An autonomous hydrophone array to study the acoustic ecology of deep-water toothed whales

For vocal animals with distinctive calls, passive acoustic monitoring can be used to infer presence, distribution, and abundance provided that the calls and calling behaviour are known. Key to enabling quantitative acoustic surveys are calibrated recordings of identified species from which the source parameters of the sounds can be estimated. Obtaining such information from free-ranging aquatic animals such as toothed whales requires multi-element hydrophone arrays, the use of which is often constrained by cost, the logistical challenge of long cables, and the necessity for attachment to a boat or mooring in order to digitise and store multiple channels of high-sample rate audio data. Such challenges are compounded when collecting recordings or tracking the diving behaviour of deep-diving animals for which the array must be deployed at depth. Here we report the development of an autonomous drifting deep-water vertical passive acoustic array that uses readily available off-the-shelf components. This lightweight portable array can be deployed quickly and repeatedly to depths of up to 1000 m from a small boat. The array comprises seven ST-300 HF SoundTrap autonomous recorders equally spaced on an 84 m electrical-mechanical cable. The single-channel digital sound recordings were configured to allow for synchronisation in post-processing using an RS-485 timing signal logged by all channels every second. We outline how to assemble the array, and provide software for time-synchronising the acoustic recorders. To demonstrate the utility of the array, we present an example of short-finned pilot whale clicks localised on the deep-water (700 m) array configuration. This array method has broad applicability for the cost-effective study of source parameters, acoustic ecology, and diving behaviour of deep diving toothed whales, which are valuable not only to understand the sensory ecology of deep-diving cetaceans, but also to improve passive acoustic monitoring for conservation and management.

Array development and PhD funding for CEM were supported by the Danish Council for Independent Research (FNU) to PTM (grant 27125). Fieldwork for CEM was funded by the Oticon Foundation (grant 18-0340), the William Demant foundation (grant 19-0386), and the Danish Acoustical Society. NAS was funded by a Ramón y Cajal fellowship from the Spanish Government. Many thanks to the researchers who generously lent us their SoundTraps: Jonas Teilmann, Magnus Wahlberg, and Jakob Tougaard. Thanks to the Zoophysiology workshop at Aarhus University (Niels Kristiansen, John Svane Jensen, Mehran Jahanara, and Lasse Vestergaard Sørensen) for preparing trawl floats, and designing both the 3D-printed moulds for the breakouts and the ping-board in the vertical array. Thanks to the collaborating students and researchers of the University of La Laguna for their help in the field, and to the crew and owners of the vessel Punta Umbría, where we navigated to find pilot whales off Tenerife. Jamie Macaulay (University of St Andrews), Magnus Wahlberg (University of Southern Denmark), Michael Ladegaard and Kristian Beedholm (both Aarhus University) are kindly acknowledged for helpful feedback and discussions.

Peer reviewed