Fast Pulse Train Sounds in a Noisy Coastal System: Structure, Distribution, and Environmental Controls

Pulsed sounds are widespread in marine soundscapes, yet they still lack systematic characterization and information on their emitting species. This study investigates the consistency and spatio-temporal distribution of Fast Pulse Train (FPT) sounds, an abundant sound type recorded in the Venice inlets during three acoustic campaigns conducted at 40 listening points in summer 2019 and 2020. Although FPTs show variability in their temporal and spectral features, clustering analyses revealed no clear separation among extracted signals, supporting the coherence of the current classification; however, when focusing on the more characteristic sounds, three slightly acoustically different groups emerged. The role of temporal, morphological, spatial, hydrodynamic and anthropogenic variables in explaining the distribution of FPT groups was statistically evaluated using generalized linear models. Background noise in the 200–630 Hz range was the only significant predictor, exerting a negative effect on FPT occurrence, likely driven by vessel noise rather than biological chorusing. This, however, does not provide additional cues for identifying the emitting species. Although FPT characteristics remain compatible with fish vocalizations, expanding comparative studies will be essential to refine the definition of this sound type, assess potential sub-categories, identify habitat associations, and ultimately develop robust hypotheses on its biological sources.