Over the last decade, scientific research and conservation work focused on sharks and rays has increased vastly due to the dramatic population and increased media attention for these species groups. Until now scientific research has mainly focused on sharks and rays on tropical coral reefs or in association with pelagic and bottom fisheries. This bias is especially true for sharks, as the majority of studies have been conducted within the Caribbean, United States, Australia or South Africa. Within the overarching project “Waders of the Bijagos”, the University of Groningen got the opportunity to look at large-scale species interactions within this relatively pristine intertidal ecosystem. So, what happens on intertidal mudflats during high tide, when the big fish like sharks and rays move in?
Before diving into how to prepare for an expedition like this, there is two more important topics that I might have to explain to you first: (1) why sharks and rays are important for ecosystem functioning; and (2) what are intertidal ecosystems and why is it needed to study sharks within these areas.
Just like large predators like big cats and African wild dogs keep small herbivore species in check on the African savanna, so do sharks in marine environments. Generally, sharks can be seen as predators feeding near or at the top of the food chain, making them top-predators or even apex predators in some cases, controlling the species lower in the food chain through top-down mechanisms. These mechanisms include the direct predation of the predator on the prey, leading to a decline in population of the prey species, or indirect effects. One of the most well-known indirect effect of predators is called the “landscape of fear”, which means that the behavior of prey species changes if their fear (i.e. fear of being preyed upon by predators) increases. A bit lower in the food chain, and often part of the diet of larger shark species, are the rays, which are closely related to sharks as their body also comprises of cartilage instead of bone. Rays, as most small shark species, feed mostly on small fish, crustaceans and bivalves, making them the meso-predators in most ecosystems, feeding at intermediate levels. Rays can often also be more specialist feeders, feeding on specific prey, in contrast to the generalist feeding strategy of most shark species. Just like wolves or big cats, sharks and rays are opportunistic and take out the easiest prey: the injured, weak or old fish and crustaceans. This top-down mechanism ensures that the overall populations of prey species are healthier, of which we even benefit on our plate. However, what would happen if these predators disappear or decline in numbers? That’s exactly what has happened to these species groups over the last decades. The targeted catch of sharks and rays (mainly sawfish and guitarfish) increased due to the demand for shark fins for the Asian market for shark fin soup. Currently, roughly 25% of all shark and ray species known to scientists are threatened with extinction. As these species disappear from marine ecosystem, so does their role that they have within the marine food web, possibly leading to cascading effects throughout the entire ecosystem.
To date, the majority of scientific studies focused on the role of sharks and rays within marine ecosystems, have concentrated on coral reef and pelagic ecosystems. We want to add a new one to that list: highly dynamic intertidal ecosystems.
These complex ecosystems provide a large variety for migrating shorebirds, which often use the areas for feeding purposes along their longitudinal migrations. But these areas, with their patterns of gullies, subtidal and intertidal mudflats and deeper waters also provide a home to early life stages of many pelagic fish species, including to many endemic species sharks and rays. Over the next years we set out to study the influence of sharks and rays on overall ecosystem health and functioning of the food web.
This research project is part of a large interdisciplinary project titled “The Waders of the Bijagos” funded by the MAVA Foundation and in collaboration with the local NGO IBAP. Within this project the Universities of Groningen (Netherlands), Aveiro (Portugal) and Lisbon (Portugal) and the Royal Netherlands Institute for Sea Research (NIOZ; Netherlands) collaborate closely to shed light on the function of the Bijagos Archipelago within the East Atlantic Flyway for migratory shorebirds. In a series of blogs on this website we will keep you up to date on our expeditions and results on mangrove and mudflat dynamics, migratory bird ecology and the ecology of sharks and rays within this area.