Baleen whales reliably produce stereotyped vocalizations, enabling their spatio-temporal distributions to be inferred from acoustic detections. Soundscape analysis provides an integrated approach whereby vocal species, such as baleen whales, are sampled holistically with other acoustic contributors to their environment. Acoustic elements that occur
concurrently in space, time and/or frequency can indicate overlaps between free-ranging species and potential stressors. Such information can inform risk assessment framework models. Here, we demonstrate the utility of soundscape monitoring in central New Zealand, an area of high cetacean diversity where potential threats are poorly understood. Pygmy blue whale calls were abundant in the South Taranaki Bight (STB) throughout recording periods and were also detected near Kaikōura during autumn. Humpback, Antarctic blue and Antarctic minke whales were detected in winter and spring, during migration. Wind, rain, tidal, and wave activity increased ambient sound levels in both deep- and shallow water environments across a broad range of frequencies, including those used by baleen whales, and sound from shipping, seismic surveys and earthquakes overlapped in time, space and frequency with whale calls. The results highlight the feasibility of soundscape analysis to quantify and understand potential stressors to free-ranging species, which is essential for conservation and management decisions.

Four Autonomous Multi-channel Acoustic Recorders (AMARs, JASCO Applied Sciences) were deployed from early June to mid-December 2016 around central New Zealand: in the South Taranaki Bight; in the narrows of Cook Strait; north-east of Kaikōura; and off the east coast of Wairarapa (henceforth referred to as STB, Cook Strait, Kaikōura and Wairarapa, respectively) (Figure 1, Table 1). Three AMARs were redeployed between late-February and early September 2017: two in the same relative Kaikōura and Wairarapa locations and one in STB, 25.2 km southeast of the first STB deployment location (Figure 1, Table 1). A recorder was not redeployed in Cook Strait in 2017. The AMARs at Cook Strait and STB were bottom-mounted on metal baseplates, with the hydrophone 75 cm off the seafloor, in water depths less than 300 m. At Kaikōura and Wairarapa, ultra-deep AMARs were deployed in water depths exceeding 1000 m and were moored on vertical line moorings, approximately 10 m above the seabed. All AMARs were retrieved via the use of acoustic releases and buoyancy aids.

Acoustic sampling was duty cycled over 900 seconds: 630 seconds at a sampling rate of 16 kHz, 125 seconds at a sampling rate of 250 kHz, and 145 seconds of sleep. Only files with 16 kHz sampling rate were considered in this study due to the focus on low frequency baleen whale sounds. Recorder sensitivity was calibrated via pistonphone, and all data were corrected by frequency- and recorder-specific sensitivity curves. The nominal pressure sensitivity level of the hydrophones was -165 dB re 1 V/µPa (± 1 dB) from 10 Hz to 100 kHz. The pressure spectral density noise floor was limited by the hydrophone preamplifier at 32 dB re 1 µPa² / Hz.

Power spectral density (PSD) was calculated with 1 Hz resolution for each 630 s, 16 kHz file using a Fast Fourier Transform with window length of 16,000 samples (Hanning window) and 80% overlap (pwelch function, MATLAB).

Data matrices contain the 1 Hz Power Spectral Density per 630-second, 16 kHz file. The first column of the Data matrices displays the Frequency value, and all other columns contain data values. The associated file names (containing recording time and date information) are provided in the separate 'Header' files. The width of the Header file is the same as the width of the Data file due to identification of the Frequency column.