Rare and cryptic fish species such as the cusk-eel Ophidion rochei (Műller, 1845) (Ophidiiformes), an endemic sand-dwelling Mediterranean fish, are likely to go undetected by traditional non-invasive monitoring techniques commonly used to survey biodiversity. Although the cusk-eel is distributed along the eastern coast of the Adriatic Sea, no study to date has reported its presence along the north-western coastline. Despite this, passive acoustic monitoring carried out during summer 2009 at the WWF-Miramare Marine Protected Area, located on the northernmost shore of the Adriatic Sea (Trieste Gulf, Italy), revealed loud sounds with characteristics similar to those of O. rochei. The sounds were long trains of low-frequency pulses, showing the typical and unique pulse period alternation pattern of O. rochei adult male reproductive calls. The consistency of these peculiar call features indicates that O. rochei is present in the marine protected area, where it is likely to reproduce. The results are further discussed in light of the occasional presence of the congeneric snake blenny Ophidion barbatum (Linnaeus, 1975). This is the first reported case in which passive acoustic monitoring enabled the identification of a cryptic fish species in a marine protected area where visual census surveys of the fish fauna, carried out for decades on a monthly basis, failed to detect the presence of this species. Passive acoustic monitoring is a powerful tool for both conservation and fishery science that should be coupled with visual surveys in order to improve the resolution of fish biodiversity assessments.
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