Determining the detection function of passive acoustic data loggers for porpoises using a large hydrophone array

Click loggers such as C-PODs are an important tool to monitor the spatial distribution and seasonal occurrence of small odontocetes. To determine absolute density, information on the detection function, the detection probability as a function of distance, and derived from this, the effective detection radius (EDR), is needed. In this study a 15 channel hydrophone array, deployed next to 12 C-PODs, was used to localize porpoises and determine their geo-referenced swim paths using the ship’s GPS and motion sensors. The detection function of C-PODs was then computed using the distance between the animals and each C-POD. In addition to this, the acoustic detection function of C-PODs has been measured by playing back porpoise-like clicks using an omni-directional transducer. The EDR for these porpoise-like clicks with a source level of 168 dB re 1 μPa pp varied from 41 to 243 m. This variation seemed to be related to the sensitivity of the devices; however, season and water depth also seemed to have an influen...