@ 1995 Elsevier Science B.V. All rights reservedWhales, seals, fish and manA.S. Blix, L. Wall~e and 0. Ulltang, editors

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Habitat use and diving behaviour of harbour seals in a coastalarchipelago in Norway

Arne Bj0rgel, Dave Thompson2, Philip Hammond2, Michael Fedak2,Edmund Bryant2, Hilde Aarefjordl, Randi Roenl and Marianne Olsenl1Norwegian Institute for Nature Research, Elindem, Oslo, Norway; and 2Sea Mammal Research Unit,Cambridge, UK

Abstract. Background: the harbour seal is coastal and non-migratory and suitable for behaviouralstudies using short-range telemetry and tracking. Methods: a combination of VHF radio telemetry andunderwater ultrasonic telemetry was used to obtain behavioural and physiological data from 13 harbourseals tagged at an archipelago in Norway. VHF signals were used to locate seals, and they were trackedat close proximity by inflatable boats. Results: transit and foraging activity were identified based ondifferences in dive profiles. When foraging, all tagged seals operated solitarily, and they returned re-peatedly to the same or approximately the same foraging sites. The radio tagged seals used differenttypes of foraging habitats, ranging from shallow kelp areas located 20 km offshore to 150-200 m deepbasins with muddy sea bed located a few kilometres from the respective haul-out sites. The seals weretypically foraging at or close to the sea bed. Display behaviour including underwater vocalization wasrecorded for sexually mature males in June and July. Conclusion: the combination of VHF and ultra-sonic telemetry is useful for studies of resident seals. Tracking free ranging seals at close proximitymade it possible to identify and describe their resting, foraging and display areas.

Key words: Phoca vitulina, telemetry, display behavior, foraging ecology

Introduction

The harbour seal (Phoca vitulina) has typically been regarded as non-migratory andlittoral in distribution [1], and exhibiting a diurnal haul-out pattern [2,3]. Recentstudies however, e.g. by Thompson and co-workers [4-6], have revealed seasonaland year to year movements between a few haul-out sites and between foraginglocations. Thompson [7] nevertheless concluded that harbour seals are resident in thesame geographical area throughout the year. The diurnal haul-out patterns and thelimited travelling speed deployed indicate that harbour seals forage within a fewkilometres of their haul-out sites. This is supported by studies of foraging move-ments of radio tagged seals where most foraging activity was less than 50 km fromhaul-out sites [5-8]. A typical harbour seal habitat should provide, therefore, suitablehaul-out sites, shelter during the parturition and lactation periods and sufficient foodwithin reach of the haul-out sites to sustain the population throughout the year.In Norway three distinct types of habitats are utilized by harbour seals: coastal ar-

chipelagos, deep fjords and fjords with estuarine sandbanks [9]. The population of

Address for correspondence: A. Bji1lrge, Norwegian Institute for Nature Research, P.O. Box 1037,Blindern, N-0315 Oslo, Norway.

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harbour seals in Norway is estimated to be at least 4,100 seals; >95% of these occurin coastal archipelagos, hauling out on intertidal rocks or small islands [9].

In the Northeast Atlantic, harbour seal breeding occurs in June and early July[1,10-13] with most pups born in mid-June [11]. Parturition is followed by a 3-5 week lactation period. The females ovulate towards the end of the lactation period,and copulation takes place in the water [14,15]. Peak moulting occurs between mid-July and mid-September. Females apparently complete their moult about 3 weeksearlier than males and the whole moulting season is completed within 2-3 monthsafter the end of the breeding season [16].

Site fidelity and restricted foraging movements make the harbour seal suitable forstudies of haul-out and foraging distribution using short range VHF radio telemetry,and such studies have been carried out at several locations since the late 1970s[5,6,17,18]. In this study, we conducted detailed investigations of the habitat use andat-sea behaviour of seals before and during the breeding season, and after the moultin a coastal archipelago typical for harbour seals in Norway. In order to obtain de-tailed real-time information on the at-sea behaviour of individual seals, VHF teleme-try was combined with underwater ultrasonic telemetry to collect data on dive depthand behaviour when seals were at sea.

Materials and Methods

The study was carried out in Froan Nature Reserve off the coast of Central Norwayat MON 9°E. Froan is a coastal archipelago with a large number of small islands andislets on a shallow water plateau. It is separated from the mainland coast to the eastby a 50 km wide and 500 m deep basin and separated from larger islands to the southby 300 m deep and about 5 km wide channels. To the north and west, Froan isexposed to the North Atlantic. The harbour seals haul-out in the southern and centralpart of the archipelago and the number of animals in the area at the time of the studyprobably exceeded 200 animals [9].

Seal capture and handling

Seals were captured in nets with mesh size of 17-19 cm (stretched mesh). The netswere set close to haul-out sites and watched continuously to minimize the risk ofdrowning seals. A total of 13 harbour seals were tagged: 2 males and 2 females inJune-July 1990; 3 males in June 1991; 1 female and 5 males in August-September1993 (not all seals had a complete set of transmitters). When necessary, seals weretranquillized by Zoletil (Laboratories Reading, Z.A.C. 17 Rue des Marronniers,94240 L'Hay-les-Roses) before further handling. After handling, seals were left onland until they voluntarily entered the water.

Tags and tagging

Small VHF radio tags transmitting on frequencies between 142.0 and 142.5 MHz

213

(produced by Marine Radar Ltd, UK) were attached to the head of the seals. Thisposition was chosen to ensure that the aerial was exposed when seals were at thesurface.

Depth-velocity tags were attached behind and above the right fore flipper. Thesetags were composed of a pressure sensor, paddle wheel, alkaline battery and an ultra-sonic transmitter. Components were made by VEMCO Ltd, Halifax, Canada, andassembled at SMRU.

A temperature tag (produced for this project by SINTEF, Trondheim, Norway)composed of a temperature sensor, alkaline battery and an ultrasonic transmitter waslowered though the oesophagus and placed in the stomach of the seal. Experimentson captive harbour seals at NINA-University of Oslo showed that the temperaturetags would remain in the stomach from 2 to about 20 days.

Ultrasonic tags transmitted on frequencies between 60 and 99.9 kHz and all exter-nal tags were glued to the fur using an epoxy resin as described by Fedak et al. [19].

Data recording and tracking

An automatic land-based VHF radio receIvmg station received and stored radiosignals from the seals when they were hauled out or at the surface. The VHF radiostation is described by Nicholas et al. [20] and was composed of a set of aerialscovering 360°, a YAESU VHF/UHF Communication Receiver FRG-9600, and aSANYO 16LT portable computer used as data logger. The station could be pro-grammed to switch from one frequency to another and thus record information frommore than one animal.

Tracking seals at sea was conducted by a directional VHF radio (a purpose builtdirectional aerial and display unit and a YAESU VHFIUHF Communication Re-ceiver FRG-9600) mounted on board an inflatable boat, type Zodiac MK IV withtwin 30 hp Johnson outboard engines. The VHF signals were used to locate andclose in on seals at sea. Information on dive duration and dive intervals and positionsof the tracking boat were automatically logged on a SANYO 16LT portable com-puter. Position of the tracking boat was obtained by a GARMIN GPS 65 PersonalNavigator.

When tracking the seal at close proximity, detailed behavioural and physiologicaldata from the ultrasonic transmitters were received by hydrophone and logged onVEMCO VR60 Ultrasonic Receivers (manufactured and marketed by VEMCO Ltd,Halifax, Canada).

Results

Haul-out patterns and haul-out sites

We considered that continuous VHF signals for 8 min or more indicated that sealswere hauled out. Although haul-out bouts were recorded during both day and night

214

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Fig. 1. The southern part of the archipelago of Froan, Norway. 1,2 and 3 are examples of foragingmovements of three adult male seals (seals 91M3, 90M2 and 91M2, respectively). 4, 5 and 6 are haul-out sites frequently used by harbour seals at Hestvrer, Sandskjrer and Masskjrer; 7 is the island of S\1r-bur\1y.

and at high and low tide, there was a tendency for seals to haul-out more frequentlyduring the day and at low tide. After a trip to sea, seals regularly returned to the same

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haul-out rock or to adjacent rocks in the same area. The radio tagged seals typicallyhauled out among other seals on intertidal rocks in clusters of small islands (e.g. atHestvrer, Sandskjrer and Masskjrer in Fig. 1). One seal (91M2) tagged at Hestvrer,however, regularly hauled out alone just outside the harbour of S0rbur0Y (Fig. 1).

Dive profiles as indicators of transit and foraging activity

When the seals were at sea, we used dive profiles to characterize their behaviour.Travelling to and from the foraging grounds, the seals moved in typical V-shapeddives (Fig. 2). Usually we observed several consecutive V-shaped dives in a constantdirection, and we defined these dives as transit dives. These dives did not alwaysreach to the sea floor.

When seals ceased directed travel at sea, a different type of dives was observed.The dive profile was U-shaped (Fig. 3), the dives usually reached the sea floor andthe swimming direction frequently changed during and between dives. We definedthese dives as foraging dives. The stomach temperature was used as further supportin identification of foraging activity. Significant drops in temperature were inter-preted as ingestion of food. An example is given in Fig. 4. On this occasion thestomach temperature of seal 93M3 was lowered stepwise from about 37°C to about30°C over a period of 30 min during a trip at sea. The temperature gradually returnedto about 37°C over a longer period of time.

On a few occasions, in particular when seals foraged in areas with complex topog-raphy, transit and foraging type dives may be mingled.

216

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Fig. 3. Swim speed and U-shaped dive profiles of an adult male harbour seal. The U-shaped dives weredefined as foraging dives.

Dive durations of transit and foraging dives were not significantly different, andthe average dive duration was 3.3 min (SD 1.9 min). The longest dive recorded was14.3 min. For both transit and foraging dives the swimming speed was typically be-tween 1.1 and 1.6 m/s.

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217

Examples offoraging trips

All recorded foraging activity was at or close to the sea floor. At the sea floor, theseals continued swimming, presumably searching for food, at their usual swimspeed. The seals foraged at water depths between 15 and 200 m and in areas withdifferent substrate. Examples of foraging behaviour of typical seals are given below.

One male seal tagged June 22, 1991 (91M3) travelled regularly about 20 km fromhis haul-out site at Sandskjrer to feed at some shallow offshore rocks (Fig. 1). He fedpredominantly at depths between 15 and 50 m. Between his haul-out site and forag-ing grounds he passed through a 200 m deep basin where other radio tagged sealswere foraging (see below). Seal 91M3, however, showed no sign of foraging activityuntil he reached the shallow rocks. His foraging trips were approximately 8-22 hlong and most of his foraging activity was during night hours (between 1800 and0600 h). The seal remained in the water until he was back at his haul-out site. How-ever, during the longer foraging trips, periods of little movement were recorded. Weinterpreted these periods as periods with resting in water.

Another male seal (90M2) tagged July 6, 1990 foraged in a deep basin 3-6 kmfrom his haul-out sites during July (Fig. 1). His foraging trips were 3-8 h long, pre-dominantly during day hours, and he repeatedly returned to his haul-out site or to asite were he remained stationary in water for several hours (see description of displaysites below).

A male seal tagged June 13, 1991 (91M2) foraged off the east side of the archi-pelago. This seal often hauled out outside the harbour of S0rbuf\<1yand was foragingwithin distances of 5 km from his haul-out site (Fig. 1). Most of his foraging activitywas at depths of about 100 m. This particular seal started or finished his foragingtrips in shallow sandy bays (depth of 15-30 m), where he caught flatfish, as verifiedby visual observation when he brought his catch to the surface. This seal foragedpredominantly during night hours, and the swimming speeds and dive profiles of hisnocturnal foraging trip from June 17 to June 18 are shown in Fig. 5.

One male seal (93M3) tagged at Hestvrer August 28, 1993 foraged on slopes atdepths between 30 and 120 m in the waters mainly north of the haul-out site. He for-aged predominantly during the day and regularly returned to Hestvrer to haul-out.However, on September 2, when he foraged on the east side of the archipelago about5-8 km from Hestvrer, he hauled out about 5 km east of his usual haul-out site. OnSeptember 4 he foraged about 15 km northeast of Hestvrer and he hauled out about10 km from his normal haul-out site.

Foraging habitats

Using the dive profiles as indicators of foraging activity, we were able to identify theforaging grounds of the individual seals. Foraging was recorded at or close to the seafloor at depths ranging from 15 to 200 m and on different substrates. The complextopography at Froan creates a diversity of habitats and the harbour seals used avariety of these for foraging.

218

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Fig. 5. Swim speed and dive profiles of an adult male harbour seal (91M2) during a nocturnal foragingtrip from June 17 to June 18, 1991. The X-axis is time in minutes from start of tracking. From 0 to120 min the seal moved at the surface close to the islands. Between 120 and 248 min he foraged atabout 100 m depth about 0.5 km from shore. From 248 min he moved into shallow water where heforaged between 362 and 480 min. The pause in tracking between 253 and 260 min was due to echo-sounding the first foraging grounds which prevented receiving ultrasonic transmitted data from the seal.

The foraging grounds of seal 91M3 can be described as shallow rocks, 15-50 mbelow surface. At depths of less than 35 m the rocks were covered by kelp forest.The main kelp species was Laminaria hyperborea where adult plants may have astipes of 2 m and a leaf of 1-1.5 m. The behavioural data did not provide informationon whether the seal was foraging within or just above the kelp forest. Echo-surveysshowed concentrations of fish surrounding the top of these shallow rocks. The fishwere thought to be young saithe Pollachius virens.

Seal 93M3 foraged on slopes from the deeper part of the kelp forest to depths ofabout 120 m. The substrate was characterized by rocks and stones in the kelp zoneand stones, gravel, shells, sand and mud as the depth increased. The foraginggrounds of seal 93M3 were not echo-surveyed, but concentrations of fish and singlefishes were recorded at similar habitat types in 1991. The recorded concentrations offish were probably young saithe.

In shallow bays and narrows between islands, the substrate consisted of shells,crushed shells, sand and clay. In shallow parts, such bays and narrows were oftendominated by sea grass Zostera marina. No concentrations of fish were recordedwhen echo-surveying this habitat type. Seal 91M2 used this habitat type for parts ofhis foraging activity. However, during most of his foraging, seal 91M2 searched forfood at depths of about 100 m on substrate of gravel, sand and mud where the seafloor was gently sloping down into the 500 m deep basin east of the archipelago.

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Echosurveys showed dense schools of fish at these foraging grounds of seal 91M2,and based on the characteristics of the echogram the fish species was thought to beherring Clupea harengus.

Seal 90M2 and several of the tagged seals not described here foraged at the seabed in 100-200 m deep basins, 2-15 km from their haul-out sites. The substrate inthese basins was soft, consisting of mud and ooze. No concentrations of fish wererecorded at these sites.

Diet composition

In 1991 the diet of harbour seals was examined by identifying fish otoliths in 20faecal samples collected at the haul-out sites. Seven species of fish were identified.The species are listed below according to their relative importance in the diet.Numbers in parentheses are percent frequency of occurrence: Norway poutTrisopterus esmarkii (25), saithe (20), Norwegian haddock Sebastes viviparus (15),dab Limanda limanda (5), herring (5), greater argentine Argentina silus (5), poor codTrisopterus minutus (5). More detailed information on diet of harbour seals at Froanis given by Olsen and Bj0rge [21].

Breeding habitats and display behaviour

In June 1991 three adult males were tagged (91M1 on June 6; 91M2 on June 13;91M3 on June 22). Until about June 27 they alternated between foraging trips andhauling out. From June 28 and July 3, seals 91Ml and 91M2, respectively, changedtheir activity patterns. Between foraging trips they spent much of their time in water

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Fig. 6. Swim speed and dive profiles of an adult male harbour seal (91Ml) displaying and vocalizing,July 3, 1991 at Froan, Norway.

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Fig. 7. Display vocalization made by male harbour seals at southern Froan in late June early July, 1991.The height of the bars indicates the number of calls per minute. Highest calls rates were recorded in thechannels between the most frequented haul-out rocks in the southwestern, part of Froan.

at particular sites with repetitive dives uniform in both profile and duration (Fig. 6).The mean duration of these dives was 3.34 min (SD 0.34) and mean surface intervalwas 0.53 min (SD 0.16). The seals vocalized at 45-60 s intervals during each divegiving 3-5 calls per dive. One seal was monitored for periods of 3 h but VHFreception indicated that he performed similar dive patterns for up to 7 h continu-ously. The characteristics of these calls are described in detail by Thompson et al.[22]. We interpreted this behaviour as sexual display activity.Similar calls were recorded over large areas in Froan. At some sites, we heard

calling at rates up to 6 calls/min. Dividing the call detection rates by the calling rateof our tagged seals indicates that between five and nine seals were calling within theaudible distance of our hydrophone at some sites. These sites were mainly in thedeeper channels penetrating into the shallow water plateau between the haul-out siteswhich were used by females with pups and other seals (Fig. 7).

Discussion

At Froan, seals haul-out on rocks and apparently suitable rocks are availablethroughout the tidal cycle. However, the haul-out pattern of harbour seals in this areawas influenced by the diel cycle and tidal cycle. The number of hauled out sealspeaked at low tide, but according to Roen and Bjprge [3], the peaks were higherduring the day than during the night. This may indicate a preference for foraging atnight and a tendency to skip one haul-out bout at low tide during the night. The radiotagged seals had different timing to their foraging trips, but at least two seals (91M2and 91M3) showed preference for foraging at night while other seals were foraging

221

during the day and night. The number of successfully tracked seals was too limitedto evaluate the 24 h distribution of foraging activity. In a sample of 17 radio taggedseals in California, 6 seals hauled out most often during day, 7 hauled out mostfrequently during the night and for the rest of the tagged seals, no preferences wererecorded [23]. In most studies, the predominant diurnal pattern indicates that sealsare foraging at night. However, Thompson [7] found that harbour seals in the MorayFirth fed more often during the day when feeding on wintering clupeids. Hesuggested that these seals preferred to forage when the herring were concentrated intight schools.

The radio tagged seals at Froan were foraging close to the sea floor. Several fishspecies do migrate vertically and are distributed deeper or closer to the sea floorduring the day. This indicates that seals may forage during the night if they arefeeding on deep water fish, such as the greater argentine, which may be availableto the seals only during their nocturnal vertical migrations. Foraging during theday may be advantageous if seals are feeding on species which are more easilycaught when they are concentrated on the sea bed and this limits the range of escaperoutes.

During June and July the tracked seals normally returned to their usual haul-out(or display) sites after a foraging trip. The tracking of seal 93M3 after moult in Sep-tember 1993 indicated that this seal frequently hauled out at alternative sites, e.g.after a period of active foraging and before returning to the usual site. However, thepresent data provide no basis to conclude whether this was due to individual behav-iour, or if it reflects a change in behaviour and site fidelity after moult. However,Thompson [7] stated that breeding may influence movement patterns of harbourseals. This is in agreement with the conclusions drawn by Thompson et al. [22]based on tracking of mature males during breeding season in the present study.

In June and July the seals showed strong site fidelity towards haul-out sites andalso towards display sites in sexually mature males. Seals tracked during this periodwere solitary when foraging and they regularly returned to the same or approxi-mately the same feeding grounds for several trips during the period of tracking. Mostof the foraging activity was within a few kilometres of the haul-out sites. Under suchconditions, intraspecific competition for food and feeding grounds may develop. Thevariety of different types of foraging habitats utilized by the harbour seals and thepattern of returning to the same feeding grounds may be an indication of individualspecialization and a means to minimize intraspecific competition and optimize forag-ing in periods of the year when site fidelity may cause constraints on the foragingmovements of these seals.

Acknowledgements

This study was made possible by funding from the Norwegian Council for FisheriesResearch as part of the Norwegian Marine Mammal Research Programme. Addi-tional funding was available from the Natural Environment Research Council, UK.

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We would like to thank S0lvi Kaarstad, Kevin Nicholas, and the fishermen RolfHeggelund, BWrn Gaarden and Magne Werkland for assisting at our field work inFroan.

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3. Roen R, Bj~rge A. Haul-out behaviour of the Norwegian harbour seal in summer. In: Blix AS,Wall~e L, Ulltang 0 (eds) Whales, Seals, Fish and Man. Amsterdam, The Netherlands: Elsevier,1995;61-67.

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5. Thompson PM, Miller D. Summer foraging activity and movements of radio-tagged common seals(Phoca vitulina L. ) in the Moray Firth, Scotland. J Appl EcoI1990;27:492-501.

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21. Olsen M, Bjl'lrge A. Seasonal and regional variation in the diet of harbour seal Phoca vitulina inNorwegian waters. In: Blix AS, Wal1l'leL, Ul1tang 0 (eds) Whales, Seals, Fish and Man. Amster-dam, The Netherlands: Elsevier, 1995;271-275.

22. Thompson D, Fedak MA, Bjl'lrge A, Bryant E, Aarefjord H, Hammond PS. Diving and callingbehaviour of male harbour seals (Phoca vitulina) during the breeding season. Mar Mammal Sci(submitted).

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