Harbour porpoise hydrophone array data recorded on a gill net

Entanglement in net fisheries (static and drift) is the largest known
cause of direct anthropogenic mortality to many small cetacean species,
including harbour porpoise (Phocoena phocoena), in UK waters. Despite
this, little is known about the behaviour of small cetaceans in proximity
to nets. We have developed a passive acoustic monitoring (PAM) system for
tracking the fine scale three-dimensional (3D) movements of echolocating
cetaceans around actively fishing nets by localising their acoustic
clicks. The system consists of two compact four-channel acoustic recorders
with sample-synchronised sensor packages that use 3D motion tracking
technology to accurately log orientation, depth, water temperature and
ambient light level. Two recorders were used in tandem, with each one
attached to and floating above the net float-line. The system can be
deployed during normal fishing operations by a trained researcher or
experienced fisheries observer. Recordings were analysed in PAMGuard
software and the 3D positions of echolocating animals in the vicinity of
the system were calculated using an acoustic particle filter-based
localisation method. We present findings from four deployments in UK
waters (each 1-2 days in duration) in which 12 distinct harbour porpoise
encounters yielded a sufficient number of detected clicks to track their
movements around the net. The tracks show a variety of behaviours,
including multiple instances of animals actively foraging in close
proximity to the fishing net. We show that a relatively inexpensive PAM
system, which is practical to deploy from active fishing vessels, is
capable of providing highly detailed data on harbour porpoise behaviour
around nets. As harbour porpoises are the one of the most difficult
species to localise, this methodology is likely to be suitable for
elucidating the behaviour of many other toothed whale species in a variety
of situations.