an AIS is dependent on navigational and/or electronic equipment, so these systems have limitations. The accuracy of AIS information received is only as good as the accuracy of the AIS information transmitted. Also, people engaged in illegal fishing may turn off the vessel’s transponder so as to not be detected.

Passive Acoustic Monitoring (PAM) sensors can also be placed in areas of concern, such as along coastlines or in marine protected areas to record underwater sounds and are used to determine if fishing regulations are being violated. PAM can detect sounds generated by fishing vessels for which the acoustic signature of vessel types is known. By combining PAM, AIS, and temporal data, the position of a vessel can be determined and used to alert law enforcement of potential illegal activity (Kline et al., 2020). In lieu of satellite-derived spatial information, PAM can provide estimations of fishing vessel presence, as well as provide continuous monitoring over long periods of time. Also, PAM can provide regulators with data about the acoustic environment (Kline et al., 2020).

PAM can be used to detect illegal fishing activities such as blast fishing. Although blast fishing is prohibited in most countries it persists in many coastal regions, especially those in developing nations (Chan and Hodgson, 2017). The blast fishing produces a shock wave releasing a large amount of energy in a short period of time. The shock wave travels faster than the speed of sound in water, attenuating over distance and time. This shock wave can injure nearby fish causing them to come to the surface to be easily collected.

Example pressure-time curves for underwater blasts. (A) Idealized curve. Adapted with permission from Gaspin JB, Goertner JA, Blatstein IM (1979) The determination of acoustic source levels for shallow underwater explosions. The J. Acoust. Soc. Am. 66:1453–1462. [92] Copyright 1979, Acoustical Society of America. (B) Example curve for an underwater blast with surface and bottom reflections. A: ambient pressure. B: peak positive overpressure. C: onset of pressure reduction from surface rarefaction wave. D: secondary peak from bottom reflection.

When the shock wave amplitude approaches ambient pressure, the wave becomes an acoustic wave traveling at the speed of sound. The resulting acoustic signal has a characteristically large peak value in sound pressure that slowly decays, which PAM can detect. This signal, along with associated gas bubbles that form during the blast, can be used to easily distinguish blast fishing from other underwater sound sources (Malige et al., 2018). A 2016–2018 study off northeastern Brazil, where blast fishing has been prohibited since 1967, provided the first acoustic record of underwater explosions from blast fishing in the region. Using a ship-deployed hydrophone, researchers recorded a distinct blast fishing signal and described the signal’s acoustic features (Melo-Souza et al., 2024). This work has enhanced the ability to detect illegal blast fishing world-wide using passive acoustics.