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

Abstract

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 (~100 m 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 6 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 h cycles with higher activity at night for all sites. All sites also had a second peak for 29.5 d, linked to the lunar cycle with higher activity during 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 is informative and appropriate for analysis of acoustic temporal data.

Low hunting costs in an expensive marine mammal predator

“…S9, and table S1). This diel foraging pattern is consistent with passive acoustic monitoring studies that document more clicking and buzzing at night ( 20 , 21 ), highlighting the importance of nighttime foraging to meet the energetic requirements of harbor porpoises in shallow waters. This elevated nighttime foraging may be related to the ease of capturing prey that are unable to see an echolocating predator in the darkness, or may result from prey moving up from the sea floor at night to feed when there is less predation pressure from visual predators such as larger fish.…”

Characterising Underwater Noise and Changes in Harbour Porpoise Behaviour During the Decommissioning of an Oil and Gas Platform

“…To assess the extent of the effect of decommissioning activities on porpoises, the proportion of detection positive hours per day (proxy for occurrence) and the proportion of buzz positive hours per number of hours present (proxy for foraging activity; Stedt et al, 2023) were modelled as a function of the interaction between the distance to Jacky and the decommissioning period (three levels: before/during/after). Since porpoises are attracted to MMS in this area (Fernandez-Betelu et al, 2022), two GAMs with a binomial family distribution (probit link function) were fitted to allow more flexibility in the predictions.…”