MARINE ECOLOGY PROGRESS SERIESMar Ecol Prog Ser

Vol. 549: 275–288, 2016doi: 10.3354/meps11677

Published May 10

INTRODUCTION

Sites where local configurations of topography andcoastline cause large tidal flows to pass through nar-row straits or around headlands are often referred toas tidal-stream habitats. Flow speeds often exceed1 m s−1 and sometimes reach 4 m s−1 or more, resultingin highly energetic conditions (Davies et al. 2012).Several authors have commented on cetaceans suchas harbour porpoises Phocoena phocoena, bottle-nose dolphins Tursiops truncatus and minke whales Balaenoptera acutorostrata exploiting energetic tidal-stream habitats (Johnston et al. 2005a,b, Pierpoint

2008, Bailey & Thompson 2010). Ephemeral yet pre-dictable oceanographic structures (e.g. fronts, boilsand eddies) may improve foraging opportunities,although other benefits, such as cost-free transport,may also contribute to their appeal (Stevick et al.2008, Embling et al. 2012, Benjamins et al. 2015, IJsseldijk et al. 2015). In apparent contrast, otherstudies (Embling et al. 2010, Booth et al. 2013) haveindicated that harbour porpoise densities, in par -ticular, are more strongly associated with low-flowhabitats, although this may be partially linked tomethodo logical differences. The influence of small-scale, ephemeral flow features on harbour porpoise

© SAMS 2016. Open Access under Creative Commons by Attri-bution Licence. Use, distribution and reproduction are un -restricted. Authors and original publication must be credited.

Publisher: Inter-Research · www.int-res.com

*Corresponding author: steven.benjamins@sams.ac.uk

Riding the tide: use of a moving tidal-stream habitat by harbour porpoises

Steven Benjamins*, Andrew Dale, Nienke van Geel, Ben Wilson

Scottish Association for Marine Science, Oban, Argyll PA37 1QA, UK

ABSTRACT: Tidal-stream habitats present periodically fast-flowing, turbulent conditions. Evi-dence suggests that these conditions benefit top predators such as harbour porpoises Phocoenaphocoena, presumably allowing them to optimise exploitation of prey resources. However, cleardemonstration of this relationship is complicated by the fact that strong tidal flows often occurnear-simultaneously across a wide area. The Great Race of the Gulf of Corryvreckan (westernScotland, UK) is a jetting tidal system where high-energy conditions persist across a broad rangeof tidal phases in a localised, moving patch of water. Porpoises can therefore actively enter oravoid this habitat, facilitating study of their usage of adjacent high- and low-energy environments.The distribution of harbour porpoises was studied using passive acoustic porpoise detectors(C-PODs) deployed on static moorings (~35 d) and on Lagrangian drifters moving freely with thecurrent (up to ~48 h). This dual approach provided complementary perspectives on porpoise pres-ence. C-PODs moored in the path of the Great Race registered a significant increase in detectionsduring the passing of the energetic tidal jet. Encounter durations recorded by drifting C-PODswere longer than those recorded by moored C-PODs, suggesting that porpoises tended to movedownstream with the flow rather than remaining stationary relative to the seabed or movingupstream. The energetic, turbulent conditions of the Great Race are clearly attractive to porpoises,and they track its movement with time; however, their structured movements in response to theevolving tidal situation cannot simply be represented as a direct relationship between currentspeed and porpoise presence.

KEY WORDS: Phocoena phocoena · Tidal-stream habitats · Drifters · Passive acoustics · Distribution

OPENPEN ACCESSCCESS

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distribution in tidal-stream habitats is increasinglyrecognised (De Boer et al. 2014, Jones et al. 2014),but it is unclear whether absolute current speed isthe most important factor determining porpoise pres-ence within these environments. Studying relation-ships between cetacean occurrence and strong tidalflows is logistically complex, limiting our current un -derstanding of the precise nature of the drivers behindsuch relationships (Wilson et al. 2013).

An important consideration when studying theecology of tidal-stream habitats is the distinctionbetween Eulerian and Lagrangian frames of refer-ence (Batchelor 2000). The former focuses on ob -serving water flowing past a stationary point, whilethe latter follows a parcel of water moving throughspace and time. Both perspectives offer insights intohow animals use these habitats, particularly whetherthey position themselves relative to the (stationary)seafloor or a (moving) parcel of water. However,many standard monitoring methods (e.g. line-transectsurveys, instrumented moorings) only provide an Eu lerian perspective, potentially providing an in -complete picture of habitat usage.

The aims of this study were to combine Eulerianand Lagrangian techniques to explore small-scalespatiotemporal variability in the distribution of vocal-ising harbour porpoises in relation to tidal currents.The Gulf of Corryvreckan system (comprising theGulf of Corryvreckan, the Great Race and the north-ern Sound of Jura in western Scotland; Fig. 1) is aprominent tidal-stream site of recognised conserva-tion significance (JNCC 2014) that provides suitableconditions to study porpoises in this manner. Por-poises are frequently observed here, and anecdotalobservations by local tourboat operators suggest thatporpoise distribution is influenced by tidal currents

(T. Hill pers. comm.). We studied porpoises using passive acoustic click detectors (C-PODs), which arewidely used to investigate spatiotemporal patterns inodontocete occurrence (Castellote et al. 2013, Dähneet al. 2013, Roberts & Read 2015).

MATERIALS AND METHODS

Study site

The Gulf of Corryvreckan is a 1 km wide tidal straitbetween the islands of Jura and Scarba (westernScotland; Fig. 1). Differences in tidal amplitude andphase between the Sound of Jura to the east and theFirth of Lorn to the west lead to a surface slope whichdrives strong tidal flows through the Gulf. Currentspeeds can exceed 4 m s−1 in either direction (UKHydrographic Office 2008). During the west-flowing(flood) tide, water accelerating through the Gulf isejected into the Firth of Lorn as a relatively narrowtidal race, known as the Great Race. The Great Raceis turbulent, displaying structure on a wide range ofscales including eddy instabilities of its flanks and apatchwork of surface structure representing turbulentbursts (boils) and convergences driven by vorticesshed from the seabed (Kumar et al. 1998, Stoesser etal. 2008). The Race progressively advances into lessenergetic waters, reaching a maximum length of~10 km. Hydrodynamic model studies (Dale et al.2011) show the Race forming a ‘vortex pair’ of counter-rotating eddies at its advancing head (Fig. 2; Fuji-wara et al. 1994, Old & Vennell 2001, Wells &van Heijst 2003). These eddies track westwards indeep water (~200 m) but largely stall as they en -counter shoaling topography (~50 m) southwest of

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Fig. 1. Study site on the westcoast of Scotland, with bottomtopography shaded. The Gulfof Corryvreckan runs east−westbetween the islands of Scarbaand Jura. The Nearfield, Far -field and Scarba moorings areshown (in green) relative to theapproximate path of the Great

Race (in black)

Benjamins et al.: Tidal-stream habitat use by porpoises

the Garvellach islands (Figs. 1 & 2). The ejection ofenergy into open water means that significant flowpersists within the Race for longer than within theGulf of Corryvreckan itself. Approaching slack waterin the Gulf, the Great Race eddies are fully devel-oped, and the greatest surface velocities exist in theouter Great Race and over the reef to the southwestof the Garvellachs (Fig. 2). During eastward (ebb)flow in the Gulf of Corryvreckan, the energy withinthe Great Race gradually decays (Fig. 2). The moreconstrained environment of the Sound of Jura, to theeast, means that the energy of the ebb tide doesnot persist in the same manner as within the GreatRace, so there is an asymmetry in the system in thisrespect.

For the present study, the key aspect of the GreatRace is that the flood tide provides an energetic pulse

of water which progressively advances into the moreopen Firth of Lorn and persists within this large areaof open water. However, these conditions could bereadily avoided by a mobile animal with an aversionto energetic environments.

The data presented here were gathered in associa-tion with the ongoing Great Race project (UK NERCGrant No. NE/H009299/1), focusing on the tidaldynamics of waters to the west of the Gulf of Cor-ryvreckan. C-PODs were deployed on both fixedmoorings and on passively drifting buoys (Wilson etal. 2012, 2013). This unusual combination of methodsprovided an opportunity to explore the influence oftidal flows on the spatiotemporal distribution andmovements of harbour porpoises in the Gulf of Cor-ryvreckan system, relative to both the seabed andmoving water.

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Fig. 2. Surface currents from a hydrodynamic model of the Great Race relative to flood and ebb in the Gulf of Corryvreckan.The interval between panels is 2 h, and they correspond to approximately 67, 125, 183 and 241 tide-degrees relative to lowtide in Oban (see ‘Materials and methods’ for details). Vectors show current direction, and underlying colours show speed.

Green discs correspond to mooring locations from Fig. 1

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Moored C-PODs

Three porpoise click detectors (C-PODs Version 1;Chelonia 2015) were deployed in and around theGreat Race from 20 July to 25 August 2011 (Table 1).Fitted with an omnidirectional hydrophone, C-PODsuse waveform characteristics to identify odontoceteecholocation clicks among broadband pulsed soundsof 20 to 160 kHz, also recording time, duration, centre frequency, loudness, inter-click interval andbandwidth of each received click. C-PODs log arecord of each detection event rather than recordactual sounds, thereby reducing data storage re -quirements. Detection ranges vary according toambient noise levels but have been estimated at sev-eral hundred metres (Dähne et al. 2013). Odontoceteecholocation signals are identified by automatedpost-processing train detection and classificationalgorithms. C-PODs have sufficient battery andmemory capacity to remain deployed and continu-ously logging for up to 3 mo.

In this study, 2 C-PODs were moored in the path ofthe Great Race jet at ~7 and ~11 km from the Gulf ofCorryvreckan, respectively (‘Nearfield’ and ‘Farfield’moorings). The Farfield mooring also contained anupward-looking acoustic Doppler current profiler(ADCP) to measure flow speeds (pinging every 4 s;15 pings used to obtain a 1 min average) mountedabove the C-POD to minimise interference. Nearfieldand Farfield C-PODs were deployed ~15 m above theseabed in waters of 114 and 95 m depth, respec-tively. A third C-POD (‘Scarba’ mooring) was de -ployed ~5 km north of the Great Race off Scarba~15 m above the seabed in water of ~60 m depth. No

ADCP data were collected from either the Nearfieldor Scarba moorings. Although the Scarba site wasadjacent to a narrow tidal channel flowing in ap -proximate synchrony with the Gulf of Corryvreckan,flows were much less strong and the site could therefore serve as a reference site (Fig. 1).

Drifting C-PODs

Although passive acoustic detectors are typicallymoored for long-term high-resolution coverage, thisapproach can cause problems in tidal-stream habi-tats (Wilson et al. 2013), including generation offlow noise as water moves past the detector (Au &Hastings 2008, Bassett et al. 2010). Therefore, addi-tional C-PODs were attached to Lagrangian drifters(Wilson et al. 2013). Advantages of this approach arethat (1) flow noise is reduced as detectors are effec-tively stationary relative to the surrounding water;(2) drifting detectors provide increased spatial cover-age; (3) logistics of repeatedly deploying/retrievingdrifters from small boats are relatively straightfor-ward. Earlier studies revealed that this approach represented an effective way to study porpoises inenergetic tidal-stream habitats (Wilson et al. 2012,2013).

Two drifter designs were used: Type 1 drifters weredeployed during 20−21 June 2011 and 18−20 August2012, and Type 2 drifters were deployed during15−17 October 2013. These were fundamentally sim-ilar but kept the C-POD at slightly different depthsbelow the surface (Type 1 at 2 m; Type 2 at 5 m). Type1 drifters transmitted their locations for tracking

using GSM mobile phone sig-nals. Due to signal coveragelimitations, the Type 2 drifterwas redesigned to broadcastpositions via the Iridium™satellite network. Drifters weredeployed for ≥24 h before re -positioning or recovery. Drifterswere released either withinthe Gulf of Corryvreckan, tocapture westward flow withinthe main jet (2011, 2013), ornear the Garvellachs to sam-ple far-field water movements(2012, 2013); no drifters weredeployed near the Scarbamooring, as the Great RaceProject did not focus on thisarea.

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Nearfield Farfield Scarba

Position 56.1581° N, 56.1770° N, 56.1951° N, 5.8235° W 5.8850° W 5.7130° W

Bottom depth (m) 114 95 60Deployment duration 36 d, 2 h, 36 d, 1 h, 35 d, 23 h,

54 min 3 min 11 minNo. of minutes exceeding 111 111 2674096 clicks min−1 limit (% of total) (0.2) (0.2) (0.5)

Total PPMs 840 486 1109Mean PPMs h−1 0.97 0.56 1.28Mean (SD) no. of click trains PPM−1 2.0 (2.6) 1.9 (2.3) 3.1 (3.9)Mean (SD) no. of click trains h−1 1.9 (5.0) 1.1 (3.8) 3.9 (10.6)Total no. of porpoise encounters 412 242 363Mean (SD) encounter duration (min) 4.4 (6.4) 3.7 (4.8) 6.4 (8.5)Min.−max. encounter duration (min) 1−45 1−34 1−51

Table 1. Summary of moored passive acoustic porpoise detector deployments. All de -tectors were deployed on 20 July 2011 and removed on 25 August 2011. PPM: porpoise-

positive minute

Benjamins et al.: Tidal-stream habitat use by porpoises

Analysis

Upon retrieval, raw data from C-PODs were pro-cessed using the CPOD.exe software (v.2.040, Chelo-nia 2015). Only click trains classified as ‘Moderate-’or ‘High-quality porpoise click trains’ were used insubsequent analyses (Carlström 2005). A randomlyselected subsample of 5% of the raw data associatedwith potential detections from moored C-PODs waschecked visually to ensure that there were no falsepositives; all potential detections from drifting C-PODs were checked in this manner. Suspect detec-tions were removed from further analysis.

All C-POD data were analysed at a resolution ofboth individual click trains and porpoise-positive min-utes (PPMs). PPMs were also arranged into encoun-ters, here defined as ≥2 click trains separated fromother encounters by ≥10 min (cf. Carlström 2005). Encounter numbers and durations are influenced bynumerous factors, including porpoise echo locationbeam characteristics, number of echolocating por-poises within detection range at any given time andambient noise. Comparing encounter durations be-tween different moored C-PODs, and between mooredvs. drifting C-PODs, allowed additional assessmentof porpoise usage of the Gulf of Corryvreckan system.

A background current that is comparable to orgreater than swimming speed has the potential tobias measures of porpoise presence. In a rapid flow,the rate at which animals are swept past a fixeddetector increases, although each individual is withindetection range for a shorter period of time than inweaker flow. If porpoises echolocate at a constantrate, the rate at which click trains are detected pro-vides an unbiased measure of the density of por-poises (more individuals but fewer click trains perindividual). PPMs, however, are biased because theincrease in the rate at which animals are swept pastthe detector leads to an increased chance that agiven minute will be a PPM.

Moored C-POD data were analysed using descrip-tive circular statistics (Raleigh’s test for circular uni-formity; Watson-Williams test to compare sites) aswell as the nonparametric Kruskal-Wallis test andcontingency table analyses (Zar 1999). Independ-ence between moorings was assumed given inter-mooring distances of >4 km. Drifting C-POD datawere matched to GPS coordinates at a temporal reso-lution of whole minutes to calculate drift speeds andmap distances travelled. C-POD data were not usedwhere corresponding GPS data were unavailabledue to signal coverage limitations. Following theseresults, both moored and drifting C-POD data were

further analysed using logistic generalised additivemodels (GAMs) and generalised estimation equa-tions (GEEs) in order to investigate the relativeimportance of different covariates on porpoise detec-tions, based on methods described in greater detailby Pirotta et al. (2011). Data from each moored C-POD (Nearfield, Farfield and Scarba) were modelledindependently, while all drifter data from eachdeployment (2011, 2012 and 2013) were combined byyear. Further details of the GAM-GEE modellingapproach and results are provided in the Supple-ment, available at www.int-res.com/articles/suppl/m549p275_ supp. pdf.

Data were aggregated by tidal cycle to study tidaleffects on porpoise click train detections. The dura-tion of each tidal cycle was derived from tidal predic-tions for the nearby port of Oban (~33 km) based onharmonic analysis (POLTIPS-3™ tidal prediction soft -ware). Times within each cycle were then assigned atidal phase angle relative to low water (0° = 360° =low tide at Oban). Tides in this area are semidiurnal(average duration 12.4 h), although individual cyclesvary in duration according to the stage of the spring−neap cycle, such that 1° of tidal phase (or tide-degree) represents 2.0 to 2.2 minutes. Importantly,although low tide in Oban was used as a reference,the timing of low water varies markedly across thearea of interest, and lateral tidal flows, rather thantidal heights per se, are relevant here. Peak flood(westward) flow within the Gulf of Corryvreckanoccurs approximately 38° after low tide at Oban, andpeak ebb (eastward) flow occurs approximately 218°after low tide at Oban.

To avoid prematurely filling C-PODs’ memory overextended deployments, an upper limit of 4096 clicksmin−1 is normally set. High ambient noise levels cancause this limit to be reached before the end of agiven minute, leading to cessation of monitoring untilthe start of the next minute (Booth 2016). Given com-paratively brief deployment periods, memory capac-ity problems were unlikely, so all C-PODs were pro-grammed with a limit of 65 536 clicks min−1 tomaximise porpoise detection probability under highambient noise conditions.

RESULTS

Moored detections

Moored C-PODs were deployed for approximately36 d, equivalent to 71 consecutive semidiurnal tidalcycles. Click train and encounter data are sum-

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marised in Table 1. A total of 6 minutes (2 fromNearfield, 3 from Farfield, 1 from Scarba) were dis-carded due to C-POD data writing errors. Despiteconcerns about high ambient noise levels, the stan-dard detection limit of 4096 clicks min−1 wasexceeded in only 489 minutes (0.3% of total; Table 1);54% of these were detected at the Scarba mooring,including 2 containing click trains. This suggestedthat the impact of high ambient noise levels at all3 moored locations was comparatively limited, butthat Scarba experienced noisier conditions, poten-tially due to larger volumes of vessel traffic. The 2click trains identified among noisy minutes at Scarbawere assessed visually and confirmed as likely gen-uine porpoise click trains. No obvious interferenceby the ADCP was apparent in the Farfield C-POD dataset.

All C-PODs regularly registered click trains through -out their deployments. Click train detection rateswere significantly different between sites (Kruskal-Wallis-test: H = 18.852, p < 0.01), with highest de -tection rates at the Scarba site and lowest at theFarfield site (Table 1). The greatest number of indi-vidual encounters (n = 412) was recorded at theNearfield site (Table 1). Most encounters (>88%)lasted ≤5 min.

The C-POD on-board tilt sensor measured instru-ment deflection from vertical (degrees; 0° = vertical).C-POD deflection varied strongly with tidal phase(Fig. 3A), due to knock-down by tidal currents.Greatest deflections were observed at Nearfield andFarfield moorings during the flood tide, when theGreat Race was predicted to flow most strongly. Anaverage lag of ~80 min was observed between thepeak deflection from vertical between Nearfield andFarfield, indicating the later arrival of the Great Raceat the Farfield site (Fig. 3A). ADCP data from the Far -field site confirmed a positive correlation betweenC-POD deflection and current speed (ANOVA: F =3084.058; df = 1851; p < 0.001). Farfield average C-POD deflection increased approximately linearlyfrom 80° at 3 to 3.5 m s−1. As Nearfield andFarfield moorings were of similar construction, re -peated similar C-POD deflections of >80° observedat the Nearfield site were assumed to correspond toflow speeds ≥3 m s−1. Although the Scarba mooringsetup allowed more C-POD movement, C-POD de -flection varied least at Scarba, indicating that condi-tions at this site were less energetic (Fig. 3A).

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Fig. 3. (A) Average deflection from vertical (0° = vertical) of moored passive acoustic porpoise detectors (C-PODs) during theentire deployment across the tidal cycle (0° = 360° = low tide at Oban, in 10° increments). (B) Percentage of total harbour por-poise Phocoena phocoena click trains detected at the 3 sites, by tidal phase. Approximate peak westward (flood) and eastward(ebb) flow within the Gulf of Corryvreckan (GoC) are indicated. Note the phase lag in both C-POD deflections and click train

detections linked to differing arrival time of the Great Race at the Nearfield and Farfield sites

Benjamins et al.: Tidal-stream habitat use by porpoises

PPMs were typically based on only a few clicktrains per minute (maximum of 42, at Scarba); 86 to95% of PPMs at moored C-PODs were based on≤5 click trains min−1. PPM identification probabilityis increased if multiple click trains per minute arereceived. However, fast movement of echolocatingporpoises in tidal flows may limit the number of clicktrains received by moored C-PODs, thereby affectingPPM detection rates and encounter lengths. The rela-tionship between flow speed and click train detectionrates could only be directly assessed for the Farfieldsite, where click trains were identified at speeds upto 1.5 m s−1. However, C-POD deflection angle couldbe used as a proxy for flow speeds at the Nearfieldsite. Based on Farfield ADCP data, click trains weresignificantly more likely to be associated with higherflow speeds (0.5−1.5 m s−1) associated with the GreatRace than with more commonly observed back-ground flow speeds of 4 m s−1) wereobserved within the Gulf of Corryvreckan duringboth westward and eastward flows. Drifters carriedwestward by the Great Race could maintain speeds>2 m s−1 as far as ~7 km from the Gulf of Corry -vreckan. Within eddies on the flanks of the GreatRace, west of Scarba and south of the Garvellachs,speeds were typically ≤0.5 m s−1 (Fig. 5). As the tideturned and eddies weakened, drifters eventuallyentered the wider Firth of Lorn.

Despite extreme flow speeds and turbulence asso-ciated with the Great Race, all drifting C-PODs wererecovered and functioned throughout. They alsoproved effective at detecting porpoises and madedetections throughout the area across different tidalphases and flow speeds.

Unlike moored C-PODs, the standard detectionlimit of 4096 clicks min−1 was exceeded regularlyduring several drifter deployments, indicating com-paratively high levels of ambient noise (Table 2). Thisnoise could be generated by turbulence (notably in

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Fig. 4. Combined data from 2011 to 2013drifting campaigns in the Gulf of Cor-ryvreckan system. (A) Observed drifterspeeds. (B) Harbour porpoise Phocoenaphocoena detections (PPM: porpoise-pos-itive min ute). Gaps in tracks representperiods when position data from Type 1drifters were unavailable (see ‘Materials

and methods’ for details on drifters)

2011 2012 2013

Deployment period 20−21 Jun 2011 18−20 Sep 2012 15−17 Oct 2013Deployment duration range (hh:mm) 21:39−23:10 22:40−45:57 22:37−23:58No. of minutes exceeding 12−829 4−652 1−124096 clicks min−1 limit (% of total) (1.3−59.9) (0.3−48.4) (0.0−0.4)No. of PPMs 33−160 1−20 155−252No. of PPMs within ‘noisy’ minutes (>4096 clicks min−1) 0−7 1−22 0Mean PPMs h−1 1.53−7.12 0.04−0.44 3.29−4.67Mean no. of click trains PPM−1 2.7−6.8 2.6−9.0 2.7−5.2Mean no. of click trains h−1 4.1−71.9 0.4−2.2 4.7−25.2Total no. of porpoise encounters 4−14 1−14 14−22Encounter duration (min) 1−124 1−20 1−82

Table 2. Summary of drifting passive acoustic porpoise detector deployments, by year. Table includes data from 3, 4 and 3drifters from 2011, 2012 and 2013, respectively; 2013 drifters were deployed twice on consecutive days. PPM: porpoise-

positive minute

Benjamins et al.: Tidal-stream habitat use by porpoises

the centre of the Great Race), waves breaking near-shore and artificial sound sources (e.g. ships). Whenchecking for false positives, 23 PPMs were consid-ered suspect and were discarded from subsequentanalyses.

The average click train detection rate across alldrifts was very low, 4.13 (95% CI: 3.93−4.33) clicktrains min−1. There was considerable variability inclick train detection rates between drifters depend-ing on their location, speed and direction of move-ment (Figs. 4 & 5). During the westward flood tide inthe Gulf of Corryvreckan and the development of theGreat Race, most click trains were detected withinthese features or adjacent eddies (Fig. 5A,B,G,H),whereas concurrent monitoring of open waters of theFirth of Lorn (as undertaken in 2013) revealed fewerclick trains. In contrast, very few click trains weredetected near the Gulf of Corryvreckan during east-ward ebb flow, with most detected in open watersfarther west (Fig. 5C−F). Drifters collectively spentlimited time in fast-flowing waters, moving 73% of total deployment time following

ejection from the Great Race (Figs. 4 & 5). However,click train detection rates were higher at speeds >1 ms−1 (Fig. 6). Porpoises were detected in flows of up to2.6 m s−1. Although click train detection rates rangedup to 68 click trains PPM−1, most (73−95%) of PPMsdetected by drifting C-PODs were based on ≤5 clicktrains min−1, comparable to moored C-PODs. Therewas no obvious relationship between drifting speedsand click train detection rates, whether by PPM or byhour (Table 2), in contrast to the moored C-PODresults.

Porpoise encounter durations varied considerablywithin and between drifter deployments, from ≤1 minto >2 h (Table 2). Several long encounters (>60 min)occurred, both within the Great Race and in openwaters of the Firth of Lorn. Significant differences inencounter duration were found across moored anddrifting platforms (ANOVA; p < 0.001), with drifterencounters often lasting considerably longer (mean ±SD = 12.0 ± 18.9 min) than those observed by mooredC-PODs (Nearfield: 4.4 ± 6.5 min; Farfield: = 3.6 ±4.8 min; Scarba = 6.3 ± 8.5 min; Fig. 7).

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Fig. 5 (Above and following page). Combined data from 2011 to 2013 drifting campaigns in the Gulf of Corryvreckan system.Data aggregated by 90° of tidal phase, centred upon peak westward (flood) flow within the Gulf of Corryvreckan (GoC) (esti-mated at 38° after low tide at Oban). Panels A,B: Flood/peak westward flow; Panels C,D: Transition Flood->Ebb; Panels E,F:Ebb/peak eastward flow; Panels G,H: Transition Ebb->Flood. Panels A, C, E and G represent drifter speeds (m s−1), while B, D,

F and H show harbour porpoise Phocoena phocoena detections (porpoise-positive minutes, PPMs)

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GAM-GEE modelling of drifter detections

Details of GAM-GEE models of the drifter detec-tions are available in the Supplement. We found sig-

nificant variability between models in terms of whichcovariates had a significant influence on the binaryresponse variable (presence/absence of detected echo -location). Model results indicated that drift speed and

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Fig. 6. Proportion of total drifter survey effort (red line) by flow speed (0.5 m s−1 bins), and harbour porpoise Phocoena phocoena click train detection rates (per minute) in each flow speed bin (purple bars)

Fig. 5 (continued)

Benjamins et al.: Tidal-stream habitat use by porpoises

tidal phase angle were important covariates duringflood tides (2011 and 2013 data) but not during ebbtide (2012 data). The 2013 model incorporated morecovariates than the others, possibly due to multipledeployments at comparable tidal phases (3 driftersdeployed twice on consecutive days, vs. 3 driftersdeployed once each time in 2011 and 2012). It isunclear to what extent variability in initial drifterplacement could have influenced the relationshipsbetween latitude and/or longitude and porpoisedetections.

DISCUSSION

Our study has illustrated the value of integratingLagrangian and Eulerian perspectives to investigatesmall-scale use of energetic tidal-stream habitats byharbour porpoises. Detection of echolocating harbourporpoises in open water to the west of the Gulf ofCorryvreckan appeared strongly influenced by tidalflow. At locations in the path of the Great Race, por-poise detections were more frequent when the GreatRace and its downstream eddy fields elevated localflow speeds (Figs. 3−5 and see the Supplement). Astrong positive relationship was observed betweenclick train detection rates and flow speeds (Farfieldsite) as well as C-POD deflection angle (Farfield andNearfield sites), suggesting that porpoise detectionrates were correlated with faster currents associatedwith the Great Race. Moreover, the tidal phase ofpeak detection rate at the Nearfield mooring oc -curred approximately 100 minutes earlier than at the

Farfield mooring (Fig. 3B), consistent with porpoisesbeing associated with the energetic pulse of theGreat Race as it advanced westwards (see also mod-elling results in the Supplement). In contrast, theScarba site, to the north of the Gulf of Corryvreckansystem, showed regular and consistent porpoisepresence with only limited influence of tidal phase,and the highest detection rate of all moored sites.

High energy in the Great Race persists duringslack water in the Gulf of Corryvreckan, and areas ofenergetic and less energetic waters can be found inrelative proximity to one another (within severalhundreds of metres to kilometres) across most ofthe tidal cycle. This contrasts with smaller, more constrained tidal channels (e.g. Wilson et al. 2013),where energetic conditions (ebb and flood) and slackwater are relatively synchronous across the system.This complicates efforts to investigate the attractive-ness of fast-flowing waters to porpoises in these smallsites since many areas are energetic at the sametime. With mean swimming speeds of harbour por-poises in the 1 to 2 m s−1 range and sprint speedsexceeding 4 m s−1 (Westgate et al. 1995, Otani et al.2000, 2001, Verfuß et al. 2005), porpoises could leaveenergetic waters within the Gulf of Corryvreckansystem should they choose to do so. The associationof elevated porpoise detection rates with the mostenergetic tidal flow within the Great Race suggestsactive selection of these conditions in preference tothe same locations when flows are reduced (Fig. 5).This does not imply that porpoises favour fast flowsunder all circumstances, as evidenced by the con -tinued regular usage of the adjacent Scarba site.

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Fig. 7. Summary of harbour porpoise Phocoena phocoena encounter durations (minutes) aggregated in 10 min bins, for allmoored sites and all drifters combined. Although brief encounters dominated, drifters observed more long encounters, partic-

ularly when compared to the Nearfield and Farfield moorings

Mar Ecol Prog Ser 549: 275–288, 2016

Instead, the Gulf of Corryvreckan system might onlyattract porpoises under certain conditions, or por-poise vocalisation rates might vary in fast-flowingwaters. Based on the results presented here, wehypothesize that porpoises spend most of their timein relatively low-energy environments (as exempli-fied by the Scarba site), but gather at the westernentrance to the Gulf of Corryvreckan as the flood tidestarts, moving downstream with the energetic watersof the Great Race as it develops. As currents slowdown, porpoises are assumed to leave the dissipatingremnants of the Race and move elsewhere. The sec-ondary peak in porpoise detections at the Farfieldsite during ebb did not coincide with increased flowspeeds (Figs. 3 & S2). Such observations might con-ceivably indicate animals returning towards theGulf of Corryvreckan in anticipation of the next tidalcycle, but further data are required to verify thisspeculation.

The complementary Eulerian and Lagrangian per-spectives allowed us to explore whether porpoiseswere actively swimming within the flow or passivelycarried downstream. The Nearfield and Farfield siteswere approximately 4 km apart, and flow speeds inthe area ranged between 1 and 2 m s−1 at peak flow(Fig. 4). A porpoise passively carried along within theGreat Race would spend approximately 35 to 65 mintravelling between the 2 sites. Importantly, the ob -served average lag of ~100 min (Fig. 3B) associatedwith detections between Nearfield and Farfield sitessuggested that, instead of passively drifting down-stream, porpoises appeared to be making some head -way against the westward current, e.g. throughzigzagging or diving (Gordon et al. 2014). Mean en-counter duration recorded by drifting C-PODs wasconsiderably longer than recorded by moored C-PODs(Fig. 7), suggesting overall downstream movement(with the drifters), rather than remaining stationary(relative to the seabed) or advancing upstream. Insummary, the results indicate that at least some por-poises periodically and repeatedly enter the GreatRace and get relocated westward whilst attempting toremain localised within this energetic system, presum -ably for foraging, before returning to calmer waters.

Drifting C-PODs detected most click trains at flowspeeds between 1.0 and 2.0 m s−1 (Fig. 6), with clicktrains detected at speeds up to 2.6 m s−1 but notabove. These observations are comparable to othertidal-stream sites (e.g. Pierpoint 2008), suggestingthat flow speeds of ~2.5 to 3 m s−1 might represent anapproximate upper limit for adult harbour porpoiseswithin tidal-stream habitats. The near-completeabsence of click trains detected by the drifters in the

Sound of Jura was interesting, although limitedresearch effort in this area to date precluded furtherinvestigation and conclusions.

As described in the Supplement, use of GAM-GEEs provided broadly comparable results to thepreceding analyses, when considered on an indi -vidual site or yearly basis. While GAMs allowed therelative significance of different covariates to bedetermined, the results should be interpreted withcare. In particular, each of the partial residual plotsincluded in Figs. S1 to S6 describes progressively lessand less residual variability, and should thereforenot be considered independently. Moreover, the verydifferent model structures generated by the differentdrifter datasets indicate considerable interlinkedvariability that may be difficult to express within thepresent limited set of covariates. The spatiotemporalevolution of the entire Gulf of Corryvreckan systemis sufficiently complex that the present GAM-GEEapproach may struggle to adequately reflect thisvariability.

Ambient sound levels in tidal-stream habitats willvary at small spatiotemporal scales due to sedimenttransport, turbulent pressure fluctuations and en -trainment of air bubbles through turbulence (Tonollaet al. 2010, Carter 2013). Given such spatiotemporalvariability in ambient sound levels, the capabilities ofpassive acoustic detectors may not be constant acrossthe tidal cycle. Drifters, in particular, often detectedhigh and variable ambient noise levels, which couldhave reduced their detection capability to an un -known extent. Independent ambient noise measure-ments were not collected during this study, com -plicating an assessment of the impact of variableambient sound levels on detector capabilities, anissue also affecting many other passive acoustic studies (Helble et al. 2013). Although porpoise echolo-cation rates were assumed to be constant in this study,specific circumstances of tidal-stream habitats couldhave led to changes in vocalisation behaviour whichwould have impacted detection patterns.

Much remains unclear about the underlying mech-anisms driving top predator presence among tidalstreams. Plankton have traditionally been assumedto become entrained within tidal structures, attract-ing fish and their predators such as harbour por-poises at particular tidal phases (the ‘tidal couplinghypothesis’; Uda & Ishino 1958, Wolanski & Hamner1988, Zamon 2002, 2003). However, cycles of preybehaviour and availability in these environmentsmay be more important in attracting predators thanabsolute prey abundance (Zamon 2002). Small-scaleephemeral and tidally driven oceanographic fea-

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tures, such as shear boundaries, eddies and boils, areclearly important (Embling et al. 2013, De Boer et al.2014, Jones et al. 2014), but the links between thesesmall-scale features and the distribution, abundanceand behaviour of both prey and top predators remainpoorly understood (Benjamins et al. 2015). Littleis presently known about porpoise prey selectionwithin energetic environments, although a widerange of forage species are known to be targetedelsewhere in Scottish waters (Santos et al. 2004).Similarly, only limited information is available onhow these fish species behave at tidal-stream sites.Although comparatively small fish (

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Editorial responsibility: Peter Corkeron, Woods Hole, Massachusetts, USA

Submitted: July 1, 2015; Accepted: February 26, 2016Proofs received from author(s): April 15, 2016

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