Seismoacoustic Localization and Source Level Estimation of Blue Whale Calls in the Gulf of St. Lawrence

Quantifying the source levels and localizing blue whale vocalizations in the Gulf of St. Lawrence is essential for effective management of this endangered population in a region of intense shipping activity. We present an innovative study leveraging data from diverse observational platforms, i.e., ocean bottom seismometers and underwater gliders. Since 2019, gliders equipped with hydrophones have been deployed every summer in the Honguedo Strait shipping lane between Gaspé and Anticosti Island, in the Gulf of St. Lawrence, to trigger mitigation measures when detecting North Atlantic right whales. These platforms have recorded acoustic signals produced by many other whale species, including the endangered Northwest Atlantic (NWA) blue whale, for which similar mitigation measures have not been established. This is because the source level of NWA blue whale vocalizations (e.g. Arch calls) and their detection range from gliders in the region remain unquantified. Yet, these parameters are necessary for cetacean density estimation and for evaluating the feasibility of using this call type for dynamic management frameworks. We take advantage of four ocean bottom seismometers (OBS) on which NWA blue whale calls were detected. We analyse a 1-hour subset of 79 Arch calls that were co-detected on glider and OBS data. The vocalizing whale is localized using a time-difference-of-arrival approach with a coupled ocean-acoustic model that incorporated spatially varying bathymetry, sound speed, and sediment properties. Received levels at each OBS were calibrated using a transfer function, derived from simultaneous particle-velocity and pressure measurements, to quantify the response of the seismometer to the waterborne acoustic wave. Source levels were then estimated using the calibrated received levels and a parabolic equation transmission loss model configured to the environment along the source-receiver path. Preliminary results from our case study demonstrate that Arch calls can be detected in the Northwest Gulf up to 125 km and 150 km from an OBS and glider, respectively, where the difference is primarily explained by the receiver’s position in the water column. The measured acoustic features of Arch calls suggest propagation similarities to infrasonic blue whale vocalizations (i.e., songs). These findings have ecological implications and can inform management strategies in an area heavily used by both whales and vessels.