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A short note on the horizontal and vertical movements of a whale shark, Rhincodon typus, tracked by satellite telemetry in the South China Sea

Yamin WANG,1 Wei LI,1,2 Xiaoguang ZENG3 and Yunchen CUI3

1College of Ocean, Shandong University at Weihai, Weihai, China, 2Chinese University of Hong Kong, Hong Kong, China and 3Ministry of Agriculture, Bureau of the South China Sea Fisheries, Guangzhou, China

AbstractWhale sharks, a global migratory species, are often reported entangled in fishing nets in coastal areas of Chi-na. The effectiveness of conservation measures has been constrained by very limited knowledge on their move-ments and preferred habitats in the coastal areas of China. For the first time, we tracked the movements of 2 whale sharks by satellite telemetry in Mainland China. The tracking results of 1 whale shark revealed that it travelled in the South China Sea in a south-eastern direction, parallel to the eastern coast of Vietnam. Total dis-tance travelled was 1018 km, in approximately 74 days, with a mean speed of 14 km per day. It appeared to head towards the cool upwelling zones in southern Vietnam at the time of the tag’s detachment. In our study, it was observed that this whale shark was a surface dweller and spent approximately 45% of its time above 10 m water depth and 90% of its time above 50 m depth. It also tended to stay in water temperatures between 27 and 30 °C, and was rarely recorded in water below 20 °C. This preliminary study indicates the importance of shal-low waters as the foraging habitat for whale sharks, and has implications for their management and conserva-tion.

Key words: pop-up archival tag, South China Sea, whale shark

Integrative Zoology 2012; 7: 94–98 doi: 10.1111/j.1749-4877.2011.00278.x

ORIGINAL ARTICLE

Correspondence: Yamin Wang, College of Ocean, Shandong University at Weihai, 180 Wen Hua Xi Lu, Weihai 264209, China.Email: wangyamin@sdu.edu.cn

INTRODUCTIONThe whale shark, Rhiniodon typus Smith, 1828, is

a global migratory species distributed throughout the

world’s tropical and warm temperate seas (Compag-no 2001). Despite this extensive range, information on their movement patterns and habitat utilization are very limited, except in several highly productive waters, such as north-western Australia, the Maldives, Seychelles (Rowat & Gore 2006), Mozambique (Cliff et al. 2007) and Mexico (Burks et al. 2006). Generally, whale sharks are found in pelagic habitats with surface temperature of 18–32 °C (Flower 2000; Eckert & Stewart 2001) and prefer nutrient-rich upwellings mingled with warm sur-face waters of salinities between 3.40 and 3.45% (Iwa-saki 1970).

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Surprisingly, little research has been carried out on whale sharks in China, despite frequent sightings along coastal areas. Knowledge on their biological characteris-tics, ecological behaviors and migration path are scarce. Existing information is mainly acquired from news re-ports on whale shark harvests and trading in Hainan, Fujian and Zhejiang provinces. These reports, along with interviews with local fishermen, confirm that whale sharks are still targets for commercial fisheries and face high rates of injury and mortality due to entanglement in fishing nets. The whale shark has a relatively slow life history (Cavanagh et al. 2003) and may not reach sexu-al maturity until 30 years of age (Taylor 1994). This sig-nifies low productivity and poor recovery potential if overexploitation occurs (Smith et al. 1998). The whale shark is considered ‘vulnerable’ by the International Union for Conservation of Nature (IUCN) and is listed in the Convention on International Trade in Endangered Species (CITES) of Wild Fauna and Flora Appendix II, which also makes it protected by the Law of the Peo-ple’s Republic of China on the Protection of Wildlife. Effective mitigation of whale shark bycatch should be based on the knowledge of their habitat utilization in the

coastal waters of China. Our study used satellite teleme-try to track horizontal and vertical movements and to re-veal the environmental conditions of the habitats occu-pied.

RESULTS AND DISCUSSIONAn individual whale shark, tracked by MK10 Pop-

up Archival Transmitting (PAT) tag (Wildlife Comput-ers, USA), was 1 of the 3 injured whale sharks that be-came than became entangled in entangled in fishing nets off the southern coast of Sanya, Hainan that were tagged and subsequently released on 29 June 2009. It was a male juvenile measuring 6.3 m long and weigh-ing 1200–1500 kg. The animal travelled southward con-tinuously after leaving Sanya on a course nearly paral-lel to the eastern coast of Vietnam (Fig. 1). The PAT tag successfully tracked the whale shark for 74 days and popped up (13.57°N, 111.6°E) prematurely for unknown reasons, well before its programmed period of 365 days. In the first 2 months, small changes in direction as well as closely spaced transmissions were observed, which might be an indicator of foraging for food. It trav-

Figure 1 Most probable horizontal movements of the whale shark derived from light-level geolocation data, fitted on bathymetry.

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elled approximately 1018.11 km in total, with an aver-age speed of 14 km per day. The whale shark appeared to apply active locomotion in its migration, rather than drift passively with surface currents. Comparing the shark’s horizontal movements with weekly geostroph-ic currents, a positive rheotaxis-swimming pattern was revealed. It first swam within a zone of minimal current speeds before turning in a south-western direction and encountering a prevailing northerly current. After that, it swam perpendicular to a westerly geostrophic current, and later against a north-west current until entering the core of a newly-formed anticyclonic eddy. This positive rheotaxis-swimming pattern of the whale shark was also observed by Rowat and Gore (2007) and is postulated to assist in feeding.

Remote sensing images showed that the shark spent the first 2 months traveling through areas of low chloro-phyll a concentration (approximately 0.05 mg/m3). Only after 3 September did the shark enter a sea area of high-er surface productivity as measured by chlorophyll a concentration (approximately 0.1 mg/m3) (Fig. 2). The formation of this area was related to the summer upwell-ings along the south-eastern coast of Vietnam caused

by the prevailing southwesterly winds. The cold water spreads northeastward and reaches maximum intensity in August (Xie et al. 2003), which could affect central South China Sea. Nutrients from upwelling zones enrich oceanic surface layers, leading to enhanced rates of bi-ological productivity. However, whether the shark was deliberately heading towards this area is not clear be-cause chlorophyll a concentration is not necessarily in-dicative of zooplankton biomass.

The whale shark swam at depths from 0 to 152 m, with a mean of 22.58 m (Fig. 3). Approximately half of the whale’s time (mean percentage of time spent diving in each 12 h period, 49%, SD = 22.03) was spent above 10 m and nearly 90% above 50 m (87.25%, SD =15.09). This further indicated that whale sharks are primarily surface dwellers, which suits with their planktivorous feeding habits. Only approximately 1.58% of the whale’s time was spent in the depth range between 100 and 200 m, and deeper diving was not observed, in contrast to oth-er studies (e.g. 800 m in Heyman et al. [2001], 979.5 m in Graham et al. [2007] and >1000 m in Brunnschwei-ler et al. [2009] and Rowat & Gore [2007]). The whale shark spent significantly more time (Mann–Whitney

Figure 2 Track of the whale shark fit-ted on an Ocean Color image from Au-gust 2009. Color bar shows chlorophyll a concentration in mg/m3.

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test, P < 0.01) above 10 m during the daytime (mean: 49.23%) than at night (mean: 39.79%). This diel pattern was also observed by Rowat and Gore (2007), who re-corded a shark tracked in the Indian Ocean that spent 44% of the daytime and 38% of the nighttime in 0–10 m. However, Brunnschweiler et al. (2009) recorded the op-posite diel pattern in diving behavior. This discrepan-cy was possibly related to the variation in prey behavior (Wilson et al. 2006). For example, another filter-feeding shark, the basking shark, was found to perform reverse diel vertical migration with a dusk descent and dawn ascent for tracking Calanoid copepods, their preferred prey, which conducted a reverse diel vertical migration to avoid invertebrate predator chaetognaths (Sims et al. 2005).

Temperature records along the migration path of the whale shark indicated that the upper 120 m of the water column was well mixed and did not have a thermocline. The ambient water temperature of the shark ranged from 16.2 to 31.6 °C, with a mean of 28 °C (Fig. 3). How-ever, the majority of the time was spent within 27–30 °C (mean percentage of time spent in each 12 h period, 73%, SD = 27.6). A little time was spent in waters below 18 °C (0.09%, SD =1.35) on 4 September, when the whale shark dived to 152 m depth and experienced cool waters of 16.4 °C (Fig. 2).

Although information generated from satellite track-ing of 1 whale shark is limited, our results may have

important implications for further conservation man-agement of whale sharks in China. The whale shark oc-cupied a relatively narrow depth and temperature range, spending approximately half its time in warm waters above 10 m and restricted its exploitation to epipelag-ic zones (0–200 m). This predominant habit of shallow dwelling would make them vulnerable to surface threats, such as commercial fishing and maritime transportation, and partially explains the frequent occurrence of its en-tanglement in fishing nets. More research should be un-dertaken to attain a better understanding of their interac-tions with fisheries activities in coastal waters of China. It is also crucial to conduct long-term tracking to eluci-date their movements on a broader scale, which would provide the basis for regional and international coopera-tion in the conservation of the whale shark.

ACKNOWLEDGEMENTSThis project was funded by the Ocean Park Conserva-

tion Foundation, Hong Kong. We thank David Rowat from the Marine Conservation Society Seychelles, Suzanne Gendron from the Ocean Park Conservation Founda-tion, and staff of the South China Sea Administration of Fisheries Enforcement and Monitoring and the Ministry of Agriculture for tagging the whale sharks. We thank www.seaturtle.org for providing tools for plotting satel-lite data.

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