Post provided by Xavier Mouy
Fish sounds and marine conservation
Many fish species produce sounds to attract mates, scare away predators or defend their territory. These sounds are very useful to us, scientists! Just by listening to the ocean, we can detect the presence of different fish species, infer their behaviour and potentially find out how many fish there are in an environment. This has many advantages over traditional monitoring methods. It is completely non-intrusive (we are listening passively), we can record sound over long periods of time (months or even years), is not affected by light conditions (day/night), and we can record over large areas. This approach is attractive for marine conservation and fisheries management objectives as it can substantially improve fish monitoring efforts in sensitive habitats such as marine protected areas.
Using fish sounds to monitor fish has a lot of potential but it is currently limited by the fact that many fish sounds have not been linked to specific species. Historically, we have known since the 4th Century BC that fish produce sounds but we still know very little about the different types of sounds fish make and what they use them for. Currently, we know that about 1000 species of fishes worldwide produce sounds. However, it is very likely that many more species produce sound, but have yet to be recorded and identified. This knowledge gap is, in part, because there is no readily available instrumentation capable of easily identifying sounds that fish produce in their natural habitat. In our paper, we propose a set of instruments that addresses this important research need.
Recording fish sounds in the wild is key
Recording sounds from different fish species could seem trivial: we just need to put fish in tanks and record them with a hydrophone. Unfortunately, the reality is rarely that simple. While this can sometimes be successful, many fish do not produce sounds outside of their natural habitat and if they do, the recorded sounds are often distorted by the complex acoustics of the tanks. So, we need to record them in the wild. This requires hydrophones to record the sounds and cameras to visually identify the species making them. But simply recording sounds is not sufficient. We need to localise precisely where the sounds are coming from to make sure the fish seen by the camera is the one producing the sound.
Design of portable audio-video arrays
To achieve this, we developed autonomous audio-video arrays composed of multiple hydrophones and video cameras that are deployed in the ocean. By measuring the time-difference of a sound’s arrival at each pair of hydrophones, we can localise fish sounds in 3D. The sounds localised with the arrays are then matched with the video recordings to identify the species and behaviour of the fish in a given video frame. One of the first challenges during the development of these audio-video arrays was to find a camera system that was autonomous, low-cost and could record videos for several days. We could not find off-the-shelf video cameras that fitted our requirements, so we designed our own. After a few iterations we had a video camera system that could record for several days and integrate with our arrays. The choice of acoustic recorders was easier as off-the-shelf products were available. We used two different models: a 6-channel AMAR recorder from JASCO Applied Sciences and a 4-channel SoundTrap from Ocean Instruments. We packaged these instruments on three different platforms: a large array, a mini array, and a mobile array. The large and mini arrays are static autonomous platforms that can be deployed on the seafloor and record audio and video for one to two weeks. The mobile array is mounted on a remotely operated vehicle with built-in video which allows remote control and real-time positioning in response to observed fish presence. Each array was designed with different constraints in mind. The large array is configured for the most accurate 3D acoustic localisation, the mini array for easier deployments in on rough/uneven seafloors, and the mobile array for dynamic real-time spatial sampling over shorter time periods (hours rather than days or weeks). These three designs allowed us to identify fish sounds in a variety of habitats. For some backstories on the development process of these platforms, we encourage the reader to explore our Fish Sound Project Blog.
New discoveries and implications for marine conservation
We deployed these three arrays at four locations off British Columbia, Canada and identified, for the first time, sounds from quillback rockfish (Sebastes maliger), copper rockfish (Sebastes caurinus), and lingcod (Ophiodon elongatus). These newly identified sounds are of high interest for fish conservation. In British Columbia, quillback rockfish are designated as threatened by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) and the population of lingcod found in the Strait of Georgia is assessed as in the cautious zone under the Precautionary Approach framework outlined by Fisheries and Oceans Canada. While more data are needed to fully describe their repertoire, these results show that passive acoustics has a high potential to improve the monitoring and management of these important fish species.
How far can we hear fish sounds?
This is an important question that we are often asked but cannot always answer. It is important because knowing how far we can detect fish sounds defines how many hydrophones are required to monitor a given environment. Since we could accurately localise fish sounds in 3D with the audio-video arrays, we were able to measure how loud the fish were (i.e., the source level) to estimate how far away from the hydrophone fish sounds can be detected. Quillback rockfish sounds can be detected at up to 33 m in a relatively quiet environment (Hornby Island) but only up to 10 m in a noisier environment (Mill Bay, close to a marina). These detection range valueswill be important to take into account when monitoring this species in the future.
Accessibility and scalability
Now that we have shown that these arrays can identify and characterize fish sounds in the wild, we need to make these instruments more easily accessible so they can be built and deployed on a larger scale in other parts of the world where passive acoustics can help monitor fish populations (e.g., coral reef ecosystems). Alongside our paper we provided detailed building instructions for the audio-video arrays as well as processing scripts and notebooks to perform the 3D localisation. Future efforts are needed to make these instruments less expensive and more user-friendly so that non-experts (e.g., hobbyists, divers, schools) can use them on a global scale to expand the worldwide catalogue of fish sounds.
You can ready the full article Identification of fish sounds in the wild using a set of portable audio-video arrays here!
Methods Blog 