Small-scale variability in vocalising harbour porpoise (Phocoena phocoena) distribution within two Scottish marine renewable energy development (MRED) sites was investigated by deploying dense arrays of C-POD passive acoustic detectors at a wave energy test site (the European Marine Energy Centre [Billia Croo, Orkney]) and by a minor tidal-stream site (Scarba [Inner Hebrides]). Respective arrays consisted of 7 & 11 moorings containing two C-PODs each and were deployed for up to 55 days. Minimum inter-mooring distances varied between ~300–600 m. All C-POD data were analysed at a temporal resolution of whole minutes, with each minute classified as 1 or 0 on the basis of presence/absence of porpoise click trains (Porpoise-Positive Minutes/PPMs). Porpoise detection rates were analysed using Generalised Additive Models (GAMs) with Generalised Estimation Equations (GEEs).
Support was received from the Scottish Funding Council, Highlands and Islands Enterprise, Scottish Power Renewables and E.ON under the Hebridean Marine Energy Futures (HMEF) programme (Grant reference number 1R042 (HFU) SPIRIT, Project Ref. HR10012), a research initiative led by the University of the Highlands and Islands (UHI) and Aquamarine Power.
Billia Croo and Scarba, Scotland, UK
To assess the significance of spatiotemporal variability in detection rates of echolocating harbour porpoises at small scales across both sites and in relation to local environmental variables.
Although there were many porpoise detections (wave test site: N = 3,432; tidal-stream site: N = 17,366), daily detection rates varied significantly within both arrays. Within the wave site array (<1 km diameter), average daily detection rates varied from 4.3–14.8 PPMs/day. Within the tidal-stream array (<2 km diameter), average daily detection rates varied from 10.3–49.7 PPMs/day. GAM-GEE model results for individual moorings within both arrays indicated linkages between porpoise presence and small-scale heterogeneity among different environmental covariates (e.g. tidal phase, time of day). Porpoise detection rates varied considerably but with coherent patterns between moorings only several hundred metres apart and within hours. These patterns presumably have ecological relevance.
These results indicate that, in energetically active and heterogeneous areas, porpoises can display significant spatiotemporal variability in site use at scales of hundreds of metres and hours. Such variability will not be identified when using solitary moored PAM detectors (a common practice for site-based cetacean monitoring), but may be highly relevant for site-based impact assessments of MRED and other coastal developments. PAM arrays encompassing several detectors spread across a site therefore appear to be a more appropriate tool to study site-specific cetacean use of spatiotemporally heterogeneous habitat and assess the potential impacts of coastal and nearshore developments at small scales.
Benjamins, S.; van Geel, N.; Hastie, G.; Elliott, J.; Wilson, B. (2016). Harbour Porpoise Distribution can Vary at Small Spatiotemporal Scales in Energetic Habitats. Deep Sea Research Part II: Topical Studies in Oceanography, 141, 191-202.