Environmental constraints drive the partitioning of the soundscape in fishes

The underwater environment is more and more being depicted as particularly noisy, and the inventory of calling fishes is continuously increasing. However, it currently remains unknown how species share the soundscape and are able to communicate without misinterpreting the messages. Different mechanisms of interference avoidance have been documented in birds, mammals, and frogs, but little is known about interference avoidance in fishes. How fish thus partition the soundscape underwater remains unknown, as acoustic communication and its organization have never been studied at the level of fish communities. In this study, passive acoustic recordings were used to inventory sounds produced in a fish community (120 m depth) in an attempt to understand how different species partition the acoustic environment. We uncovered an important diversity of fish sounds, and 16 of the 37 different sounds recorded were sufficiently abundant to use in a quantitative analysis. We show that sonic activity allows a clear distinction between a diurnal and a nocturnal group of fishes. Moreover, frequencies of signals made during the day overlap, whereas there is a clear distinction between the different representatives of the nocturnal callers because of a lack of overlap in sound frequency. This first demonstration, to our knowledge, of interference avoidance in a fish community can be understood by the way sounds are used. In diurnal species, sounds are mostly used to support visual display, whereas nocturnal species are generally deprived of visual cues, resulting in acoustic constraints being more important. Significance More and more studies stress the potential deleterious effect of anthropogenic sounds on fish acoustic communication. Paradoxically, how the communication between fishes in a community is organized remains extremely poorly known, as studies using passive acoustic recordings are typically restricted to one or two species. At a single site, we were able to follow 16 different vertebrate sounds for 15 days. We demonstrate that the fish population can be distributed into two groups: one diurnal and one nocturnal. Most interestingly, fish calling at night do not show overlap at the level of the main calling frequency, in contrast to fish calling during the day. This shows that at night, in the absence of visual cues, sound communication is more important.