Monday 14 May 2012

Satellite tracking Manta Rays off the coast of Mexico.

Manta Rays, or Devilfish (Manta birostris) are the worlds largest Batoid Fish (bilaterally symmetrical fish with cartilaginous skeletons descended from Sharks), reaching widths of up to 7.1 m. They are considered to be Vulnerable by the International Union for the Conservation of Nature, due to their low breeding speed (Manta Rays produce one or two live young after a pregnancy lasting twelve months) and increasing targeting by fishermen; Manta Rays are eaten and used as bait for sharks, they are also increasingly used in traditional Chinese medicine. Many Manta Ray habitats also coincide with shipping lanes, and reefs where the animals aggregate are in places becoming popular with tourists, both of which are likely to have some effect on them. To make matters more complicated scientist have recently recognized a second species of Manta, Manta alfredi, or the Reef Manta Ray, which is slightly smaller, but which may be the species found in most large groups on reefs, making the Giant Manta (M. biostris) somewhat rarer than previously thought; a third species may also exist.

A diver swimming with a Manta Ray off the coast of Mexico; it is unclear how this behavior affects the Rays. Franco Banfi/Caters/The Telegraph.

In a paper published in the journal PLoS One on 10 May 2012, a team of Scientists led by Rachel Graham of the Wildlife Conservation Society's Gulf and Caribbean Sharks and Rays Program and the Centre for Ecology and Conservation at the University of Exeter describe the results of the first study of the movements of Manta Rays using satellite tracking technology.

The team tagged a six Manta Rays during a thirteen day research cruise off the Yucatan Peninsula, Mexico. The Rays were then tracked by satellite for as long as the tags survived; between 2 and 64 days. During this time they were able to observe the movements, and asses the feeding strategies used by the Mantas by observing how they interacted with resources. 

Maps showing the movements of three Manta Rays, a female (g), a male (h) and a juvenile (i), during the period their tags remained attached. The colours represent Surface Sea Temperature. Dotted contour lines represent seafloor depth. Graham et al. (2012).

During the time of study the Mantas spent the majority of their time in surface waters; 83% of their time above 50 m deep, and 92% of their time above 100 m deep. They also favored warmer water, spending 95% of their time in water warmer than 26.1°C. They were conservative in their movements, spending the majority of their time in areas of known population density. This confirms the results of studies made using other techniques (principally visual identification) in other parts of the world.

Map showing known area of Manta population density (grey); darker colours representing denser populations. Blue areas represent established marine conservation areas. Crosses represent Mexican tourism ports. Graham et al. (2012).

There are a number of existing marine protected areas around the Yucatan Coast, set up to conserve other species, but the areas favored by Manta Rays are mostly outside of these. Manta Rays are in theory protected from exploitation by fishing vessels in Mexican waters, but these restrictions are thought to be widely ignored. In theory additional reserves could be established to include the areas used by Manta Rays, but there are clearly limits on how many such such reserves could be established and effectively policed; if fishermen find too much of the sea is closed to them they may stop respecting any of the reserves.

During the study the Mantas were observed using two feeding strategies. Firstly the visited an area being used for spawning by a shoal of Little Tunny (Euthynnus alletteratus), where large numbers of planktic eggs were being produced. Secondly they visited areas of seasonal upwelling, where currents were brining nutrient-rich waters up from the deep, fueling a phytoplankton (single-celled marine algae) bloom, and consequent boom in population in the animals which feed on these, typically Sergestid Shrimp and Calanoid Copepods and the predatory Chaetognaths (Arrow Worms) that feed upon them.

The areas used by the Manta Rays coincide with some of the busiest shipping lanes in the Caribbean, potentially putting the Rays at risk both from direct strikes, and from marine litter (particularly plastics) and oil or other spills.

Map showing shipping lanes in the West Caribbean. Graham et al. (2012).

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