Archival tagging of a basking shark, Cetorhinus maximus,in the western North Atlantic

Gregory B. Skomal*$, Grayson WoodO and Nick CaloyianisP

*Martha’sVineyard Research Station, Division of Marine Fisheries, Commonwealth of Massachusetts, PO Box 68,Vineyard Haven, Massachusetts 02568, USA, e-mail: Gregory.Skomal@state.ma.us. OCoastwatch Program,

National Oceanic and Atmospheric Administration, 28 Tarzwell Drive, Narragansett, Rhode Island 02882, USA,e-mail: grayson.wood@noaa.gov. P442 Chalfonte Drive, Baltimore, Maryland 21228, USA, e-mail: iultima@earthlink.net

$Corresponding author, e-mail: Gregory.Skomal@state.ma.us

A 6.1-m long female basking shark (Cetorhinus maximus) was tagged 73 km east of Nantucket Island,Massachusetts on 27 September 2001 with a pop-up archival transmitting tag. The tag detachedprematurely on 6 December 2001 in an area approximately 800 km south-west of the tag site o¡ the coastof North Carolina. The basking shark was vertically active for the 71-d tracking period, moving throughdepths and temperatures ranging from the surface to 320m and 5.8 to 21.08C, respectively. The sharkdisplayed temporal variation in its residence depth and exhibited a marked temperature preference, with72% of the temperature observations between 15.0 and 17.58C. This track provides evidence that thebasking shark associates with the continental shelf and shelf edge o¡ the south-eastern United Statesduring autumn. Moreover, it corroborates previous studies indicating that the basking shark remainsactive and does not hibernate during autumn.

INTRODUCTION

The basking shark, Cetorhinus maximus (Gunnerus), is theworld’s second largest ¢sh, but little is known of its lifehistory and ecology. The species is broadly distributed innorthern and southern temperate waters of the Atlanticand Paci¢c oceans, and found from brackish coastallagoons and shallow coastal waters to the open ocean(Francis & Du¡y, 2002). In spring and summer, thebasking shark moves inshore to feed on planktoniccrustaceans, but little is known of its autumn and winterhabitats in most of its range (Francis & Du¡y, 2002). Thepaucity of winter observations and the discovery of severalsharks lacking gillrakers prompted Parker & Boeseman(1954) to propose that basking sharks hibernate duringwinter because zooplankton densities are too low to makefeeding energetically pro¢table. This hibernation hypoth-esis has persisted for decades, though there are nowtheoretical considerations (Sims, 1999) and bycatchrecords (Francis & Du¡y, 2002) to the contrary. Recently,Sims et al. (2003) investigated the seasonal movementsand behaviour of the basking shark in the eastern NorthAtlantic using pop-up archival transmitting (PAT) tags.They found that basking sharks remain vertically andhorizontally active in late autumn and early winter.Moreover, they suggested that basking sharks exploitcontinental shelf and slope regions associated withmesopelagic and epipelagic zooplankton habitats.

In the western North Atlantic, the basking sharkconcentrates in spring and summer in areas of highproductivity and along thermal fronts on the continentalshelf from southern New England to Newfoundland(Templeman, 1963; Owen, 1984). Although North Atlanticstock structure has yet to be de¢ned for this species,tagging data suggest separate eastern and western stocks

(Kohler et al., 1998). The autumn and winter habitat ofC. maximus in the western North Atlantic remains largelyunknown despite limited evidence that the species maymove into waters o¡ the south-eastern United States(Schwartz & Burgess, 1975; Springer & Gilbert, 1976).Moreover, the seasonal movements, vertical behaviour,and possible habitat preferences of basking sharks relativeto oceanographic features remain a mystery during thattime of year.

MATERIALS AND METHODS

On 27 September 2001 a *6.1-m total length femalebasking shark was tagged 73 km east of Nantucket Island,Massachusetts (41.298N 69.088W) using a PAT tag(Wildlife Computers, Redmond, WA, USA). The tag wasprogrammed to record swimming depth (minimumresolution: 0.5m), water temperature (minimum reso-lution: 0.058C), and light level (measured as irradiance at550 nm wavelength) every minute. These data werecompiled and stored by the tag as three types ofsummary data over a set interval of three hours. First,depth and temperature measurements were aggregatedinto 12 pre-speci¢ed depth bins (510m, 10^25m, 25^50m, 50^75m, 75^100m, 100^150m, 150^200m, 200^300m, 300^500m, 500^750m, 750^1000m, 41000m)and 12 pre-speci¢ed temperature bins (55.08C, 5.0^7.58C, 7.5^108C, 10.0^12.58C, 12.5^158C, 15.0^17.58C,17.5^20.08C, 20.0^22.58C, 22.5^25.08C, 25.0^27.58C,27.5^30.08C, 30.0^60.08C), respectively. Second, the tagcompiled a temperature^depth pro¢le of the watercolumn inhabited by the shark during each three hourinterval. This comprised minimum and maximum watertemperatures at eight equally spaced depths, which

J. Mar. Biol. Ass. U.K. (2004), 84, 795^799Printed in the United Kingdom

Journal of the Marine Biological Association of the United Kingdom (2004)

ranged from the shallowest to the deepest swimmingdepths. Finally, the tag processed the light level data tocorrect for depth and then estimated times of dawn, dusk,and midnight or midday.

The tagged shark was part of an aggregation of 10^12basking sharks. The sea surface temperature in thetagging area was 16.78C and none of the sharks in theaggregation was observed feeding. The PAT tag wasapplied by a diver along the dorsal midline just posteriorto the ¢rst dorsal ¢n with an intramuscular stainless steeldart. The dart was placed approximately 7^10 cm intothe dorsal musculature, with the tag trailing on a 20 cmlength of heavy mono¢lament nylon line with cha¢ng

protection. During the tagging process, the entire groupof sharks descended.

The PAT tag was programmed to release from the sharkand to transmit data summaries on 14 February 2002, atime when the distribution of the basking shark in thewestern North Atlantic is unknown. Moreover, we chosea relatively short tag deployment to assess tag attachment.To construct the track of the shark, estimates of its long-itude and latitude positioning were computed from lightlevel messages with an algorithm developed by the PATmanufacturer (PAT Decoder, Wildlife Computers).Assuming an accuracy of �0.98 longitude and �1.28 lati-tude (Welch & Eveson, 1999), position estimates were

796 G.B. Skomal et al. Basking shark archival tagging

Journal of the Marine Biological Association of the United Kingdom (2004)

Figure 1. Estimated track of a 6.1-m basking shark tagged 27 September 2001 plotted on an AVHRR SST image of 6 December2001, the date of detachment (several estimated positions were not plotted due to point overlap); lines delineate 100m and 200mbathymetry curves (black) and isotherms (white). Star indicates tagging location.

re¢ned using AdvancedVery High Resolution Radiometer(AVHRR) sea surface temperature (SST) images andbathymetry data coupled with depth and temperaturepro¢le data collected by the tag.

RESULTS

The tag began transmitting data on 14 February 2002from an area in the central North Atlantic (36.488N45.818W) about 2100 km south-east of the tag site.However, depth and temperature data collected by thetag indicated that the tag detached prematurely between0900 and 1200 on 6 December 2001. The cause of theearly detachment remains unknown, but in December2002 we received noti¢cation from the tag manufacturerof a potential problem with the stainless steel release pin.The eastward movement of the £oating tag after its releaseclosely followed the £ow of the Gulf Stream.

The light data collected by the tag allowed the estima-tion of 40 positions along the track of the basking sharkusing the algorithm developed by the PAT manufacturer.However, in consultation with the tag manufacturer,many of these were discarded due to light degradationassociated with the depth of the animal and time of year,or they could not be corroborated with satellite imageryand behavioural data. This ¢ltering process resulted in 15position estimates that were used to construct the track ofthe shark. These estimates indicated that the tag releasedfrom the shark in an area approximately 800 km south-west of the tag site, o¡ the Outer Banks of North Carolina(near 36.008N 75.308W) (Figure 1). The basking sharkmoved out of southern New England waters shortly aftertagging, yet remained on the continental shelf for theduration of the track and, at times, was associated withthe shelf edge (Figure 1). When position estimates weresuperimposed on satellite SST images, the southern

Basking shark archival tagging G.B. Skomal et al. 797

Journal of the Marine Biological Association of the United Kingdom (2004)

Figure 2. Swimming depth and water temperature data from a PAT-tagged basking shark tracked in the western North Atlantic:(A) minimum and maximum swimming depths and average water temperatures collected every three hours; (B) percentage time-at-temperature histogram of measurements collected over the duration of the track.

movements of the animal appeared to coincide with theseasonal cooling of more northerly continental shelfwaters. By 5 October, the shark had moved south-westfrom the Gulf of Maine to shelf waters in the mid-AtlanticBight east of New Jersey (Figure 1). The shark continuedto move south-west as shelf waters cooled and on 21October the shark was on the outer shelf south-east ofDelaware Bay. By 1 November, the shark was north-eastof the Outer Banks of North Carolina. It remained in thatgeneral region for the next month, moving slightly north-east in late November, and back to the Outer Banks by 6December.

The basking shark was vertically active for the entiretracking period, moving though depths ranging from thesurface to 320m (Figure 2). Although the shark encoun-tered water in the temperature range of 5.8 to 21.08C,72% of the observations were within the range 15 to17.58C (Figure 2). Over the 71-d track, the shark displayedsome temporal variation in residence depth (Figures 2 &3). In late September and October, tracking data indicatedthat the shark resided within a depth range of 25 to 75mfor 83% of the time and there was little di¡erence betweenday and night observations (Figure 3). During this period,the shark made occasional forays to the surface. InNovember, the vertical behaviour of the shark was more

variable and deeper, particularly during the night, withfewer surface forays. Although most of the track was shal-lower than 100m, the shark made a single dive to 320mon 15 November and was likely on the edge of the shelf atthis time. For the balance of the track, the shark displayedshallow behaviour and remained in the top 25m for 99%of the ¢rst week in December (Figures 2 & 3).

DISCUSSION

The track reported herein indicates that the taggedCetorhinus maximus moved south on or near the edge of thecontinental shelf in autumn from its northern summerfeeding areas in the western North Atlantic. This southerlymovement appears to coincide with the seasonal cooling ofmore northerly continental shelf waters. The SST datacollected in the Gulf of Maine by the National Data BuoyCenter showed a rapid cooling coincident with the depar-ture of the shark from these waters in early October. Themean daily water temperature dropped from 15.88C on26 September to 12.78C on 30 September 2001, which isbelow the thermal preference exhibited by the shark(Figure 2). Using aerial survey data, Owen (1984) investi-gated the seasonal movements of the basking shark in theGulf of Maine and southern New England. He found thatthe temporal and spatial distribution of the basking sharkwere largely functions of seasonal water strati¢cation,temperature, and prey abundance. Sharks were sighted ina SST range of 11 to 248C, but peak densities occurred inwaters of 22 to 248C; well above the thermal range of theshark we report here. All of Owen’s (1984) sightings in themid-Atlantic Bight occurred on the continental shelf, buthe reported only three sightings in autumn and found thatbasking shark abundance decreased before zooplanktondensities declined. Many of the 156 basking sharksreported in Kohler et al. (1998) were tagged on or closelyassociated with the edge of the continental shelf fromsouthern New England to Chesapeake Bay. Similarly, ¢vePAT-tagged basking sharks in the eastern North Atlanticexhibited broad movements con¢ned to the continentalshelf and remained within discrete areas of high produc-tivity, which occur seasonally along large-scale frontalfeatures (Sims et al., 2003).

The general region o¡ the Outer Banks of NorthCarolina may represent the over-wintering habitat of thebasking shark in the western North Atlantic. This is adynamic oceanographic region where the northward£owing Gulf Stream closely approaches the coast ofNorth Carolina. Steep thermal gradients and frontalzones are common in the area and our data indicatedthat the tagged basking shark was associated with thesepotential areas of high productivity (Figure 1). The sharkremained in this area from early November until thetermination of the study in early December, presumablyforaging in areas of high productivity. This is furthersubstantiated by Schwartz & Burgess (1975), who reportedthat the species occurs o¡ North Carolina from Decemberthrough March, with seasonal movements north as waterwarms above 108C.

The seasonal movements of this basking shark may havebeen mediated by temperature as it a¡ects foragingopportunities. Plankton concentration data collected bythe National Marine Fisheries Service (NMFS) from

798 G.B. Skomal et al. Basking shark archival tagging

Journal of the Marine Biological Association of the United Kingdom (2004)

Figure 3. Monthly depth histograms from PAT-taggedbasking shark during the day (white) and night (black).

Per cent Time

30 October to 16 November 2001 indicated that theconcentration of copepods on the continental shelf fromCape Hatteras to the Gulf of Maine remained well abovethe minimum threshold foraging density (*400 copepodsm73) estimated by Sims (1999) for the basking shark(J. Prezioso, Ecosystems Monitoring Group, NMFS,28 Tarzwell Drive, Narragansett, RI, personal communi-cation). However, feeding opportunities rapidly dimin-ished in the Gulf of Maine during the subsequent wintermonths. Copepod abundance data collected at 47 stationsin this area from 23 to 30 January 2002 indicatedconcentrations below this threshold at all but two stations.Moreover, water temperature data collected during thesurvey ranged from 4.0 to 9.38C, well below the tempera-ture range exhibited by the shark. The seasonal decline ofcopepod abundance exhibited in the western NorthAtlantic sharply contrasts with the eastern North Atlantic,where plankton densities remain above basking sharkfeeding threshold levels in the winter (Sims et al., 2003).

The winter habitat of the basking shark has been thesubject of long debate over the last century and it hasbeen hypothesized that basking sharks hibernate in deepwater o¡ the continental shelf (Parker & Boeseman,1954). Based on catch records, Francis & Du¡y (2002)provided evidence that basking sharks o¡ New Zealanddo not hibernate, but overwinter in deep water on thecontinental slope. Sims et al. (2003) concluded that thehabitat of the basking shark in the eastern North Atlanticremained similar in the summer, autumn, and earlywinter. This ¢nding is corroborated by the basking sharkin our report, which remained on the continental shelf andwas vertically active throughout the tracking period. Simset al. (2003) found that three PAT-tagged basking sharksremained on the shelf during December and early January.Two of these sharks were tagged and recovered in theEnglish Channel, the third shark exhibited southernmovements similar to those demonstrated in the currentstudy. That shark left the tagging site in the Clyde Sea inmid-September and moved 555 km south over the nextmonth to continental shelf and shelf-edge areas o¡ south-west England where it remained until the tag jettisoned inearly January. Similarly, our basking shark exhibitedmovements in October from northern summer feedinggrounds o¡ New England to potential winter habitat inthe mid-Atlantic Bight. The basking sharks tagged bySims et al. (2003) in the eastern North Atlantic exhibiteda broader vertical range (0^1000m) than the shark in thecurrent study, but this is largely due to the shallowerdepths of the area inhabited by the latter. In both studies,the basking sharks appeared to exploit the entire watercolumn, which may be attributed to prey-searching beha-viour (Sims et al., 2003).

This study provides a ¢rst glimpse into the vertical andhorizontal movements and temperature preferences of thebasking shark in the western North Atlantic, butadditional PAT tagging of basking sharks is needed tofurther elucidate the seasonal movements and habitatrange of this species in this region.

We thank Captain Bill Chaprales of the FV ‘Rueby’ and spot-ter pilot TimVoorheis for locating basking sharks and facilitatingthe tagging process. We are grateful to the following individualsfor ¢eld assistance and other technical support: Captain RobertGoodwin, Jarrod Paquette, and James Lodge of the FV‘Kuttamy’; Dan Dau of Wildlife Computers; Molly Lutcavage ofthe New England Aquarium; Jerome Prezioso of the NMFSEcosystems Monitoring Group; Clarita Berger and KimberlyWilson. A grant from the Elga and Peter Gimbel Foundationand other support provided through the Sport¢sh RestorationAct made this study possible. This is Massachusetts Division ofMarine Fisheries Contribution no. 11.

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Kohler, N., Casey, J.G. & Turner, P.A., 1998. NMFS CooperativeShark Tagging Program, 1962^93: an atlas of shark tag andrecapture data. Marine Fisheries Review, 60, 1^87.

Owen, R.E., 1984. Distribution and ecology of the basking shark

Cetorhinus maximus (Gunnerus 1765). MS thesis, University ofRhode Island, Kingston, RI, USA.

Parker, H.W. & Boeseman, M., 1954. The basking shark,Cetorhinus maximus, in winter. Proceedings of the Zoological Societyof London, 124, 185^194.

Schwartz, F.J. & Burgess, G.H., 1975. Sharks of North Carolina andadjacent waters. Morehead City, NC: Information Series. NorthCarolina Department of Natural and Economic Resources.

Sims, D.W., 1999.Threshold foraging behaviour of basking sharkson zooplankton: life on an energetic knife-edge? Proceedings ofthe Royal Society B, 266, 1437^1443.

Sims, D.W., Southall, E.J., Richardson, A.J., Reid, P.C. &Metcalfe, J.D., 2003. Seasonal movements and behaviour ofbasking sharks from archival tagging: no evidence of winterhibernation. Marine Ecology Progress Series, 248, 187^196.

Springer, S. & Gilbert, P.W., 1976. The basking shark, Cetorhinusmaximus, from Florida and California with comments on itsbiology and systematics. Copeia, 1976, 47^54.

Templeman, W., 1963. Distribution of sharks in the CanadianAtlantic.Bulletin of the FisheriesResearchBoard of Canada,140,1^77.

Welch, D.W. & Eveson, J.P., 1999. An assessment of light-basedgeoposition estimates from archival tags. Canadian Journal ofFisheries and Aquatic Sciences, 56, 1317^1327.

Submitted 5 January 2004. Accepted 25 March 2004.

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