What drives sharks and rays?

In the ten or so years since completing his Master’s degree in Marine Biology, Guido Leurs has rapidly established himself as an expert on sharks and rays. He is a member of the IUCN Shark Specialist Group, in which experts pool their knowledge and data and assess whether species qualify for inclusion on the international Red List of Threatened Species. His status recently took him to Sri Lanka, where he delivered a keynote speech at the Sharks International 2026 conference, attended by all the leading academics in his field of research. ‘Fortunately, I don’t suffer from imposter syndrome, but I was still quite nervous about telling my story to all these leading figures.’

Leurs had never seen a shark or a ray in the wild when he decided to focus on them whilst studying for his Bachelor’s degree in Applied Biology at HAS. His interest continued to grow during his Master’s degree in Biology at WUR: ‘In Jaws and other shark films that I had watched with my father as a child, sharks were portrayed as monsters. Documentaries like Rob Stewart’s Sharkwater and the work of Steve Irwin gave me a completely different perspective. And I realised just how hard life is for these animals. I then decided to study them in more depth. During my bachelor’s degree, I went in search of wild sharks. That was obviously exciting, but I also discovered that sharks are particularly dangerous to people who assume that sharks are harmless.’

In scientific terms, sharks and rays are treated as a single group because they are evolutionarily related: they are cartilaginous fish. ‘From an evolutionary point of view, rays are flat sharks,’ Leurs explains, ‘But in ecology, sharks are classified as primary predators, whilst rays are predators lower down the food web.’

In his view, however, the reality is more nuanced: ‘Young sharks are just as much a part of the food web as rays. They are hunted relentlessly by other sharks - even those of the same species - and by marine mammals.’

Leurs studies the role of sharks and rays in the ecological system to which they belong. ‘In many of my projects, I study how sharks migrate, where they are at a particular time of year and what they eat. That may be because of the presence of prey, the temperature of the seawater or other reasons. But we’re not really sure.’

Of the approximately 540 species of shark and 670 species of ray in the world, only a fraction are being studied. On behalf of WUR, he personally carries out research in West Africa, off the coast of Guinea-Bissau, for example. He trained local researchers to record all the sharks and rays caught in the fishermen’s nets and brought ashore. Leurs collects all the information and compiles it into a dashboard for the national fisheries authority.

One of the species found in West African waters is the critically endangered blackchin guitarfish. ‘This is one of the three most commonly caught species of shark and ray in Guinea-Bissau. We know that the archipelago off the coast of mainland Guinea-Bissau serves as a nursery for this species. Together with local researchers, we fitted twelve young blackchin guitarfish with satellite transmitters. We received a signal from ten of them from fishing villages. Within nine months, those animals had been captured. That means that these animals don’t really stand a chance and have no time to contribute to the conservation of the species.’

This kind of knowledge contributes to the global Important Shark and Ray Areas programme, which identifies ecologically important protected areas for sharks and rays. ‘Based on this programme, government agencies and public bodies recognise that if we want to protect rays and sharks, we need to be present in those areas. Our data therefore make a practical contribution to decision-making.’

In West Africa, where sharks often play a ceremonial role in local cultures, there is a strong desire to provide better protection for sharks and rays. At the same time, the fishing industry is vital to the food supply. ‘That makes it very difficult to introduce protective measures. I expect that the blackchin guitarfish may become extinct during my career.’

On the other side of the Atlantic is the Yarari Marine Mammal and Shark Sanctuary, a marine nature reserve that protects sharks and rays around Bonaire, Saba and Sint Eustatius. Here, Leurs studies how sharks move: ‘We fitted fifteen silky sharks with transmitters and found that they remained on the Sababank, near Saba and Sint Eustatius, for the first two years. This atoll appears to be an important nursery for these species. During those first few years, the juveniles are dependent on prey in the reef. They then head out into the open ocean, where they spend the rest of their lives wandering in search of food. In terms of food, they then switch to a diet consisting of tuna and other pelagic species. This is something we see with guitarfish too: as their bite force increases, they start to eat more fish and larger crustaceans.’

During their migration across the ocean, silky sharks may also pass through areas that are not protected. The question is: what is the best way to protect sharks and rays? According to Leurs, this poses a dilemma: ‘Measures now often focus on areas where the largest populations of sharks and rays are found, or which we know serve as nurseries. They do not focus on the animals that head out to the open sea and connect populations. These are the animals that are important for the conservation of species. Ideally, we’d do both, but we still need to find out much more about the movements of these migratory animals.’

He adds: ‘We often don’t know which habitats and food sources a shark or ray needs during certain stages of its life. So we often don’t yet know exactly how to manage protected areas for these species, or which habitats and food sources require additional protection.’

Leurs is also involved in research close to home: in the South-Western Delta of the Netherlands. On behalf of Rijkswaterstaat, Wageningen Marine Research uses satellite transmitters to study the role of sharks and rays in the food web. ‘We want to find out how their role changes throughout their life cycle. We know, for example, that the Eastern Scheldt is an important nursery for species such as the school shark, the starry smooth-hound and the whiptail stingray. But we want to find out more about their role in the ecosystem and what happens to them when they grow up.’

A species like the school shark is critically endangered worldwide, but seems to be faring relatively well in the North Sea: ‘I’m not saying there hasn’t been a decline, but at least there is no active fishing for school shark in our waters.’

All the knowledge contributions from different parts of the world increase our understanding of the challenges faced by sharks and rays. And Leurs adds: ‘If we see that the school shark is doing quite well in the North Sea and we know why that is, then that knowledge can help ensure that the species is better protected in other areas.’

It’s about finding good news: ‘If I only looked at the research findings from West Africa, I would be pessimistic about the future of sharks and rays. But the situation in the North Sea seems to be quite good. In Guinea-Bissau too, all parties are committed to doing more to ensure protection. So I’m keeping my hopes up.’