From sky to underwater: Can passive acoustic monitoring replace plane-based aerial photography for quantifying human use of inland waters?

Large inland waters concentrate most of the alterations linked to human activities and management strategies are needed to maintain a good level of ecosystem services. However, assessing human presence is often challenging - particularly in large socio-ecosystems - due to detection thresholds, spatial resolution limits, and the high cost of traditional monitoring methods. Although passive acoustics-based technologies have advanced considerably, they remain underutilized in freshwater management and we lack direct comparisons between sound levels and exhaustive counts from traditional methods. In the present study, we tested whether underwater Passive Acoustic Monitoring (PAM), which consists of deploying acoustic recorders underwater to record the soundscape, could be used to monitor boating activity on a large multi-use lake. We deployed acoustic recorders at five different locations of Lake Bourget, the largest natural lake in France, where Plane-based Aerial Photography (PAP) was carried out during the summer to provide comprehensive counts of human presence. Sound pressure levels increased with PAP-derived motorboat numbers and the strength of the relationship was high (Pearson's correlation coefficients between 0.60 and 0.79) to very high (0.80 < r < 1) at four locations, and very low (-0.19 < r < 0.19) to low (0.20 < r < 0.39) at the fifth. Averaging sound pressure levels over the one-third octave band centered at 1584 Hz rather than the 0.01-20 kHz frequency band slightly improved the correlations. Conversely, motorboat numbers estimated from spectrogram visual inspection underestimated the actual numbers of motorboats. Compared to PAP, underwater PAM does not provide exhaustive counts, has a low spatial resolution, and assessing detection thresholds can be challenging. However, it can be deployed almost continuously over much longer periods and may prove more cost-effective in the long term. From the managers' perspective, underwater PAM could be used instead of or in combination with visual approaches to quantify inland water use.