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Rhincodon Typus) in the Gulf of Tadjoura, Djibouti

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Rhincodon Typus) in the Gulf of Tadjoura, Djibouti Environ Biol Fish DOI 10.1007/s10641-006-9148-7 ORIGINAL PAPER Aggregations of juvenile whale sharks (Rhincodon typus) in the Gulf of Tadjoura, Djibouti D. Rowat Æ M. G. Meekan Æ U. Engelhardt Æ B. Pardigon Æ M. Vely Received: 23 April 2006 / Accepted: 16 September 2006 Ó Springer Science+Business Media B.V. 2006 Abstract A total of 23 whale sharks were ARGOS (Splash) satellite tag for 9 d. During this identified over a 5 d period in the Arta Bay time the shark traversed to the shoreline on the region of the Gulf of Tadjora, Djibouti. Most of opposite side of the Gulf (a distance of 14 km) the sharks aggregating in this area were small and then returned to the Arta Bay area before (<4 m TL) males. Individuals were identified retracing his path to the other shore. The shark using photographs of distinctive scars and spot spent most of the daylight hours at the surface, and stripe patterns on the sides of the animals. Of while at night dives were more frequent, deeper these, 65% had scarring that was attributable to and for longer durations. boat or propeller strikes. Most of the whale sharks we encountered were feeding on dense Keywords Whale shark Á Rhincodon typus Á accumulations of plankton in shallow water just Aggregation Á Feeding off (10–200 m) the shoreline. This food source may account for the aggregation of sharks in this area. One 3 m male shark was tagged with an Introduction D. Rowat (&) Whale sharks, Rhincodon typus, are found Marine Conservation Society Seychelles, P.O. Box throughout the world’s tropical oceans (Compag- 1299, Victoria, Seychelles e-mail: [email protected] no 2001) with the first scientific description being made from a specimen from the Indian Ocean M. G. Meekan (Smith 1829). Seasonal aggregations of these Australian Institute of Marine Science, P.O. Box animals have been recorded at a number of 40197, Casuarina MC, Darwin, NT 0811, Australia coastal localities in the Indian Ocean such as off U. Engelhardt north-western Australia (Taylor 1989; Taylor Reefcare International Pty. Ltd., P.O. Box 1111, 1996; Colman 1997; Meekan et al. 2006); India Townsville, QLD 4810, Australia (Silas 1986; Hanfee 2001), the Maldives (Ander- B. Pardigon son and Ahmed 1993) and off Seychelles (Colman Marine Conservation Society of Djibouti, BP 4476 1997; Newman et al. 1997; Rowat 1997). At many Djibouti, Djibouti Republic of these places, the predictable timing of these aggregations has allowed significant ecotourism M. Vely Megaptera, 4 rue Pierre et Marie Curie, 77186 Noisiel, industries to develop that support regional econ- France omies (Newman et al. 1997), while in others, 123 Environ Biol Fish aggregations have been targeted by fishermen Red Sea, is an inlet of the Indian Ocean caused by (Hanfee 2001). In view of the long distance the fault line of the northerly end of the East migrations of this species (Eckert and Stewart African Rift Valley that transects Djibouti, Ethi- 2001; Eckert et al. 2002; Wilson et al. 2006) and opia and Kenya. The area is geologically and their relative economic importance in some areas, volcanically active as evidenced by the 1978 it is valuable to characterise these aggregations eruption of the Ardoukoba volcano. The seabed and the movements of sharks within them. shelves steeply from the coast of the Gulf Coastal aggregations of whale sharks are usu- dropping to 100 m depth around 2 km offshore ally dominated by immature males of around 5– and to around 450 m depth in the centre of the 7 m in length (Heyman et al. 2001; Meekan et al. Gulf of Tadjoura. 2006). Spatial and/or temporal segregation of Anecdotal reports suggested that whale sharks populations by size and gender is typical of sharks occur during the months of October to February (Springer 1967; Sims et al. 2000), but raises in the Arta Bay area, (latitude 11° 35¢N, longitude questions as to the location of the remainder of 42° 49¢E) on the southern coast of the western the population, and the reasons for these aggre- end of the Gulf of Tadjoura some 33 km from gations. Coastal aggregations of small whale Djibouti city (Fig. 1). sharks in the Sea of Cortez appear to occur in response to seasonal aggregations of their plank- Demographic study tonic food (Clarke and Nelson 1997), as is the case with larger members of the species (Wilson et al. From the 20th to the 24th of January 2006, whale 2001). There are, however, very few records of sharks along a 3–5 km stretch of coastline cen- whale sharks from size at birth (around 58 cm to tered on Arta Beach were found by haphazard 64 cm), (Joung et al. 1996) to 3 m (Colman 1997). surface searches from a small (6 m) boat. The New-born whale sharks have occasionally been sharks were found either by sighting their dorsal found in the guts of blue sharks, Prionace glauca, or tail fins breaking the surface or by watching for and blue marlin, Makaira mazara, suggesting that the swirling, vortex patterns on the water caused very small sharks may be pelagic (Colman 1997). by the sharks’ suction feeding just below the In 2004, anecdotal reports by a local conserva- surface. On sighting a shark, a recorder aboard tion organisation suggested that a previously the boat would note the time and location and the unknown aggregation site for whale sharks shark would be photographed using an underwa- existed in coastal waters of the Gulf of Tadjoura, ter digital camera by a snorkeller. Where possi- Djibouti, at the northern eastern margin of the ble, images were recorded of the area behind the Indian Ocean (Staszewski et al. in press). This gills and forward of the dorsal fin for both sides of aggregation appeared to be unusual in that it was the shark, as well as of any obvious scarring. composed of relatively small juvenile (3–4 m) Details of each shark’s size, sex, behaviour and sharks, similar to the aggregation reported in the any other associated fauna were recorded, when Sea of Cortez, and the first reported in the Indian possible, for each encounter. Size estimation was Ocean. Here, we describe the demography, made by one of three trained researchers, each behaviour and short-term (daily) movements with at least 10 years of experience, by compar- and diving patterns of whale sharks in this ison of the shark against a nearby object of known aggregation in the Gulf of Tadjoura, Djibouti. length, such as the boat or snorkeller, or by the use of a graduated 1.5 m pole spear as a measur- ing aid. These by-eye estimates of length were Methods usually within 25 cm of those made by measuring sharks with a tape measure. Study area Short-term shark movements were determined by evaluation of the daily sighting log and digital The Gulf of Tadjoura, (latitude 11° 40¢N, longi- photography. Spot and stripe patterns on the tude 43° 00¢E) at the southern entrance to the dorsal surface were used to individually identify 123 Environ Biol Fish Fig. 1 Study area, Arta Bay on the Gulf of Tadjoura, Djibouti, north east Africa showing ARGOS satellite track of a 3 m male whale shark sharks from photographs (Meekan et al. 2006). depth and temperature at 2 s intervals. Summary Photographs were first separated into individual histograms of these depth and temperature data encounters, each encounter relating to a single sets were uploaded to the satellites when the shark. Obvious matches were then made by eye shark was at the surface. based on characteristics such as major scars and Histogram data was decoded using the ‘SAT- the presence of marker tags or tag remnants. The PAK2003’ software, provided by the tag manu- remainder were compared using spot and stripe facturer, and the tag’s location was plotted daily patterns. The record of sightings based on using the ‘STAT’ Satellite Tracking and Analysis identification photographs was analysed to estab- Tool (Coyne and Godley 2005)1. Distances lish the frequency of sighting and resighting of between locations were calculated by STAT on individuals. high quality ARGOS location codes (1–3) only, to Long distance movement and diving patterns minimise errors and allow speeds to be accurately were investigated by tagging one shark with a calculated. Splash satellite tag (Wildlife Computers, Seattle). Water temperatures were recorded using a The tag was attached through base of the dorsal ‘Hobo Tid-bit’ temperature logger (Onset Tech- fin of the shark on a 1 m stainless steel wire trace nologies). Where groups of sharks were found in of 150 kg breaking strain, by a stainless steel a confined area the logger was lowered on a line anchor dart applied in-water by a modified spear- to the sea-bed and then raised in 5 m increments gun. The tag recorded position via ARGOS satellites when the shark was at the surface and 1 www.seaturtle.org 123 Environ Biol Fish to record a temperature depth profile. Weather the steeply-shelving Arta Beach. Feeding behav- and in-water visibility were recorded for each iour recorded included both ram filter feeding and shark encounter. vertical feeding at the surface. The zooplankton on which the sharks were feeding were dense concentrations of chaetognaths, salps and mantis Results shrimp larvae. Temperature profiles at the 20 m depth contour off the beach where whale sharks Nearly all the whale sharks were sighted along a were seen feeding varied from 27°C at the surface 1.5 km stretch of coastline centred on Arta Beach to 25.5°C at 20 m in the morning, while in the and less than 500 m from shore.
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