Methodology and results of calibration of tonal click detectors for small odontocetes (C-PODs).

Static acoustic monitoring (SAM) is one major technology for observing small cetacean species. Automatic click loggers deployed for long time periods (>2 months) with a single hydrophone are a standard solution. Acoustic properties, like detection thresholds of these instruments, are essential for interpretation of results, but have nevertheless received little attention. A methodology for calibrating tonal click detectors in small tanks consisting of the determination of the horizontal directivity pattern and detection thresholds including a transfer function is presented. Two approaches were tested to determine detection thresholds by (a) determining the 50% detection threshold and (b) fitting a linear regression model to the recorded relative amplitudes. The tests were carried out on C-PODs (Cetacean PODs, tonal click detectors), the most commonly used instrument for SAM in Europe. Directivity and threshold were tested between 60 and 150 kHz. Directivity showed a maximum variation of 8.5 dB in the horizontal plane. Sensitivity is highest between 80 and 130 kHz and linear (± 3 dB) in this frequency range for most of the instruments tested. C-PODs have a detection threshold (calculated with the linear model) of 114.5 ± 1.2 (standard deviation) dB re 1 µPa peak-peak at 130 kHz.