Observations on the Short-Term Movements and Behaviour of Whale Sharks (Rhincodon Typus) at Ningaloo Reef, Western Australia

Marine Biology (1999) 135: 553±559 Ó Springer-Verlag 1999

J. S. Gunn á J. D. Stevens T. L. O. Davis á B. M. Norman Observations on the short-term movements and behaviour of whale sharks (Rhincodon typus) at Ningaloo Reef, Western Australia

Received: 19 January 1999 / Accepted: 22 June 1999

Abstract The short-term movements and behaviour of formation, and morphometric data, with a few obser- whale sharks (Rhincodon typus Smith, 1828) during vations on feeding and reproduction (see Silas 1986 and March 1994 and April 1997 are reported from data Wolfson 1986 for overviews). collected by acoustic tracking and archival tags at Nin- Each March and April, whale sharks are known to galoo Reef on the north west coast of Western Australia. aggregate on the continental shelf of the central Western Sharks were tracked for up to 26 h and generally swam Australian coast, particularly in the Ningaloo Reef area slowly at '0.7 m s)1 parallel to the reef edge; occa- (22°00¢S; 113°50¢E: Fig. 1). It has been hypothesized sionally they swam in a wide arc adjacent to passes in the that they are attracted to the area by a localised increase reef. All tracked sharks made regular dives through the in productivity associated with coral-spawning events water column, mostly from the surface to near the bot- that occur each year following the full-moon periods in tom. These dives did not appear to be related to hy- March and April (Taylor 1994a). The whale sharks are drographic features, and the sharks were probably often found close to the reef front, within a few miles of searching the water column for food. Most sharks were the shore and in water of <50 m deep. A local tourist accompanied by other ®shes, usually the golden trevally industry, based on snorkelling with the sharks, has de- Gnathanodon speciosus. veloped rapidly. Information on the distribution and movements of these sharks is of importance to this industry, and to our knowledge of whale shark ecology. Introduction Possible impacts of snorkellers on whale shark behaviour must also be considered. The whale shark (Rhincodon typus Smith, 1828) has a Limited aerial surveys and marking with dart tags cosmopolitan distribution in tropical and warm-tem- have been carried out to estimate population numbers at perate seas (Compagno 1984; Last and Stevens 1994). It Ningaloo (Taylor 1994a), and some unsuccessful at- is epipelagic, found in both oceanic and coastal envi- tempts have been made to track sharks with both sat- ronments, grows to a huge size (at least 12 m), and feeds ellite and acoustic tags (Taylor 1994b). on small prey ranging from plankton to small ®shes and In March 1994 and April 1997, we spent a total of cephalopods (Compagno 1984). 25 d observing and tracking whale sharks at Ningaloo Our knowledge of the biology, ecology and behaviour Reef. This paper provides information on their short- of whale sharks is very limited. The scienti®c literature is term movements and behaviour. mainly restricted to locality records, distributional in-

Communicated by G.F. Humphrey, Sydney Materials and methods J.S. Gunn á J.D. Stevens (&) á T.L.O. Davis CSIRO Marine Research, Data on movements and behaviour of Rhincodon typus Smith, 1828 GPO Box 1538, in shelf waters o Ningaloo Reef were collected between 21±31 Hobart, Tasmania 7001, Australia March 1994 and between 1±14 April 1997. Behavioural observations were made while snorkelling in an area of '30 km2 adjacent Fax: 0061 (0)36 232 5000 to the reef edge (Fig. 1) at Tantabiddi. Whale sharks were observed e-mail: [email protected] on 30 occasions in 1994 and 7 occasions in 1997. Limited hydro- B.M. Norman graphic data from the tracking areas were recorded during our Murdoch University, visits. South Street, Three whale sharks were tracked (two in 1994 and one, on two Perth, Western Australia 6150, Australia separate days, in 1997) using acoustic telemetry. The equipment 554

Table 1 Rhincodon typus. Summary of acoustic tracks. Track No. Start Finish Duration Surface Av. speed Speed is m s)1 (D/N data for time (%) (D/N) day and night) Time Date Time Date (D/N) (hrs) (hrs)

1994 Track 1 13:34 28 Mar 17:15 28 Mar 3 h 41 min 53 0.7 Track 2 14:00 29 Mar 16:00 30 Mar 26 h 52/73 0.5/0.7 1997 Track 1 13:58 6 Apr 14:56 7 Apr 24 h 58 min 17/65a 0.8/0.7a Track 2 16:25 10 Apr 07:00 11 Apr 14 h 35 min 17/20 0.8/0.9 a Based on ®rst 18 h of track comprised a Vemco 40 KHz V22TP-01 transmitter, a V-11 hy- sharks are known to return in succeeding seasons to Ningaloo, it drophone and VR-60 receiver, and a Garmin 100 Global Posi- was hoped the archival tags would provide information on longer- tioning System (GPS). Data on swimming depth and water term movements. One of these tags was retrieved by a whale shark temperature from the multi-channel tags were recorded at 3 s in- tour-operator '24 h after deployment. tervals together with GPS position. The tags had a range of In 1994, temperature and salinity pro®les were taken with a '1.8 km and a battery life of >1 wk. Shark location and swimming Platypus submersible data-logger (SDL) along a transect consisting speed were assumed to be the same as that of the tracking vessel. of three stations extending from close to the reef front to '9 km Swimming speed was determined from the straight-line distance out to sea (i.e the area in which the majority of whale sharks are travelled in 10 min. The hydrophone was mounted over the side of the vessel and rotated electrically to maximise signal strength. Two methods were used to attach tags. In 1994, they were attached to the ®rst dorsal ®n by means of a detachable stainless-steel head mounted on a spear that was propelled using a Hawaiian sling. In 1997, the tag was attached below the base of the ®rst dorsal ®n, again with a detachable stainless-steel head, but using a powerful spear gun to penetrate the thick skin. Both attachment procedures were carried out underwater, at depths of 0 to 8 m, by divers using snorkel. Between 26 and 30 March 1994, Zelcon SBT100 archival tags were attached to six sharks. These tags measure and store data on time, date, swimming depth, light levels, and water temperature; light levels can be used to provide estimates of geolocation (Gunn et al. 1994; Klimley et al. 1994). Since some individual whale

Fig. 1 Rhincodon typus. Tracks of two whale sharks followed by Fig. 2 Rhincodon typus. Acoustic telemetry track of whale sharks acoustic telemetry outside reef at Ningaloo (thick line 3 h 41 min followed for 3 h 41 min (A) and 26 h (B) at Ningaloo (upper plots track; thin line 26 h track). Inset shows study area in Western swimming speed determined from straight-line distance travelled in Australia 10 min; lower plots swimming depth; shading sea bottom) 555

Fig. 3 Rhincodon typus. Proportion of time spent at dierent depths during day and night by whale sharks tracked for 26 h (A) 18 h (B) and 14.5 h (C) and whale shark to which archival tag was attached for 22 h (D)

observed near the surface). At each of these stations, the SDL was associated with the sharks, with the smallest size classes lowered to the bottom allowing the entire water column to be always swimming close to the sharks' mouths in groups sampled. In 1997, temperature and salinity pro®les were taken using the same SDL at three sites in the same area. The 1997 of 2 to 15. Trevally estimated to be '100 to 120 mm in pro®les were taken while tracking a shark rather than at pre-de- fork length were generally seen above the heads and termined positions along a transect. around the gill slits of the sharks, and were usually In 1994, plankton samples were collected at seven sites along solitary or in groups of <3. Trevally ³150 mm were the seaward side of the reef in an area in which several whale sharks rarely seen, but when present were solitary and ranged were sighted during the day. Samples were taken opposite either reef passes or mid-reef sections, and over bottom depths of 30 to widely over the bodies of the whale sharks. 50 m. A 333 lm-mesh ring net with a 0.75 m-diam opening, rigged Three whale sharks were tracked for periods of be- with 50 kg weights and deployed as a drop-net (Heron 1982), tween 3 h 41 min and 26 h (Table 1). On 28 March sampled the top 25 m of the water column. Each cast sampled 1994, a whale shark was tracked for 3 h 41 min (Table 1, '11.05 m3. To provide an estimate of relative plankton abundance, samples were assigned an index of abundance according to the Figs. 1 and 2A) before the tag became detached. During number of organisms present: 0 = zero catch; 1 = 1 to 30 or- the track, the shark spent '53% of its time at or near ganisms; 2 = 31 to 60; 3 = 61 to 90; 4 = 91 to 120; 5 = >150. the surface; it made '10 dives, mostly to near the bottom which varied from '20 to 70 m (Fig. 2A). Swim- ming speed tended to increase during the track, Results averaging 0.7 m s)1 and reaching a maximum of 1.3 m s)1 (Fig. 2A). In 1994, Rhincodon typus were observed on 30 occasions On 29 March 1994, a second whale shark, estimated over a 10 d period, but only one individual was seen to be '5 m total length was tracked for 26 h (Table 1, actively feeding (29 March, see below). All other sharks Figs. 1 and 2B). The proportion of time that this shark swam slowly ('0.5 m s)1), mostly at or near the surface. spent at dierent depths by day and night are shown in The reaction of the sharks to snorkellers varied about Fig. 3A. Overall, 52% of the daylight hours were spent equally between ignoring them to slowly diving. Most at or near the surface compared to 73% during the sharks were accompanied by other ®shes, usually the night. During the track, the shark made regular dives to golden trevally Gnathanodon speciosus. Golden trevally both the bottom and to mid-water. The longest time it ranging in size from '30 to 200 mm fork length were spent on the surface was '3.5 h between sunset and 556

Fig. 4 Rhincodon typus. Tracks of whale shark followed on two separate days for periods of 25 and 14.5 h by acoustic telemetry outside reef at Ningaloo Fig. 5 Rhincodon typus. Acoustic telemetry track of whale shark followed for 25 h (A) and for 14.5 h (B) at Ningaloo (upper plots swimming speed determined from straight-line distance travelled in 22:00 hrs, and the longest time it spent away from the 10 min; lower plots swimming depth; shading sea bottom) surface was '40 min. At '21:00 hrs, the shark was observed under moonlight feeding at the surface; it ap- peared to be at an angle in the water, with the top of its 0.7 m s)1 at night. Over the course of the 26 h track, the head clear of the surface. During this period the tracked whale shark moved north and then south on a track shark came into close contact with a second whale shark. parallel to the reef (Fig. 1). On two occasions it deviated For '15 min, the two individuals were observed swim- from its long-shore movement to swim in a wide circle at ming head to tail in a tight circle on the surface. At the mouth of the reef-pass east of Jurabi Point. 16:00 hrs, a snorkeller was deployed to remove the tag On 6 April 1997, a 6 m male whale shark was fol- and the track was terminated. Shortly after removing lowed for 24 h 58 min before the track was terminated the tag, while the vessel still had visual contact with the because of poor weather (Table 1, Fig. 4). However, shark, it was again observed feeding. For >45 min the after 18 h we had had to transfer to another vessel, and shark swam around in circles with its head, ®rst dorsal the last 7 h of the track was carried out using a hand- and sometimes upper caudal ®n out of the water. Esti- held hydrophone and manually calculating the depth of mated swimming speed was 0.5 to 1.5 m s)1. During the the shark from the pulse interval of the tag. Fig. 3B track, shark swimming speed varied from '0.1 to shows the proportion of time the shark spent at dierent 1.5 m s)1, with the fastest speeds being recorded during depths by day and night for the ®rst 18 h of the track. the night at '21:00 hrs, after which time its speed gen- Overall, 17% of daylight hours were spent at or near the erally declined until '09:00 hrs. It then tended to in- surface compared with 65% during the night. During crease through the rest of the day, reaching a maximum the track, the shark made regular dives to near the of 1.4 m s)1 at 21:00 hrs (Fig. 2B). Mean swimming bottom or mid-water and, as for the sharks we tracked speed during the day was 0.5 m s)1 compared to in 1994, these dives consisted mostly of a quick descent 557 02:00 hrs until the track was terminated at 07:00 hrs, the shark rarely ascended to depths of <5 m (Fig. 5B). Its swimming speed varied from '0.1 to 1.8 m s)1 (Fig. 5B). Its mean swimming speed during the day was 0.8 m s)1 compared to 0.9 m s)1 at night. As on its previous track, the shark moved predominantly parallel to the reef over bottom depths of 10 to 70 m for most of the 14 h 35 min we followed it. At '20:00 hrs, it made a large circuit oshore slightly to the north of a reef-pass (Fig. 4). Data from the recovered archival tag are presented in Fig. 6 which show the same regular diving behaviour as displayed by the tracked sharks. This individual spent almost all of the night below the surface, with the depth of its dives increasing from sunset through to 03:00 hrs, suggesting that it was moving oshore during this period. The 1994 SDL transect carried out on 31 March is shown in Fig. 7A, B. The salinity/depth pro®les for the three stations were very similar. The temperature pro- ®les were dierent at the three stations, but there was a clear thermocline at each site. In 1997, water tempera- tures at the three stations were generally cooler and the water was less saline than in 1994 (Fig. 7C, D). A weak thermocline was present at the 33 and 70 m stations, but there was no clear thermocline at the 35 m site. Plankton abundance indices of 1 to 3 were recorded at 6 of the 7 sites sampled. The seventh sample had an abundance index of 5, and was ®xed and subsequently examined and counted in the laboratory. It contained 165 crab megalopae, 74 crab zoea, 32 mysids, 14 eu- Fig. 6 Rhincodon typus. Archival tag data recovered from whale phausids, 13 sergestids, 5 siphonophores, 4 chaetog- shark after 22 h, showing shark swimming-depth and water temperature naths, 3 stomatopods, 3 copepods, 1 medusae and 1 hyperid amphipod. The wet weight of the sample was 4.277 g, providing an estimate of plankton biomass of followed by a quick ascent (Fig. 5A). In the night, the organisms >333 lm in that area of 0.39 g m)3. As- shark tended to spend more time near the surface, par- suming a similar relationship between number of ticularly from midnight until 03:40 hrs, during which plankton organisms and wet weight, the other six sta- period it remained at the surface. At one stage between tions would have had approximate biomasses of <0.04 sunrise and 08:00 hrs, the shark spent 60 min away to 0.11 g m)3. from the surface, apart from a quick ascent to 13 m. Its swimming speed varied from '0.1 to 1.8 m s)1 (Fig. 5A). Its mean swimming speed during the day was 0.8 m s)1 compared to 0.7 m s)1 at night. Over the 25 h Discussion we followed this shark, it swam predominantly parallel to Ningaloo Reef. The only deviations were three large Our observations and data on the movement of Rhin- circuits, ®rst oshore and then back towards the reef. codon typus at Ningaloo Reef portray a pattern of slow, The ®rst circuit took place north of the reef-pass at Low long-shore movement along the inner portion of the Point, the other two were adjacent to the reef-pass at narrow continental shelf between the front of Ningaloo Jurabi Point (Fig. 4). Reef and the Leeuwin Current that ¯ows southward On 10 April 1997, two days after we terminated our along the shelf break. On six occasions, the tracked ®rst track, we re-established contact with this shark and sharks swam in large circles adjacent to passes in the tracked it for a further 14 h 35 min (Table 1, Fig. 4). reef. This occurred at times when water was ¯owing out Overall, it spent 17% of daylight hours at or near the from the reef lagoon, possibly transporting potential surface compared with 20% during the night (Fig. 3C). prey outside the reef. The telemetry and archival tag As with all previous tracks, this shark made regular data show numerous dives throughout the 24 h period. dives through the water column. The dives generally Similar regular diving behaviour has been noted in te- lasted in the order of 15 to 20 min and the longest time lemetry experiments with other pelagic sharks and tele- the shark spent on the surface was '15 min. From osts, and has been related to the position of the 558

Fig. 7 Temperature and salinity pro®les in 1994 (A, B) and 1997 (C, up and down through the water column were associated D). A, B Three stations along transect running oshore of reef about with searching for food. Of some 30 whale sharks we half way between Jurabi and Low Point, Ningaloo (+ Station 1 at observed, only one individual (on two separate occa- 30 m bottom depth; s Station 2 at 50 m bottom depth; m Station 3 at 70 m bottom depth); C, D three stations during telemetry tracks [h sions) was seen actively feeding on the surface. The Station 1 at 14:52 hrs on 6 April at 35 m bottom depth (Fig. 5A); feeding behaviour of this shark, which swam in close s Station 2 at 06:03 hrs on 7 April at 79 m bottom depth (Fig. 5A); n circles often with its head held high in the water, was Station 3 at 07:06 hrs on 11 April at 37 m bottom depth (Fig. 5B)] similar to that observed by Taylor (1994a) for other whale sharks at Ningaloo and reported for wild and thermocline or oxygen minima/maxima (Carey captive whale sharks from other parts of the world and Robinson 1981; Carey and Scharold 1990; Holland (Compagno 1984). et al. 1992; Holts and Bedford 1993). The whale sharks It is possible that a tracking vessel may have an eect for which we have data did not appear to be orientating on the behaviour of a slowly-swimming whale shark. We to the thermocline. Tracked sharks spent most of the endeavoured at all times to maintain a distance of 200 to day and night within the mixed layer, while the shark 500 m between the boat and the tracked individuals. with the archival tag spent the day in the mixed layer but This separation was estimated from calibrations of sig- a signi®cant part of the night below it. Given the fre- nal strength on the Vemco receiver versus distances. quency of the behaviour, it seems likely that movements However, on a number of occasions when the whale 559 sharks were on the surface and either circling in one area M. Sherlock for help in preparing the telemetry equipment, D. or moving very slowly, the boat came to within a few Kube and the CSIRO workshop sta in Hobart for making equipment, and R. Bennett, S. Visser, A. Pearce and G. West for metres of them, prompting them to either move o or to help with the project. B. Bruce and J. Young reviewed the manu- dive. Whether or not the constant noise of an engine and script and we thank them for their useful comments. The Com- occasional close encounter with a boat is enough to monwealth National Ecotourism Program funded the 1997 work, modify behaviour is not known, but must be considered additional funds were provided by the Australian Geographic So- a possibility, not just for whale sharks but for all ®shes ciety. tracked in telemetry experiments. The possibility of the modi®cation of the behaviour of whale sharks in re- sponse to boats and other human activities such as References diving and snorkelling is an important issue for man- agers of the Ningaloo Reef Marine Park. They control Carey FG, Robinson BH (1981) Daily patterns in the activities of sword®sh, Xiphias gladius, observed by acoustic telemetry. Fish the activities of the tourist operators who take out divers Bull US 79: 277±292 to observe the whale sharks. Considering the possible Carey FG, Scharold JV (1990) Movements of blue sharks (Prionace eects on whale shark behaviour of human interactions, glauca) in depth and course. Mar Biol 106: 329±342 diving of the sharks during ``contact'' time might be Compagno LJV (1984) FAO species catalogue. Vol. 4. Sharks of interpreted as an avoidance reaction. However, our the world. An annotated and illustrated catalogue of shark species known to date. Part 1. Hexanchiformes to Lam- observations show that regular diving is normal behav- niformes. FAO Fish Synopses 125: 1±250 iour of the sharks; this needs to be borne in mind when Gunn JS, Polacheck T, Davis TLO, Sherlock M, Betlehem A interpreting shark/human interactions. (1994) The development and use of archival tags for studying Whale sharks are thought to migrate to Ningaloo the migration, behaviour and physiology of southern blue®n tuna, with an assessment of the potential for transfer of the Reef each year to take advantage of the high zoo- technology to ground®sh research. Int Counc Explor Sea plankton biomass associated with large-scale synchro- Comm Meet (ICES Symp Fish Migration) Mini 21: 1±23 nous coral spawning events over the March and April Heron AC (1982) A vertical free fall net with no mouth obstruc- full moons (Taylor 1994a). In 1994, we were at Ningaloo tion. Limnol Oceanogr 27: 380±383 Holland KN, Brill RW, Chang RKC, Sibert JR, Fournier DA two weeks before the ®rst coral spawning event, which (1992) Physiological and behavioural thermoregulation in big- occurs 7 to 9 d after the March full moon (Simpson eye tuna (Thunnus obesus). Nature, Lond 358: 410±412 1991), and thus, presumably, before the associated ma- Holts DB, Bedford DW (1993) Horizontal and vertical movements jor blooms or peaks in zooplankton biomass. Since of the short®n mako shark, Isurus oxyrinchus, in the southern whale sharks were relatively abundant during our 1994 California bight. Aust J mar Freshwat Res 44: 901±909 Klimley AP, Prince ED, Brill RW, Holland K (1994) Archival tags visit, we wonder whether they assemble in anticipation 1994: present and future. NOAA natn mar Fish Serv tech of the ``big event'', or whether there are other sources of Memo US Dep Commerce NMFS-SEFSC 357: 1±31 production in the area that support them at this time. Last PL, Stevens JD (1994) Sharks and rays of Australia. CSIRO Our limited plankton sampling in 1994 suggested that Melbourne, Australia Silas EG (1986) The whale shark (Rhiniodon typus Smith) in Indian zooplankton biomass close to Ningaloo Reef was high in coastal waters: is the species endangered or vulnerable? Marine comparison to that of the main water body of the east- Fisheries Information Service, Technical and Extension Series. ern tropical Indian Ocean o Ningaloo (Tranter and Central Marine Fisheries Research Institute, Cochin, India Kerr 1977). Simpson CJ (1991) Mass spawning of corals on Western Australian reefs and comparisons with the Great Barrier Reef. J Proc R Soc West Aust 74: 85±91 Acknowledgements We are particularly grateful to L. and A. Wight Taylor G (1994a) Gentle giants of the deep. Aust Geographic 34: of ``Underwater Discoveries'' for providing the opportunity to 93±103 carry out this work, and for their help in the ®eld. We also thank Taylor G (1994b) Whale sharks. The giants of Ningaloo Reef. H. Morrison, S. Jones and the crew of ``Lion®sh 3'', R. Allum, Angus & Robertson, Sydney R. McGuinness, K. Deacon and G. Taylor for help in the ®eld. Tranter DJ, Kerr JD (1977) Further studies of the plankton eco- P. Lake, H. Shave and N. Gruber of the vessel ``Wildthing'' pro- systems in the eastern Indian Ocean. III Numerical abundance vided much assistance with the tracking work. The Department of and biomass. Aust J mar Freshwat Res 28: 557±583 Conservation and Land Management (CALM) provided the re- Wolfson FH (1986) Occurrences of the whale shark, Rhincodon search permit for this work, the use of their vessel ``Pseudorca'', typus Smith. In: Uyeno T, Arai R, Taniuchi T, Matsuura K funds for spotter-plane time, and personnel for assistance in the (eds) Indo-Paci®c ®sh biology. Proceedings of the Second In- ®eld; we especially thank D. Myers, C. Williams, D. Noble, ternational Conference on Indo-Paci®c Fishes. Ichthyological P. Lambert, D. Coughran and A. Darbyshire. We also thank Society of Japan, Tokyo, pp 208±226