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An  international  team  led  by  researchers  from  the University  of Liège used underwater audio recordings for understanding fish diversity in the Mediterranean Sea.  The research team included a MSCA-COFUND postdoctoral fellow (Dr. Marta Bolgan) based in the European leading laboratory in the field of functional morphology of fish sound production mechanisms (MORFONCT; Prof. Eric Parmentier) and a long list of international collaborators. This MSCA COFUND fellowship was called BEIPD, which means “BE an International PostDoc” and the fellow had surely taken this literally; she has travelled all around Europe for collecting her data, proving that international collaborations are the lifeblood of a successful (and joyful) research career.


The BEIPD MSCA COFUND fellow, Dr Marta Bolgan, while auditioning fish in the wild. Because we must know how each fish species….sounds!

Oceans have been long assumed to be quiet, as shown by the title of Jacques Cousteau’s 1956 movie “The Silent World”. Nothing but a silent world, oceans are actually filled with numerous sound sources: abiotic sources generate geophonies, sounds emitted by living organisms generate biophonies and man-made noise generates anthrophonies[1]. All together, these sounds form the acoustic environment (soundscape)[1]. Each aquatic habitat is characterised by a unique, localised soundscape, where sonic sources may consistently change over relatively short periods and geographical scales [2-4]. Each localised soundscape conveys a complex sonic narrative loaded with critical information for the survival of any sentient animal which inhabits it[2][5-7].

Many species of fish produce sounds (alone or in chorus) under a variety of conditions, such as when engaging in reproductive activities, defending territory or offspring, competing for food, responding to threats, synchronising mating, calling for conspecifics or as a by-product of their activities[8] (see sound libraries of both MORFONCT and CHORUS) Fish have evolved the largest diversity of sound-generating mechanisms among vertebrates[9]. This in its turns results in a wide diversity of sounds emitted by different species, which features are species-specific in most cases. In other words, we should be able to recognise different fish species only on the basis of their calls, exactly as many naturalists identify birds by listening to their songs. Listening to fish calls, we can achieve information about which species is present in a specific environment and where and when it prefers to aggregate[10].


Prof. Eric Parmentier, head of the Laboratory of Functional and Evolutionary Morphology (MORFONCT) at the University of Liège and supervisor of Marta Bolgan. Prof. Parmentier is implementing new technologies for listening to fish in the deep-sea.

Passive acoustic monitoring (PAM) involves the use of hydrophones to receive and record all the components of underwater soundscapes, including fish calls[9]. PAM represents a non-invasive way to assess temporal and spatial patterns of distribution of calling individuals. Several studies used PAM to investigate different aspects of vocal fish populations, such as presence, distribution, relative abundance, diel, lunar and seasonal cycle of activity as well as for delimitating spawning areas and for studying wild fish spawning behaviour [11-21]. Since sound emission is associated with reproduction in many species, this method became especially useful for monitoring spawning sites at both temporal and geographical scales, and is now considered a powerful tool for conservation studies[15-16][22-23]. The knowledge that can be gathered using PAM is indeed of fundamental importance for monitoring vulnerable or commercially important fish species, for determining appropriated fishing periods as well as for informing effective conservation plans, by protecting the animals during their reproductive season. Furthermore, PAM can provide critical information for understanding the impact of human activities on fish populations, such as how anthropogenic activities (fishing, pilling, wind farm, etc) influence fish vocal behaviour, displacement and activities [24-26] or how climate change impacts on biodiversity[7]. Natural sounds collected using PAM, especially those from vocal animals, can be used as proxies to learn about the diversity of species, habitat quality and the phenology of biological events.

Eric presentations

Figure 3- Prof. Parmentier explains that…size matters! Fish sounds can indeed contain information about the size of the singer. Presentation given to a group of high school students during summer classes activities at STARESO research station (Corsica, France).

The BEIPD fellowship “Sonata for Teleosts: fish sounds as proxies to learn about the diversity of species” aimed to address three work packages:

  • Work package 1: SOUNDS CATALOGUE of FISH INHABITING the CALVI BAY (Corsica, France);

Work package 1: Audition of recordings previously collected by MORFONCT included recordings collected at -40 m in sandy habitats in front of STARESO research station (Calvi Bay, Corsica, France). This analysis, which was carried out during the initial phases of the BEIPD, was aimed to catalogue fish sound types and to test recent, commonly applied automated procedures for the analysis of large acoustic datasets. This work resulted in two international peer-reviewed publications; the first describes the fish vocal community at -40 m (July) in sandy habitats of the Calvi Bay and tests the performance of the Acoustic Complexity Index to provide information about fish vocal dynamics [27].

40 m depth

40 m depth2

Figure 4- The fish vocal community at 40 m depth in sandy habitats in front of  STARESO research station. These results were presented at the STARECAPMED workshop in April 2018.


On the left, Dr. Marta Bolgan (BEIPD fellow, ULiège, Belgium) and, on the right, Dr. Marta Picciulin (Italy). The two MartaS used PAM to reveal the presence of a cryptic fish species in the Trieste Gulf (Italy)

The second publication was carried out thanks to international collaboration and data sharing. In particular, MORFONCT had previously characterised in details the sound types, the vocal dynamics as well as the morphology of the sound producing apparatus of the cryptic fish species Ophidion rochei [28-32]. A sub-set of O. rochei sounds recorded by MORFONCT was used for comparison with previously collected recordings of Dr. Marta Picciulin in the Trieste Gulf (Italy). This comparison allowed to identify the presence of O. rochei thanks to its calls in the Marine Protected Area of Miramare (Italy), an area in which visual census of the fish fauna has been carried out for decades but the presence of O. rochei has always gone unnoticed [33]. We cannot see it….but we can hear it!

During the BEIPD, new acoustic data have been collected in the Calvi Bay, specifically at -20 m in Posidonia oceanica meadows and at -140 m, at the head of a submarine canyon. This latter dataset was added to that collected at similar depths in different seasons by the Research Institute CHORUS; analysis is now concluded and a manuscript is almost ready for submission. To the best of our knowledge, this represents the first description of potential fish sounds recorded in Mediterranean waters below 100 m depth.  Altogether, these studies will soon allow to produce a catalogue of fish sounds recorded in different environments of the Calvi Bay.

Work package 2:  Although initial aims were restricted to the Calvi Bay (Corsica, France), the geographical scale of the investigation has been enlarged, focusing on one of the least described Mediterranean vocal fish community, i.e. the one inhabiting the endemic environment of  Neptune seagrass (Posidonia oceanica) meadows. In particular, this work will shed light on how different vocal fish species share the same acoustic space. During 3 months of consecutive fieldwork, Dr. Bolgan used a combination of vessel-based Passive Acoustic Monitoring (PAM) and Static Acoustic Monitoring in the Tyrrenian (Calvi, Corsica), Balearic (Mallorca, Spain) and Aegean Sea (Crete, Greece). A true acoustic Odyssey! The underlying hypothesis were; i) the vocal community of P. oceanica is composed of different sound types which shows frequency and temporal partition; ii) some sound types are present along the entire Mediterranean axis, where their sound features variation is related to environmental conditions and taxonomic diversity; iii) some other sound types are present in one site only (“acoustic endemism”); 4) the highest number of acoustic endemisms may be found in Crete, potentially highlighting Lessepsian migrations. Analysis of these recordings is ongoing and it is carried out in collaboration with CHORUS Research Institute. A manuscript will be soon ready for submission.

STARESOCAPMED200318final-full lenght

Figure 8- Some preliminary results. Each of these long-term spectrograms represents a month of acoustic recordings collected at 20 m depth in three Neptune seagrass meadows along a Mediterranean axis (Mallorca, Corsica and Crete). Although depth and environments were the same, it is easy to appreciate that each site is characterised by a unique acoustic signature.

Work package 3:  PAM is dependent on the evidence that sounds recorded in the wild actually belong to a specific species. Evidence for the identity of the species that produces a particular sound is often obtained by comparing the remotely recorded PAM sounds with known sounds recorded either in captivity, or, preferably whenever possible, in the field with in situ methodologies. Furthermore, if a precise relationship between sound characteristics and spawning can be found in specific species, this could greatly contribute to the conservation of these species, as we could be able to locate and to protect their spawning areas thanks to a completely not invasive methodology such as PAM. In this context, three investigations have been carried out;

  • Together with a Master student, Miss. Justine Soulard, the identity of the unknown species emitting the most abundant fish sound in Neptune seagrass meadows, (the so-called “Kwa”) has been investigated by using an inter-disciplinary and multi-approach. A publication is almost ready for submission.
  • Together with another Master student, Miss. Aurora Crucianelli, the potential of Mediterranean fish sounds to provide information about fish status, readiness to spawn and reproductive success has been investigated in fish species of high commercial value (Sciaenidae spp). This investigation was carried out thanks to data previously collected by MORFONCT and in collaboration with HCMR (Crete). In particular, two Mediterranean Sciaenidae species (Argyrosomous regius and Umbrina cirrosa) are housed at HCMR Crete where breeding is induced and monitored. A publication is envisaged by early 2019.
Io e le fie

The BEIPD fellow (in the centre) together with the Master students Aurora Crucianelli (on the left) and Justine Soulard (on the right) at their graduation ceremony. The co-supervision experience was for Dr. Bolgan one of the most learning dense experience of her BEIPD.

  • MORFONCT has long-term experience in the recording and characterization ofsound production mechanisms in Ophidiiformes. However, sounds recordings and morphological investigations are currently lacking for one Mediterranean species, Parophidion vassalli. During the BEIPD, some specimens of Parophidion vassalli have been recorded in captivity at MORFONCT and morphological examinations will be soon carried out. Furthermore, Passive Acoustic Monitoring has been carried out where these specimens were fished; the hypothesis is that Parophidion vassalli can be located in the wild thanks to the sounds recorded at ULiege. The analysis is ongoing.

In synthesis, many questions have been and are still tackled during “Sonata for teleosts”. The supervisor Prof. Parmentier has provided Dr. Marta Bolgan with the perfect intellectual environment for deepening her knowledge of fish sound production (from mechanisms to diversity) and for expanding her range of skills in Passive Acoustic Monitoring (from coastal to deep waters). During the BEIPD, Dr Marta Bolgan had the possibility of strengthening previous international collaborations and to initiate new ones. International, collaborative scientific networks permit to avoid to replicate results between research groups and allow for a more effective questioning- answering around biological questions, which is the core of biology research. As in nature, everything is connected to everything, and the most diverse environments are the most resilient and productive, so researchers in different field of biology gain amazing benefits by working in collaborative networks and by sharing their knowledge.

international collaborators

International collaborations are the lifeblood of a successful (and joyful) research career. From left to right; Dr. Lucia Di Iorio of the CHORUS Research Institute (France), expert in underwater soundscape decomposition and interpretation; Prof. Clara Amorim of Lisbon University (Portugal), expert in the behavioural significance of fish sounds; Dr. Marta Bolgan, BEIPD fellow at MORFONCT (ULiège, Belgium) and Prof. Paulo Fonseca of Lisbon University (Portugal), expert in the physiology underlying acoustic communication in fish and insects.

its time consuming

Acoustic analysis is extremely time-consuming! Fish sounds must be selected, counted and identified in each acoustic track. This figure represents 5 min of acoustic recording (only!) and each blue box was manually made by Dr. Bolgan around each fish sound….she took almost 1 hour for counting all fish sounds in 5 min of recording!

Concluding, it has to be considered that the analysis of acoustic datasets is an extremely time-consuming task, and therefore some of the most exciting findings of this BEIPD will be published in the next years. Stay tuned 😀