Abstract
Marine habitat heterogeneity underpins species distribution and can be generated through interactions between physical and biological drivers at multiple spatiotemporal scales. Passive acoustic monitoring (PAM) is used worldwide to study potential impacts of marine industrial activities on cetaceans, but understanding of animals׳ site use at small spatiotemporal scales (<1km, <1 day) remains limited. 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 and 11 moorings containing two C-PODs each and were deployed for up to 55 days. Minimum inter-mooring distances varied between ~300–600m. 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).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 (<1km diameter), average daily detection rates varied from 4.3 to 14.8PPMs/day. Within the tidal-stream array (<2km diameter), average daily detection rates varied from 10.3 to 49.7PPMs/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.
Highlights
Detection ranges of passive acoustic monitoring methods (PAM) detectors are negatively impacted by high ambient noise levels, such as might occur during storms or during peak tidal flows (Tonolla et al, 2010; Carter, 2013).The assumption that data from sparse fixed PAM efforts, or even solitary PAM sensors, adequately describe ecologically relevant variability in harbour porpoise distribution and habitat use across a development site remains largely untested
There was no significant difference in Porpoise-Positive Minutes (PPMs) detection rates per day between the paired collected using autonomous passive acoustic detectors (C-PODs) at single moorings
While it is not necessarily surprising that a heterogeneous tidal habitat is not used uniformly by a mobile species such as harbour porpoise, these results indicated significant spatiotemporal heterogeneity in porpoise habitat use amongst closely spaced moorings hundreds of metres apart
Summary
High-resolution data can be gathered through visual observation (e.g., De Boer et al, 2014), equipping animals with telemetry tags (e.g., Johnston et al, 2005) or passive acoustic monitoring methods (PAM; e.g., Tyack et al, 2011); all of these methods require considerable effort to obtain sufficient observations for robust assessment of status and/or risk For this reason, studies often undertake monitoring at a limited number of locations (often at a single location) within a site. The present study combines observations from studies at two separate, and contrasting, energetic sites in Scotland, UK (exposed to waves and tidal currents, respectively) Both studies used relatively dense arrays of moored passive acoustic detectors to assess use by harbour porpoise of sites suitable for marine renewable energy development (MRED; Benjamins et al, 2015). The aims of the present study were to assess the significance of spatiotemporal variability in detection rates of echolocating harbour porpoises at small (o1 km, within hours) scales across both sites and in relation to local environmental variables
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