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.

Spectrogram of the explosion recorded in the Baia de Todos os Santos, Northeastern Brazil (From Melo-Souza et al.)

Another acoustic tool that has shown promise for detecting illegal fishing is the use of passive acoustic telemetry with tags. These tags transmit identifying acoustic signals from tagged fish to receivers. Researchers use acoustic tags to monitor fish populations and behaviors, as well as those of other marine species.

A study by Tickler et al. (2019) demonstrated that acoustic tag data could be used to estimate the number of sharks taken illegally in the British Indian Ocean Territory (BIOT) Marine Protected Area (MPA). From 2013 through 2014, the scientists inserted acoustic tags in 47 gray reef sharks (Carcharhinus amblyrhynchos) and 48 silvertip sharks (Carcharhinus albimarginatus) near BIOT MPA reefs. These shark species remain in their usual habitat throughout the year. Using passive acoustic telemetry, they documented that 2.6 ± 1.0 tags per month over 21 months (March 2013 to November 2014) could no longer be detected. However, in December 2014, 15 of the remaining 43 active tags were lost over just ten days. During this time, BIOT patrols caught 17 vessels illegally fishing in the MPA. Two of the fishing vessels were found with a total of 359 sharks, of which gray reef and silvertip sharks constituted 47% of the catch. The corresponding loss of tag detections and the evidence of illegally caught sharks suggest that the use of acoustic tag data may be an effective tool for detecting the presence of illegal fishing.

The use of acoustic technologies to monitor marine environments can provide evidence for suspected illegal fishing, such as in blast fishing. This can allow for rapid enforcement of regulations and assist in the sustainable management of fisheries across the globe.

Resources

  • Chan, A. and Hodgson, P.A., 2017, February. A systematic analysis of blast fishing in South-East Asia and possible solutions. 2017 IEEE Underwater Technology (UT), Busan, Korea (South), 2017, pp. 1-6, DOI: 10.1109/UT.2017.7890330.
  • Cheng, X., Zhang, F., Chen, X. and Wang, J., 2023. Application of Artificial Intelligence in the Study of Fishing Vessel Behavior. Fishes, 8(10), p.516. DOI: 10.3390/fishes8100516
  • Kalaiselvi, V.K.G., Ranjani, J. and SM, V.K., 2022, March. Illegal fishing detection using neural network. In 2022 International Conference on Communication, Computing and Internet of Things (IC3IoT) (pp. 1-4). IEEE. DOI: 10.1109/IC3IOT53935.2022.9767876.
  • Kline, L.R., DeAngelis, A.I., McBride, C., Rodgers, G.G., Rowell, T.J., Smith, J., Stanley, J.A., Read, A.D. and Van Parijs, S.M., 2020. Sleuthing with sound: Understanding vessel activity in marine protected areas using passive acoustic monitoring. Marine Policy, 120, p.104138. DOI: 10.1016/j.marpol.2020.104138
  • Malige, F., Patris, J., Buchan, S.J. and Glotin, H., 2018, June. Acoustical analysis of submarine explosions in northern Chile on long terms continuous recordings. In DCLDE 2018. DOI: 10.13140/RG.2.2.26498.53441
  • Melo-Souza, V., Gavrilov, A. and Rossi-Santos, M.R., 2024. Dropping a bombshell: Acoustic characterization of blast fishing in Todos os Santos Bay, Brazil, and its implication for marine conservation. Marine Pollution Bulletin, 202, p.116332. DOI: 10.1016/j.marpolbul.2024.116332
  • Tickler, D.M., Carlisle, A.B., Chapple, T.K., Curnick, D.J., Dale, J.J., Schallert, R.J. and Block, B.A., 2019. Potential detection of illegal fishing by passive acoustic telemetry. Animal Biotelemetry, 7, pp.1-11. DOI: 10.1186/s40317-019-0163-9