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Swim Like an Eagle

TCI’s eagle rays on “Near Threatened” list

By Dr. Aaron C. Henderson, Resident Lecturer, The School for Field Studies, Center for Marine Resource Studies, South Caicos

Eagle rays flap their pectoral fins like birds use their wings. Jan Lupton

Eagle rays flap their pectoral fins like birds use their wings. Jan Lupton

The spotted eagle ray, known scientifically as Aetobatus narinari, is one of the largest species of fish inhabiting the waters around the Turks & Caicos Islands. Growing up to 300 cm across (almost 10 feet), a close encounter with an eagle ray is not quickly forgotten. Unfortunately, eagle ray populations have experienced large declines throughout their range in recent decades due to both targeted and accidental capture in commercial fisheries, and the species is now classified as “Near Threatened” by the International Union for the Conservation of Nature (IUCN).
Furthermore, very little is known about the biology of this magnificent behemoth, to the extent that it is even unclear exactly how many species of eagle ray occur throughout the world’s oceans. Just a few years ago, it was assumed that there was just a single species, namely Aetobatus narinari, but recent research employing DNA analysis has found that there are at least two species, and possibly as many as four, each inhabiting different geographical regions.
Despite their large size, eagle rays feed on relatively small invertebrate prey such as crabs and whelks. Their teeth form flattened plates which are strong enough to crunch through hard shells, but they also feed on soft-bodied organisms such as worms and small fish. These feeding activities bring eagle rays into the shallow waters of the Turks & Caicos Islands, where they rummage in the soft sand for their preferred prey, while expertly maintaining position using their large pectoral fins. Unfortunately, these excursions into shallow waters also bring the rays into closer contact with human activities such as boat traffic, which can potentially disrupt their natural behaviour or even cause physical injury to them.
In order to gain a better understanding of eagle ray ecology and the potential for human disturbance, The School for Field Studies, Center for Marine Resource Studies on South Caicos is currently conducting research into their movement patterns and behaviours. Biologists normally conduct such studies by attaching tags to their study animals, thereby assigning an identity to each individual; subsequent encounters with these individuals can then be recorded and, over time, a picture of their movement patterns starts to emerge. However, this approach usually requires capturing and restraining the study animal in order to attach the tag, and this can be a very stressful experience.
So, we are using an alternative approach which allows us to identify the rays without ever having to touch them. The white spots that cover each ray’s back form a unique pattern on each individual, akin to a fingerprint. Instead of physically capturing the rays, we take a photograph of the spot pattern, which we then load into a piece of software that converts the pattern into a digital map, which is stored in the software’s database. Every time we add a new photograph to the software, it searches through its database to see if the spot pattern matches any previous entries. In this way, we can identify re-sightings of individual rays, and gradually build a picture of their movements (as long as we have information relating to where and when the photos were taken).
The results so far have been very interesting. A number of rays have been re-sighted on multiple occasions over the course of the study, suggesting that they are resident in our local waters. Furthermore, most of these rays return to the same study locations time after time, indicating that they follow very specific routes when they are travelling. On the other hand, there are some rays that we have seen only once, and these are likely to be transient individuals that are just passing through our study area. Additionally, it seems that the large schools formed by the rays are quite dynamic, with smaller groups or individuals regularly breaking away to do their own thing.
So far, our study has been limited to the waters around South Caicos, but we are very keen to expand it to cover as much of the Turks & Caicos Islands as possible. It will be interesting to see if there are discreet groups of rays inhabiting different areas or if they move between islands, utilising much larger areas. It will be particularly interesting to see if any rays traverse the deep waters of the Columbus Passage between the Caicos Islands and the Turks Islands.
As many of the visitors to this country spend at least some time in the water, often with cameras, this is the perfect opportunity for some “citizen science” to help us expand our study. All that we need is a photograph taken directly above the ray, together with information regarding where and when the photograph was taken. These don’t need to be professional-quality photographs; as long as the spot pattern is visible, that is the most important thing. The photograph and its associated information can then be uploaded to our project website at www.fieldstudies.org/eagle-ray-project. The photographs will remain the property of the people who took them, and they will not be used for any purpose other than identifying the rays.
So the next time you go into the water anywhere in the Turks & Caicos Islands, whether snorkelling or diving, make sure you have your camera with you! You might end up making a very important contribution to our understanding of eagle ray ecology, which in turn might go a long way to helping conservation efforts.
However, please be careful not to get too close to the rays! Although they are generally shy creatures that move away from humans, they are extremely powerful swimmers and can cause serious injury if they accidentally collide with a person while trying to swim away. Additionally, like many other ray species, they possess venomous stinging barbs at the base of their tail, which can also cause serious injury.
Further information about The School for Field Studies in general can be found at www.fieldstudies.org while information on the Center for Marine Resource Studies is at www.fieldstudies.org/tci.



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