Unlike its smaller cousin, the reef manta ray, the oceanic giant manta ray rarely ventures into shallow coastal waters to have cleaner fish nibble the parasites off its skin. For most of his life he glides through the depths of the open ocean. That makes it complicated for marine conservation organizations to study its behavior.
But now researchers around the marine ecologist Anna Knochel from the King Abdullah University of Science and Technology in Thuwal near Mecca in Saudi Arabia are reporting an important success: Like them in the magazine Aquatic Conservation to reportthey have identified a place in the Red Sea where the animals may give birth to their young.
The protection of the offspring of the giant rays is of particular importance due to their conservative reproductive strategy. Unlike the cod, for example, which lays many thousands of eggs at the same time several times a year and can thus recover quite quickly after a phase of overfishing, a female manta ray only becomes sexually mature at the age of ten to 15 years. As a result, it only gives birth to a young about every three years, so the reproduction rate is extremely low.
In the Gulf of California, where the manta ray population was so depleted by the early 21st century that fishing had to be halted, giant manta rays are now virtually non-existent.
The animals react to divers by twisting their head fins
Global populations of the giant rays have shrunk in just a few decades to levels that have earned them an IUCN “Vulnerable” classification. Definition: “Critically Endangered”. This is mainly the result of targeted fishing.
Unfortunately for the giant manta rays, traditional Chinese medicine ascribes all sorts of healing powers to their gill plates, which is why they have been massively hunted down in recent decades. However, the ray often ends up as by-catch in the huge deep-sea nets of tuna fleets or gets caught in the nets of local fishermen near the coast.
But in order to be able to better protect the animals, one must first understand their living habits. What is known: The giant manta ray (Mobula birostris) occurs in almost all waters of the subtropics and tropics. With a fin span of up to eight meters, it reaches the dimensions of a small airplane and is probably around 40 years old. Like many of the largest sea creatures, it feeds primarily on plankton – microscopic plants, larvae or crabs that float in the water.
In search of it, he dives to depths of 1500 meters and has developed some special techniques. One of them is to perform several somersaults while scooping the microorganisms into its mouth with its overly long head fins.
Scientists recently discovered that these fins could do more than that also used for communication. A slight twitch of the lobe tip has been observed, for example, when the rays greet conspecifics, a curling of the tip when they chase each other for fun – and even a corkscrew-like twist when they hover towards divers. However, the precise meaning of these gestures has yet to be deciphered.
Rays are ovoviviparous, meaning they produce eggs that are hatched inside. It takes more than a year for a single living young animal to be born, its fins folded like a roulade. Unrolled, it already measures almost two meters and is completely independent from the first moment. Such a birth could only be observed in an aquarium in Japan.
Where free-living giant manta rays give birth to their offspring remained a mystery for a long time – until 2018 the first “giant manta nursery” was discovered on a remote reef in the Gulf of Mexico. The researchers around Knochel could now have found a second one.
The scientists combined a proven method with a creative approach. Like some shark species, giraffes or the human fingertip, the manta ray has a unique pattern on its skin that allows each individual to be uniquely identified.
Project leader Anna Knochel and her team combed through thousands of posts on YouTube, Facebook and Instagram for photos and videos of giant manta rays shot by tourists in the Red Sea. The researchers were able to identify 267 different individuals – and in many cases also determine their age. To their amazement, the team noticed a large number of juveniles in the waters off the Egyptian city of Sharm El-Sheikh, sightings of which are otherwise extremely rare.
The local population is often dependent on manta ray hunting
In 2013, the 184 member states of the Convention on International Trade in Endangered Species committed to granting limited protection to the oceanic manta ray in their waters. It can only be hunted using sustainable fishing methods, which is hardly possible in practice. However, in order to be able to take targeted measures, the authorities are dependent on research projects such as that of Anna Knochel.
However, illegal fishing remains a problem, as the local population in the affected regions often depends on the income from ray hunting. On the Indonesian island of Lamakera, a bloody clash almost broke out in 2018 when a village community refused to hand over illegal fishermen to the authorities.
Local and international organizations are therefore trying to create alternative and sustainable sources of income – for example in tourism. Guy Stevens of the Manta Trust explains that people all over the world long for an encounter with charismatic big fish like manta rays or whale sharks. Stevens calculates in a study that business with manta rays alone brings in the Maldives more than 15 million euros annually.
The local population also benefits from this money because new jobs are created. However, scientists warn, tourism can also become a stress factor for the animals. Ecotourism must therefore be designed to be sustainable. In addition, critical habitats must be identified and protected.
Much more research is needed for this. Maybe then we will also find out what the wagging of the head fins is all about when a manta ray meets a human. It probably just means: Pull the leash, dude!