Article:Ruppé, Laëtitia, et al. “Environmental constraints drive the partitioning of the soundscape in fishes.” Proceedings of the National Academy of Sciences112.19 (2015): 6092-6097.
Most of us probably don’t think of fish as vocal animals, but in fact they are quite noisy (to find out about how fish actually make noise, check out this link). Fish use vocalizations and sound during mate attraction and in territorial battles to assert dominance (Hear what kinds of sounds toadfish make!). The combined noise produced by all organisms in the ocean can create an underwater soundscape (think of stepping into a forest and hearing the bird calls). Being in an environment like this often means animals have a hard time being heard, but there are a few strategies that allow these animals to communicate successfully without losing their voice. Imagine you are at a party trying to have a conversation with your friend; the room is loud as everyone seems to be having their own conversations. Often, we subconsciously start to talk louder to allow ourselves to be heard – this is called the Lombard Effect. The Lombard Effect is described as an increase in vocal intensity and has been exhibited by birds, mammals, and fish. Animals can also increase the length of their vocalization or increase the amount of noise they produce. These strategies, however, aren’t as helpful in a diverse soundscape; figuring out how to communicate in a diverse environment a greater challenge (Fig. 1).
To cope with background noise created by other species, animals need to insert their vocalizations in a way that makes them distinct, or signal interference avoidance. Signal interference avoidance could include any strategy that ensures sounds made will not be lost in the noise, this could be accomplished by changing when noise is made or how that noise is made (Fig. 2). Competition for acoustic space leads to changes in the timing and frequency of sounds as has been shown in birds and amphibians. While there has been research focusing on fish vocalizations and communication, these studies have only focused on single species. It is still unclear how fish vocalize as part of a larger fish community. Here, researchers set out to understand how, or even if, fish find their niche in the soundscape and avoid losing their vocal signals.
In the spring of 2013, researchers deployed an autonomous recording device off the coast of South Africa at 120 meters depth. The area they recorded was known to harbor a large diversity of fish species with a census highlighting the presence of 136 bony fish species and 19 cartilaginous fish species (like sharks and rays). Researchers recorded all of the sounds at this site for 19 days, recording in 9 minute intervals every 10 minutes using a passive audio recording device (Fig. 3). The sounds recorded were then analyzed for distinct acoustic parameters such as sound frequency, duration, and number of sound pulses. Researchers found almost 2,800 different sounds and then divided them into 17 groups based on sound similarity (Fig. 4). Based on the timing of sound groups, researchers were able to distinguish two larger groups: diurnal fish (who are making sound during the day) and nocturnal fish (who are making sound at night).
To understand if these fish have the ability to avoid signal interference, diurnal and nocturnal fish were broken down by sound and placed into one of the 17 sound groupings. There were 6 unique sound groups associated with both nocturnal and diurnal fish. The sound groups were then plotted by peak frequency (Hz) and pulse rate. Plotting sounds this way highlights any overlap in sound or sound avoidance. Researchers found an interesting pattern when looking at the results this way. They found that in diurnal species there was quite a bit of overlap between the sound groups in terms of both peak frequency and pulse rate, indicating no clear pattern and no avoidance (Fig. 5). Conversely, nocturnal fish exhibited sounds that were distinct in both peak frequency and pulse rate (Fig. 6).
This work shows that there are major differences in how fish handle the soundscape and it is correlated with the time of day they are actively vocalizing. Fish that are active in the day (diurnal) exhibited no strong signal avoidance with their sounds. This makes sense as visual cues play a strong role during daylight hours. As it turns out, many of the fish species at the study site have previously been shown to have vocal and visual cues tied together, thus being seen is equally important to being heard. As night falls, sound becomes increasingly important as a method of communication due to the loss of most visual cues. As such, nocturnal fish appear to have adapted a signal avoidance within their soundscape, changing both pulse rate and peak frequency in order to stand out from the noise. This research is the first indication that fish have developed this strategy in dealing with communication. The idea of understanding sound production by marine organisms is critical as we continue to flood these soundscapes with human noise, causing problems for many species. Human-produced noise, like that from boats, has the potential to drown out fish vocalizations and could cause major issues with communication. This research shows that fish have already figured out a way to be heard through the crowd, but dealing with noise from humans may be a whole new challenge (Fig. 7).