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.