Micro-scale spatial preference and temporal cyclicity linked to foraging in harbour porpoises

Habitat heterogeneity is a crucial driver for species distribution across
scales. Harbour porpoise Phocoena phocoena basin-wide
distribution is linked to prey availability, and small-scale (kilometres
to tens of kilometres) differences in distribution are prevalent. However,
information on porpoise distribution and foraging-behaviour variations on
a micro-scale (hundred meters to kilometres) is limited. To monitor
harbour porpoise distribution and foraging activity on a micro-scale we
deployed passive acoustic dataloggers, logging porpoise acoustic activity
at six sites in a small, high porpoise-density area in southern Sweden.
Data were collected for almost a year, giving detailed time series on
porpoise activity. The time series were analysed using dynamic time
warping to compare activity patterns between sites. Large differences were
found between sites separated by only a few hundred meters, indicating
micro-scale spatial preference. Spectral analysis for temporal cyclicity
in activity revealed a dominant peak for 24-hour cycles with higher
activity during the night for all sites. All sites also had a second peak
for 29.5 days, linked to the lunar cycle with higher activity during the
full moon. Activity was overall highest during autumn and winter
(September–December). Spatial and temporal patterns were linked to
foraging, showing a positive correlation between porpoise presence and the
percent of time present with detected foraging. The study demonstrates
that harbour porpoise spatial distribution on a micro-scale should be
considered in e.g. behavioural, management, and conservation studies and
actions. In addition, we show that time series statistical methodology,
such as dynamic time warping and spectral analysis, are informative and
appropriate for analysis of acoustic temporal data.