Spatiotemporal variation in harbor porpoise distribution and foraging across a landscape of fear

Laura D Williamson,Paul M Thompson,Fabian E Bachl,Kate L Brookes,Janine B Illian,Beth E Scott,Megan R Laxton

doi:10.1111/mms.12839

Abstract

AbstractUnderstanding spatiotemporally varying animal distributions can inform ecological understanding of species' behavior (e.g., foraging and predator/prey interactions) and support development of management and conservation measures. Data from an array of echolocation‐click detectors (C‐PODs) were analyzed using Bayesian spatiotemporal modeling to investigate spatial and temporal variation in occurrence and foraging activity of harbor porpoises (Phocoena phocoena) and how this variation was influenced by daylight and presence of bottlenose dolphins (Tursiops truncatus). The probability of occurrence of porpoises was highest on an offshore sandbank, where the proportion of detections with foraging clicks was relatively low. The porpoises' overall distribution shifted throughout the summer and autumn, likely influenced by seasonal prey availability. Probability of porpoise occurrence was lowest in areas close to the coast, where dolphin detections were highest and declined prior to dolphin detection, leading potentially to avoidance of spatiotemporal overlap between porpoises and dolphins. Increased understanding of porpoises' seasonal distribution, key foraging areas, and their relationship with competitors can shed light on management options and potential interactions with offshore industries.

Highlights

Between July and September each year, PO was highest on Smith Bank in the outer Moray Firth (MF) (15–23 hr/day; Figure 2), but porpoise detections in this offshore area decreased in October to 10–12 hr/day (Figure 2 and Table 3)
Harbor porpoise detections did still occur in coastal areas with higher bottlenose dolphin detection (7.6% of hours in inner MF area and 18.8% in coastal MF), suggesting that porpoises and dolphins may be interacting in these areas
In the coastal MF, there was a strong decrease in porpoise detections when dolphins were detected; in central and offshore regions, there was no trend in porpoise vs. dolphin detections

Summary

Introduction

Previous studies in areas where these interactions occur have shown that porpoises and dolphins are often detected in the same locations, but there are fine-scale differences in temporal patterns of area use (Nuuttila et al, 2017; Thompson & White, 2004). We used data from a large-scale array of passive echolocation-click detectors (C-PODs) to investigate spatial and temporal variation in occurrence and foraging activity of porpoises, and how this may be influenced by both daylight and the presence of bottlenose dolphins.

Results

Conclusion