Friday 29 August 2014

Satellite tagging Whale Sharks in the Red Sea.

Whale Sharks (Rhincodon typus) are the largest extant Shark species, and indeed the largest living Fish species of any kind, often exceeding 10 m in length. They are found in tropical and subtropical waters across the world, but there life-cycle and biology are poorly understood;  they are filter feeding planktivores, however unlike the slightly smaller Basking Sharks (Cetorhinus maximus) they do not feed directly on phytoplankton, instead actively feeding on zooplankton and small fish. Whale Sharks are listed as Vulnerable on the International Union for the Conservation of Nature’s Red List of Threatened Species. They have a long life cycle, a slow reproductive rate and a migratory lifestyle that moves them across many political boundaries, making them hard to protect for their entire life cycles. They appear to change their movement patterns at different stages in their life-cycles, with observed aggregations of Whale Sharks typically consisting of individuals of similar age. Whale Sharks have been targeted by fisheries in many parts of the Indo-Pacific region, and while many countries have now introduced laws to limit or ban exploitation, these are seldom enforced. Tellingly several countries have reported a drop in catch without a reduction in fishing effort, which is usually a sign of a declining population.

In a paper published in the journal PLoS One on 30 July 2014, a team of scientists led by Michael Berumen of the Red Sea Research Center of the King Abdullah University of Science and Technology discuss the results of a satellite tagging program carried out on Whale Sharks at an aggregation site near Al-Lith, on the Saudi Arabian Red Sea coast, and the data this revealed about the biology and movements of the Sharks.

A diver approaching a Whale Shark, Rhincodon typus. Red Sea Research Center.

The study was initiated after a number or reported Shark sightings by commercial dive boats, carrying tourists to popular locations on the reefs of the southern Red Sea. Juvenile Whale Sharks (2.5-7.0 m in length) were found to be gathering on the northern Shi’b Habil Reef, about 4 km off the coast of Al-Lith during the spring period, which is roughly March to May in the southern Red Sea. Sharks were approached by divers in 2009, 2010 and 2011, who attempted to assess their size and sex before attaching a satellite tag to the dorsal fin with a long pole. 47 of the 59 tags deployed operated for 11-135 days, with a failure rate of 20.4% (12 tags that did not work at all); such a failure rate is typical for such experiments, though why it occurred is unclear.

Study sites for Rhincodon typus in the Saudi Arabian Red Sea. (A) Location of the study area within the Red Sea. (B) Locations of 59 satellite tag deployments on juvenile Rhincodon typus near Al-Qunfudhah (n = 2) and Al-Lith (n = 57). (C) Detail of tag deployments around Shi’b Habil near Al-Lith (n = 55). Symbol color indicates the year of tag deployment. Berumen et al. (2014).

The Sharks were found to have a roughly evenly balanced sex ratio (18 male, 21 female and 18 undetermined individuals). Studies of older Whale Sharks in aggregations have found these gatherings to be heavily skewed in favour of one sex or the other, though at what point in their lives the Sharks segregate, or why they do so, remains unknown.

The majority of the Sharks remained within the southern Red Sea, at least for as long as the tags remained operational, apparently following a regular cycle in which they spend spring on the Saudi Arabian coast, then move to the Sudanese coast during the summer, then south to the Eritrean Coast in autumn, before recrossing the Red Sea to spend winter off the coast of Yemen, moving northwards back into Saudi Arabian waters in the spring again.

Movements of 47 Rhincodon typus tagged with satellite tags in the Saudi Arabian Red Sea: Most individuals (n = 39) made basin-scale movements within the southern Red Sea. Berumen et al. (2014).

Three of the tagged individuals moved northwards, reaching as far as Sharm el-Sheikh on the Egyptian Coast. Exactly why they did this is unclear; the waters of the southern Red Sea are considered to be more productive, and therefore presumably present better feeding opportunities to Whale Sharks, and were clearly favoured by the majority of individuals.

Movements of 47 Rhincodon typus tagged with satellite tags in the Saudi Arabian Red Sea: Three individuals performed excursions into the northern Red Sea as far as Sharm el-Sheikh. Berumen et al. (2014).

Five individuals moved out of the Red Sea altogether, moving through the Gulf of Aden then northward into the northern Indian Ocean. These individuals were in the 3-5 m size range (roughly in the middle of the sample size range, rather than at the upper end which might imply an age-related change in behaviour), and four of the individuals were of indeterminate sex, the remaining one being male, making it impossible to determine if this movement related to sexual segregation in maturing individuals. It does, however, imply that the population of Whale Sharks in the Red Sea is not separate from that in the western Indian Ocean, and that individuals move between the two groups.

Movements of 47 Rhincodon typus tagged with satellite tags in the Saudi Arabian Red Sea: Five sharks departed the Red Sea and moved into the Gulf of Aden and northern Indian Ocean. Berumen et al. (2014).

The Sharks were all spotted and tagged close to the surface, and spent the majority of their time in the upper 50 m of the water column, though only 16% of their time within 2 m of the surface. The Sharks occasionally went through periods of deeper foraging and would spend about 80% of their time in the 200-400 m zone. Deeper excursions were also fairly common, with Sharks making individual dives below 400 m, and in the case of three individuals, below 1000 m (maximum recorded depth 1360 m). The waters of the Red Sea are unusual in that there is little vertical temperature differentiation, with waters ranging from up to 34˚C at the surface to about 21.7˚C at 400 m, then remaining constant to a depth of about 3 km. This meant that diving Sharks within the Red Sea were never encountering temperatures lower than 21.7˚C, and that the depth to which they dove was probably more influenced by oxygen availability. This is interesting, as it has been suggested that the movements of Whale Sharks are largely influenced by water temperature. However the Sharks which left the Red Sea continued to dive regularly, encountering a minimum temperature of 8˚C in the Gulf of Aden, suggesting that the Sharks are able to tolerate at least short periods at much lower temperatures.

It is not clear exactly why the Whale Sharks congregate around the Shi’b Habil Reef in the spring, though it may be associated with Coral spawning, which in the southern Red Sea takes place around the full moons in April to June. This is supported by the presence of Manta Rays in the area at the same time (also large plankton feeders), though the Mantas spend the majority of their year in inshore waters around Al-Lith, and do not engage in deep-diving behaviour, suggesting there is only a limited overlap in diet between the two species.

See also…

Sawsharks (Pristiophoridae) are highly specialized...

Eagle Rays (Myliobatidae) are large Batoid Fish (bilaterally symmetrical fish with cartilaginous skeletons descended from Sharks), that predominantly live as free swimming animals in the water column rather than on the sea bottom. They are strong swimmers, and some...

Manta Rays, or Devilfish (Manta birostris) are the worlds largest Batoid Fish (bilaterally symmetrical fish with cartilaginous skeletons descended from Sharks)...

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