Inside Stories

Unravelling stingray secrets

SOSF D’Arros Research Centre

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St Joseph Atoll in the Seychelles is teeming with stingrays, and Chantel Elston is there to study them, especially what they eat. Her initial results suggest that their diet consists largely of marine annelid worms – related to earthworms – from the lagoon’s sandy bottom.
Photo by Rainer von Brandis

I’d like you to stop for a moment and think of everything you know about stingrays. For most people, the answer is probably not much. In fact, I bet the first thing you’ll think of has something to do with the late Steve Irwin. But you needn’t worry; the truth is, the scientific community doesn’t know much about these animals either.

What we do know is that there are more than 70 species of whiptail stingrays belonging to a family called Dasyatidae. These animals have wide-ranging distributions and generally occur in shallow tropical waters. Evolutionarily they are closely related to sharks and share many similar features, such as having only cartilage in their skeletons. Unfortunately, though, the unanswered questions, including basics such as the size at maturity for many species, far outweigh the answered ones. Human curiosity is naturally attracted to what it finds most interesting and the larger and more charismatic animals, such as sharks, have been in the forefront of elasmobranch research. Consequently research on stingrays has lagged behind.

The Save Our Seas Foundation D’Arros Research Centre (SOSF–DRC) might be a pinprick in the Indian Ocean, but despite its tiny size and remote location, it’s in just the right spot to start closing this knowledge gap on stingrays. It is situated only one kilometre from St Joseph Atoll, a place that could easily be nicknamed ‘Stingray City’. Shallow sand flats surrounding a deeper lagoon teem with three stingray species, from juveniles through to adults.

The first question that SOSF–DRC scientific director Rainer von Brandis asked about these stingrays was, ‘What do they eat?’ Not surprisingly, he couldn’t find an accurate answer. Although numerous references in guide books and scientific literature state that the three species – cowtail ray Pastinachus sephen, mangrove whiptail Himantura granulata and porcupine ray Urogymnus asperrimus – eat benthic organisms and fish, only one detailed dietary study has been published, and that was on the porcupine ray in Australia. Rainer’s simple enquiry gave rise to a research project in 2014 that aims not only to delve into the three species’ diets, but to find answers to many other questions too.

Two successful field trips were conducted in 2014, during which we used the relatively novel non-lethal technique of gastric lavage (or stomach flushing) to collect stomach contents from 55 porcupine rays. In addition, we surgically implanted acoustic transmitters into 17 mangrove whiptails and 13 porcupine rays. At predetermined intervals, these transmitters send out pings that are recorded by a network of receivers in the lagoon. This will enable us to monitor the movements of these stingrays for the next couple of years, even when the researchers are away from the island.

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Chantel Elston Project Leader

Rainer von Brandis Scientific Director of the SOSF D'Arros Research Centre

Preliminary results have been very enlightening and Rainer finally has part of the answer to his question. After sifting through what was essentially stingray vomit, we found that porcupine rays eat mostly annelid worms (which are very common in our sediment samples). Every now and then a prawn or shrimp is found among the worms and we hypothesise that when a porcupine ray finds one of these crustaceans it’s like hitting the jackpot. On future field trips stomach contents from the two other stingray species will be collected and blood and muscle samples will be taken for stable isotope analyses. These analyses will provide insight into the long-term variation in the stingrays’ diets.

The surgically implanted acoustic trackers also provided a wealth of information. After the stingrays were tagged in March, the data from the receivers were downloaded in November and more than 200,000 pings were recorded. Even a quick glance at this data provided some surprises. We found that the smallest tagged juvenile (a mere 30-centimetre disc-width) ventured into the dangerous deep of the lagoon where large, predatory sicklefin lemon sharks lurk. This was contrary to our hypothesis that small juveniles always remain on the shallow sand flats where it is relatively safe. Also, larger individuals were found to leave the atoll; in fact one was recorded about 15 kilometres away. These preliminary results lead to even more questions, such as why do the stingrays leave the atoll? Is it because there is not enough food for the larger individuals? Do they leave the atoll to mate? We hope to find some answers over the next two years.

‘But what does it matter?’ one might ask. Why is it important to know what stingrays eat or where they spend their time? Well, something to store in your stingray fact collection is that they are crucial to the ecosystems in which they occur. They fall in the middle of the food web and are important in the transfer of energy from lower trophic levels, such as shrimp, to higher trophic levels, such as sharks. They also play a major role in modifying the soft sediments over which they occur, shifting and aerating the sediments as they feed.

Unfortunately, many stingray populations are in decline due to their susceptibility to overfishing. Like sharks, stingrays grow slowly, breed late and give birth to few young. This means they can’t replace their numbers quickly enough to compensate for the numbers being removed both by targeted fisheries and through by-catch. In fact, the porcupine ray is listed as Vulnerable by the IUCN, and St Joseph Atoll is one of the few places in the world – if not the only location – where it occurs in such high abundance. We’re really lucky to be working on this species at St Joseph; Andrew Chin, who is also funded by the SOSF to research porcupine rays, has a much harder job finding these rays on Australia’s Great Barrier Reef as they are rare and difficult to locate there.

A Global Shark Red List assessment that was published last year highlighted the plight of stingrays; they are the fifth most threatened of all the shark, ray, skate and chimaera families, with 30% of species considered threatened and 39% of species lacking enough data to make an assessment. Thus any knowledge gathered about these animals is a gain. What we learn from this project will be beneficial not only when trying to manage the St Joseph population of stingrays, but when trying to understand and conserve these species throughout their tropical range.