Every time I see a group of Chilean devil rays Mobula tarapacana, I am as amazed as I was the first time I ever saw these intriguing and captivating creatures. The more I observe them, the more questions arise. They are such big animals, and yet we know so little about them. According to the IUCN Red List of Threatened SpeciesTM, the species is Data Deficient; we don’t even know the current status of the global population. A recent telemetry study conducted in the Azores revealed that Chilean devil rays dive to extreme depths, reaching at least 1,800 metres. Every summer they come to the Azores, in the middle of the Atlantic Ocean, and gather in large numbers at shallow sea mounts.
But why? What is so important about these shallow sea mounts that highly migratory Chilean devil rays aggregate in such specific locations? That is yet to be unveiled…
A sea mount is regarded as shallow when its summit reaches a depth of less than 100 metres. Such sea mounts can be important aggregation sites for migratory pelagic species. However, we have little idea of what it is about them that attracts the aggregations, mainly because it is often difficult to determine the drivers of distribution in large, highly migratory species. Sea mounts create characteristic conditions, such as an increased vertical flow of nutrients or the retention of the water masses that promote local productivity – and thus support organisms of the higher trophic levels that feed on it.
Sea mounts may also have distinctive magnetic signatures (features that interrupt the earth’s magnetic field) that numerous migratory pelagic species could use in navigation and as cues to stop. It is most likely, though, that the explanation for aggregations lies in a combination of factors that make sea mounts an ideal location for them.
In any case, it is clear that adult Chilean devil rays aggregate at these particular sites in much higher numbers than anywhere else. Most of the individuals seen in these aggregations are adults, including numerous pregnant females, which suggests that these locations may well play a role as mating and/or pupping grounds for this species. The rays are also observed feeding, an indication that they use local resources when at aggregations.
There is a gradual increase in the number of individuals in the aggregations at the beginning of the season and a gradual decrease towards the end. Changes in water temperature, day length and food abundance may all be factors explaining this seasonal aggregation.
Another really interesting behaviour is the rays’ obvious interaction with divers. They are incredibly curious and while circling the groups of divers hanging onto the cable in the deep blue they approach very closely, almost to the point of touching them.
When I first heard about these aggregations I was told that a few years ago they used to comprise 50, 60 and even more than 80 individuals at the same time. With the help of dive centres, I have been collecting data over the past three years and we have recorded a group of 40 only once or twice every season.
Are the rays coming in lower numbers? Are they dispersing to different locations? Or is the population decreasing?
To answer these questions, further research is urgently needed. It is crucial to learn more about the movements of Chilean devil rays and how they use their habitat if we are to evaluate possible threats and identify areas of particular ecological importance for their population. Identifying the location of aggregation sites and migration routes can provide important information for the implementation of conservation measures.
Aggregation sites like the ones in the Azores are going to be fundamental to increasing our knowledge about the least studied species of the family Mobulidae.