Listening to the unseen: Passive acoustic monitoring reveals the presence of a cryptic fish species

1. 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.
2. 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.
3. 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.
4. 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).
5. 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.
6. 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.