Passive acoustic monitoring reveals cryptic species colonization following introduction of an artificial reef

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Rebecca V. Van Hoeck and F. Joel Fodrie, Institute of Marine Sciences, University of North Carolina at Chapel Hill, Morehead City, NC, Avery B. Paxton, CSS-Inc., Fairfax, VA, Avery B. Paxton and J. Christopher Taylor, National Centers for Coastal Ocean Science, National Ocean Service, National Oceanic and Atmospheric Administration, Beaufort, NC, DelWayne R. Bohnenstiehl, Department of Marine, Earth, & Atmospheric Sciences, North Carolina State University, Raleigh, NC, DelWayne R. Bohnenstiehl, Center for Geospatial Analytics, Raleigh, NC

Background/Question/Methods
Habitat enhancement, often accomplished through the introduction of artificial structures, is a common strategy used by marine resource managers to provide habitat subsidies, protect sensitive habitat, and create new fishing opportunities. Traditional monitoring methods that assess habitat enhancement outcomes face numerous limitations, including dependence on environmental conditions and trade-offs between sampling frequency and duration. Passive Acoustic Monitoring (PAM) is not subject to these same limitations and offers many advantages as a complement to traditional monitoring methods. Our team opportunistically monitored the soundscape and community development of a newly deployed artificial reef and compared it to that of a nearby established artificial reef using PAM and underwater time-lapse videos. Specifically, we compared the sound pressure level (SPL) timeseries, dusk peak in SPL, and dusk power spectrum between the two artificial reefs to evaluate whether and on what timescale the soundscapes converged. Additionally, we tracked temporal patterns in species-specific vocalizations to identify the trajectory of community development on the new reef. Lastly, we compared the qualitative conclusions drawn from PAM to previously published results from video monitoring of the same two artificial reefs.
Results/Conclusions
The results of PAM identified minimal difference in mean low frequency SPL between the two reefs at the onset of monitoring. Though the timeseries correlation, dusk SPL, and dusk power spectra all varied across sampling periods there were periods of low frequency soundscape alignment at five- and eleven-months following artificial reef deployment. The high frequency timeseries on each reef were well correlated during all sampling periods despite an initial SPL difference of 17 dB. Throughout monitoring, high frequency sound levels became more similar between the reefs but did not converge. Video monitoring suggested that demersal species did not colonize the reef until five-months post deployment; however, patterns in species-specific vocalizations suggested that toadfish (Opsanus sp.) a cryptic, demersal species may have colonized the new reef within two-weeks. Our findings demonstrate that passive acoustic monitoring is a useful complement to traditional methodologies and can provide a more holistic view of community development than visual monitoring alone.