Sharks and Rays in Troubled Waters: Threatened Species in Dynamic Intertidal Ecosystems

I am glad to announce that my thesis has been published both in print and online. For the past five years, I have worked with a great team of colleagues, fishers, conservationists and local communities to study the impact of fisheries on sharks and rays in large intertidal areas, and how this may affect their role and interactions within these highly important and dynamic coastal areas.

(Read or download the full thesis below)

I will defend this thesis on the 4th of June in the Acadamy Building of the University of Groningen. Before the defense there will be a symposium with four presentations:

  • dr. Anieke van Leeuwen (the Royal Netherlands Institute for Sea Research) – A protected fish community under pressure: long-term trends in the catches of a marine protected area, from teleosts to threatened elasmobranchs.
  • Prof. Colin Simpfendorfer (James Cook University, Australia) – The ins and outs of shark and rays in mangrove habitats(online)
  • dr. ir. Erwin Winter (Wageningen Marine Research/Wageningen University) – Sharks in the Shallows: role of Dutch temperate and tropical waters as nurseries.
  • Guido Leurs (University of Groningen/NIOZ) – Laymen’s talk: Sharks and Rays in Troubled Waters – Threatened Species in Dynamic Intertidal Ecosystems

If you would like to join this event or for any other inquiries, get in touch!

Abstract. This thesis addresses the impact of fisheries on the role of sharks and rays as intertidal predators in large intertidal areas, with a focus on the Banc d’Arguin in Mauritania and the Bijagós Archipelago in Guinea-Bissau. Through a multidisciplinary approach, I determine how fisheries affect these species, their potential interactions with other predators, and the consequences of their removal for ecosystem functioning.
Findings reveal a severe decline in shark and ray populations in both study areas, likely due to the combined effects of industrial and small-scale fisheries operating on the border and within these protected areas, respectively. This has led to changes in shark and ray communities and catches of fisheries. This thesis shows that highly threatened hammerhead sharks and guitarfish still use these areas but that continued exploitation will likely cause these to disappear. Moreover, the research underscores the importance of the intertidal habitat for sharks and rays as feeding refugia or nurseries. This also highlights their role in ecosystem functioning and their interaction with migratory shorebirds, as an overlap between prey species of rays and shorebirds suggests their shared reliance on intertidal prey.
The thesis underlines the need to include ecologically important areas for sharks and rays in marine conservation and emphasizes the need for community involvement in data collection and management strategies. Recommendations include improving data collection, identifying ecological areas for protection, and enforcing fishing regulations. Urgent action is needed to halt the continued loss of sharks and rays from these vital ecosystems, stressing their integral role in coastal livelihoods and the need for their integration into future conservation strategies.

Citation: Leurs G. (2024) Sharks and Rays in Troubled Waters: threatened species in dynamic intertidal ecosystems. PhD thesis, University of Groningen, Groningen, The Netherlands. 345p.

Download thesis files:

Collaboration with local fishers is key throughout my work. This video shows how, in 2021, we put the first satellite transmitter on a critically endangered blackchin guitarfish (Glaucostegus cemiculus) in the Bijagós Archipelago (Guinea-Bissau). Read more

Chapters published as papers:
Chapter 2: Leurs, G., van der Reijden, K. J., Cheikhna Lemrabott, S. Y., Barry, I., Nonque, D. M., Olff, H., Ledo Pontes, S., Regalla, A., & Govers, L. L. (2021). Industrial Fishing Near West African Marine Protected Areas and Its Potential Effects on Mobile Marine Predators. Frontiers in Marine Science, 8(March), 1–13.

Chapter 5: Leurs, G., Verkuil, Y. I., Hijner, N., Saalmann, F., Dos Santos, L., Regalla, A., Ledo Pontes, S., Yang, L., Naylor, G. J. P., Olff, H., & Govers, L. L. (2023). Addressing data-deficiency of threatened sharks and rays in a highly dynamic coastal ecosystem using environmental DNA. Ecological Indicators, 154(March).

Chapter 6: Clements, O. N., Leurs, G., Witbaard, R., Pen, I., Verkuil, Y. I., & Govers, L. L. (2022). Growth, maturity, and diet of the pearl whipray (Fontitrygon margaritella) from the Bijagós Archipelago, Guinea-Bissau. PeerJ, 10, e12894–e12894.

Chapter 7: Leurs, G., Nieuwenhuis, B. O., Zuidewind, T. J., Hijner, N., Olff, H., & Govers, L. L. (2023). Where land meets sea: Intertidal areas as key-habitats for sharks and rays. Fish and Fisheries, 24(3), 407–426.

Chapter 9: Nauta, J., Leurs, G., Nieuwenhuis, B. O., Mathijssen, D. R. A. H., Olff, H., Bouma, T. J., Van Der Wal, D., Hijner, N., Regalla, A., Pontes, S. L., & Govers, L. L. (2024). Bioturbation by Benthic Stingrays Alters the Biogeomorphology of Tidal Flats. Ecosystems.

Box F: Hyde, C. A., Notarbartolo di Sciara, G., Sorrentino, L., Boyd, C., Finucci, B., Fowler, S. L., Kyne, P. M., Leurs, G., Simpfendorfer, C. A., Tetley, M. J., Womersley, F., & Jabado, R. W. (2022). Putting sharks on the map: A global standard for improving shark area-based conservation. Frontiers in Marine Science, 9.

Chapter 10: Leurs, G. (2024). Opportunities and challenges in value chain analysis for sustainable management of sharks and rays. Marine Policy.

This work was done in collaboration and with the support of many people and organizations, especially in Guinea-Bissau (IBAP & INIPO) and in Mauritanina (PNBA & IMROP) and the local communities working and living in both study areas. Please see the full acknowledgments of the thesis for a word of thank you to those who made this work possible.