Of the 29 oceanographic surveys that Prince Albert led between 1885 and 1915, 13 were conducted in the waters around the Azores. As well as being remote, these nine islands have a complex geological background that, together with the surrounding deeper waters, results in a unique diversity of marine habitats and of life within them. The objectives of the prince’s surveys were primarily to investigate the deep-sea fauna, but also to study the oceanographic currents and meteorology and to map the sea floor. He ended up observing all manner of creatures, from bacteria and plankton to fishes, turtles and marine mammals, between the surface and a depth of 6,000 metres (20,000 feet). His surveys contributed immensely to the progress of oceanography, a fairly new science at the time, and also played a part in devising innovative methodologies, leading to the discovery of life forms and ecological relationships that hadn’t been known about before.

Although Prince Albert realised at the time that the Princesa Alice sea mount would be an important fishing ground, it’s unlikely that he could have imagined that, more than a century after its discovery, it would become one of the most important dive sites in the North Atlantic and that the revenue generated from diving would surpass by far the revenue that came from fishing. The main reason for this is the presence of majestic Chilean devil rays, which visit the sea mount every summer. The predictability of their aggregations, coupled with the rays’ curious attitude towards divers, provided an opportunity for the rapid growth of a targeted diving industry.

Princesa Alice is, in fact, one of only a few places in the world where Chilean devil rays are known to aggregate in large schools. Another important aggregation site in the Azores is Ambrósio, also a sea mount, which lies three nautical miles off the north coast of the island of Santa Maria. Elsewhere, the devil rays reportedly amass in Brazil’s St Peter and St Paul Archipelago and around Cocos Island, Costa Rica. Aggregations such as these offer an incredibly rare opportunity to study the species in the wild. Nevertheless, collecting data on such oceanic and migratory animals is no easy task.

The Chilean devil ray is large – more than three metres (10 feet) from tip to tip – slow-growing and one of the least fecund of the elasmobranchs, producing a single pup every two or three years. These pelagic rays occur around the world in sparse locations in the Pacific, Atlantic and Indian oceans, appearing to visit productive areas around oceanic islands and offshore sea mounts on a seasonal basis. The size of the species’ global population, like that of the other Mobula species, is not known. However, low fecundity, together with other life-history and ecological traits, make it particularly susceptible to overexploitation.

A combination of different approaches has enabled us to shed a little light on the rays’ mysterious lives, especially while they are here in the Azores. We have learnt from data collected by dive operators that they start arriving in the region in June and stay until the very end of October, so they are here when the water is warmer and less productive. Satellite tagging has revealed that when they leave our sea mounts they head south; tagged Chilean devil rays have travelled distances of up to 3,800 kilometres (2,360 miles) in seven months, demonstrating their truly migratory nature. However, what they do and where they go after that remains a mystery.

When Chilean devil rays and giant mantas occur together, they typically interact; usually the devil ray is observed following the manta. The third species is seen more rarely, and when it is encountered it is closer to shore and in groups of three to five individuals. We don’t know where these animals come from or where they go when they leave our waters. Further research and the development of new methodologies will be key if we are to gain a better understanding of the ecology of devil rays.

What we do know is that the biggest threat mobulid rays currently face is fishing. In Asian markets there is a growing demand for their gill plates, which are used in a health tonic, and the animals are taken as by-catch as well as in targeted fisheries. In fact, the ever-increasing demand has led to a shift from incidental by-catch to targeted fishing. Throughout its wide range, the Chilean devil ray is caught both incidentally and as a targeted species, and by both artisanal and commercial fishers. It has the largest – and therefore most valuable – gill plates of all the mobulid rays and is thus in greatest demand. Its gill plates can be sold for up to US$557 per kilogram in China, and a single ray can produce up to 3.5 kilograms (almost eight pounds) of dried gill plates.

Because the locations and timing of its aggregations are predictable, and because it is a highly migratory species, the Chilean devil ray is vulnerable to both coastal and offshore fishing. Even though it is not fished in the Azores, the species knows no boundaries and once the rays that we see at the sea mounts leave our waters, it is likely that they become targets for fisheries.

At the global level, M. tarapacana is listed as Vulnerable on the IUCN Red List of Threatened Species, while in South-East Asia, the Eastern Pacific and the Indian Ocean it is classified as Endangered. We suspect that its global population has declined by at least 30% over the past 30 years. Only a few decades ago, the aggregations seen in the Azores were much larger than those reported in the past five years. If the demand for gill plates continues to drive unsustainable fisheries, these local aggregations may be at risk of disappearing altogether.

In September and October 2016, representatives of governments from around the world gathered at the 17th Conference of the Parties to the Convention on International Trade in Endangered Species (CITES) in Johannesburg, South Africa. Here they debated a proposal presented by Fiji to include the nine Mobula species in Appendix II.