Detection of invasive fish species with passive acoustics: Discriminating between native and non-indigenous sciaenids.

Invasive alien species have been rising exponentially in the last decades impacting biodiversity and ecosystem functioning. The soniferous weakfish, Cynoscion regalis, is a recent invasive sciaenid species in the Iberian Peninsula and was first reported in the Tagus estuary in 2015. There is concern about its possible impacts on native species, namely the confamiliar meagre, Argyrosomus regius, as there is overlap in their feeding regime, habitat use, and breeding behaviour. Here, we characterised the sciaenid-like sounds recently recorded in the Tagus estuary and showed that they are made by weakfish as they have similar numbers of pulses and pulse periods to the sounds made by captive breeding weakfish. We further demonstrate that breeding grunts from weakfish and the native sciaenid, recorded either in captivity or Tagus estuary, differ markedly in sound duration, number of pulses and pulse period in the two species, but overlap in their spectral features. Importantly, these differences are easily detected through visual and aural inspections of the recordings, making acoustic recognition easy even for the non-trained person. We propose that passive acoustic monitoring can be a cost-effective tool for in situ mapping of weakfish outside its natural distribution and an invaluable tool for early detection and to monitor its expansion.

Sound generation in the searobin (Prionotus carolinus), a fish with alternate sonic muscle contraction.

The Northern searobin (Prionotus carolinus) contracts its paired sonic muscles alternately rather than simultaneously during sound production. This study describes this phenomenon and examines its effect on sound production by recording sound and EMGs during voluntary and electrically stimulated calls. Sounds produced by a single twitch resulted in a two-part sound representing contraction and relaxation sounds. The relaxation sound of one twitch coincides with the contraction sound of the next twitch of that muscle. Maximum amplitude of evoked sounds occurs between 100 Hz and 140 Hz, approximately half the fundamental frequency of a voluntarily calling fish. The muscle is capable of following electrical stimulation at frequencies of up to 360 Hz. Rapid damping and response over a wide frequency range indicate that the swimbladder is a highly damped, broadly tuned resonator. A consequence of alternate contraction is a 3.3 dB loss in acoustic pressure due to the contraction of a single sonic muscle at a time. This decrease in amplitude is offset by a doubling of fundamental frequency and a constructive interaction between the sides of the bladder, resulting in increased amplitude of each unilaterally produced sound. The alternate contraction of the bilateral sonic muscles represents a novel solution to the inherent trade-off between speed and force of contraction in rapidly contracting sonic muscles.