Monitoring illicit activities at sea with covert subsurface hydrophones (underwater listening devices that record sounds over a designated period of time).

Here at CSIRO, we are developing low-cost, simple units tailored to specific surveillance needs.



Sound has long been used to monitor marine activities. Existing technologies can log vessel sounds, allowing detection and warning of activities in important areas. However, challenges occur when the surveillance technology must be hidden, or provide real-time data in remote locations. We are using CSIRO’s expertise in marine engineering and mooring design to expand existing hydrophone equipment to covert applications.

Left: A typical hydrophone setup with an acoustic release with sound trap and external battery and a remote control deck unit. Right: An infographic of a typical hydrophone deployment unit

Range and applications

Subsurface hydrophones are designed specifically to detect dynamite fishing and different types of vessel activity to inform patrols. They can detect loud noises (e.g. dynamite) within 15 kms, and accurately identify vessel activities within a 2-5km range. Analysis of acoustic data shows the location of the vessel or activity and additional information that can be useful to improve surveillance efficiency, such as the type of vessel (e.g. inboard versus outboard engines) and movement (e.g. transit, loitering, stopping and starting behaviours indicative of certain types of fishing). Over time, the hydrophone data can provide managers with detailed information on the locations, times, and types of vessel activities in their area.

Hydrophones are typically deployed for three months to avoid fouling, although the battery may last longer. The acoustic release is rated to 400m depth. The units have been successful even in strong currents and can be deployed and retrieved without a winch. Each unit is less than 20kg shipping weight, and includes all necessary materials apart from the mooring weight.


Preparing a hydrophone unit for deployment



The acoustic files are large, so data is either moved via a hard drive, or we provide users with the analysis software we have developed. We can then run analysis remotely and provide outputs. Additionally, we have tools for users to do a preliminary visual exploration of the sound data themselves. Many sounds are easily identifiable (e.g. dynamite or large vessels).

Example of data output from a covert hydrophone operation

Machine learning based algorithms for sound event detection are currently being developed. A brief overview of how this system works can be seen in this video.

Real-time covert hydrophones

We have developed a hydrophone prototype that can send alerts in near real-time to help patrols and enforcement of illicit activities. Certain types of noise (e.g. a vessel or a detonation) recorded on the hydrophone will trigger a message to an external microcontroller. A buoyancy-controlled antenna moves to the surface and transmits the information via an Iridium satellite. The messages can then be sent to phones, web servers, or emails.

Infographic showing the process of monitoring with real-time hydrophones

Real-time alert

Hydrophone lending library

We currently have a number of hydrophone units available for loan for up to a year, with shipping being the only cost to the users. We hope to be able to provide longer-term or permanent units in the future.
The current units function like other subsurface acoustic telemetry stations. The recording unit is attached to a weight for deployment, and retrieved using a remote to trigger the acoustic release.


Download our hydrophones factsheet here