by Hawksbill turtles are critically endangered, found in all oceans and are the most tropical sea turtles. Adult clams have long been thought to be closely associated with shallow seas (less than 15 meters deep) where coral reefs grow.
But new research by my colleagues and I shows for the first time that hawksbill turtles feed at much deeper reef locations than previously thought.
Young falcons drift in currents during their pelagic (open water) stage of development before moving to benthic (seabed) habitats. Peregrine falcons are usually seen foraging in coral reefs where the diet consists mainly of sponges. They also feed on a variety of algae, coralmorphs (anemones like corals), tunicates and more.
To study their feeding habits in more detail, my team at Swansea University used high-precision global positioning system (GPS) satellite tags to track 22 adult female peregrine falcons from their nesting site on Diego Garcia in the Chagos archipelago in the Indian Ocean to some hunting. lands.
Three of the tags included a pressure transducer that was programmed to record depth every five minutes and relay the measurement to the satellite system each time the turtle surfaced. This gave us information about where the turtles were and how deep they were diving to feed while swimming.
We predicted that the peregrine falcons tracked in our study would likely migrate to shallow coral reefs around the seven atolls of the Chagos archipelago. Many studies have shown the sparse nature of these reefs and we previously noted that hawks often foraged in reef habitats there.
But, surprisingly, all the turtles migrated to remote deep banks and submerged reefs in the archipelago, staying at these deep sites for more than 6,000 combined days of tracking. By looking at sea charts for the turtle locations, we could see that the hunting habitat was located at a depth of more than 30 meters.
Over 183,000 depth measurements from the tags on three turtles showed an average depth of between 35 and 40 metres. Most of the mounds reached a depth of between 30 and 60 metres. That’s a lot deeper than we expected.
It is vital for conservation
The coral reefs located at depths between 30 – 150 meters below the waves are known as mesophotic (or low light) ecosystems. Now, knowing that these habitats are so vital to critically endangered sea turtles suggests that the marine life deep down on the sea floor is much richer – and more nutritious food for turtles to eat – than previously thought.
We would expect to find an abundance of colorful sponges and other invertebrate prey, such as soft corals which make up a large part of the hawk’s diet. Our results add to the growing evidence that submerged banks at these mesophotic depths may host diverse communities of life, including sponges and seagrasses that are primary foods for green turtles that breed and forage in the Western Indian Ocean too.
Mesophytic ecosystems cover a huge area and should therefore be a significant part of conservation. We estimated that submerged banks (at depths of 30 to 60 metres) in the Western Indian Ocean stretch across more than 55,000 km² – around three times the size of a small country like Wales.
Scientific understanding of mesophotic ecosystems is very poor, in part because they are difficult to explore. They are usually remote and far from land, and the depth is often beyond the limit of normal scientific scuba diving.
There is huge scope for more interesting research to explore the ecology of these poorly understood marine habitats. Recent studies have indicated that biodiversity is rich and that many fish, corals and sponges live at depths of over 30 metres.
reef sanctuaries
With the pressures of climate change and warming seas, mesophotic reefs may become a refuge for corals and sponges that normally live in shallow coral reefs. For example, coral cover in mesophytic reefs in the Caribbean (30 to 40 meters deep) remained stable during hurricane, bleaching and disease events in 2017 to 2019 while coral cover in shallow and mid-water depths declined. This shows the importance of these mesophotic reefs as a reproductive refuge for corals.
The results of our study highlight that submerged banks and mesophotic depths are important foraging grounds for critically endangered marine animals such as turtles and can support a rich array of marine life. Although the metaphotic reefs used by the peregrine falcons in our study are located within one of the largest marine protected areas in the world, protected from industrial fishing, the future conservation management of this region is an ongoing negotiation.
These submerged banks in the Chagos Archipelago, and probably others around the world, should be prime areas for conservation focus. The resilience of marine ecosystems, and all that live within them, may depend on the health of these deeper, uncharted habitats, particularly in light of climate change.
This article from The Conversation is republished under a Creative Commons license. Read the original article.
Nicole Esteban receives funding from the Bertarelli Foundation as part of the Bertarelli Program in Marine Science (grant numbers BPMS-2017-4 and 820633).
