Hawksbill turtles are critically endangered, are found in every ocean, and are the most tropical of sea turtles. Adult hawksbills have long been thought to have a close association with shallow (less than 15 meters deep) seas where coral reefs thrive.
But new research conducted by my colleagues and I reveals for the first time that hawksbill turtles are feeding much deeper in reef areas than previously thought.
Young hawksbills drift in currents during the pelagic (open water) phase of their development before moving into benthic (seabed) habitats. Hawksbills are often seen foraging on coral reefs, where their diet is predominantly sponges. They also feed on various algae, corals (coral-like anemones), tunicates, and more.
To examine their feeding habits in more detail, my team at Swansea University used high-accuracy global positioning system (GPS) satellite tags to track 22 adult female hawksbills as they foraged from their nesting grounds at Diego Garcia in the Chagos archipelago in the Indian Ocean. on grounds.
Three of the tags had a pressure transducer 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 dived to feed while swimming.
We predicted that the hawksbills we tracked in our study would likely migrate to shallow coral reefs around the seven atolls of the Chagos archipelago. Many studies have demonstrated the pristine nature of these reefs, and we have previously observed hawksbills frequently foraging in reef habitats here.
But surprisingly, all the turtles migrated to deep, remote shores and submerged reefs in the archipelago and remained in these deep areas for a total tracking period of more than 6,000 days. When we looked at marine charts for the turtles’ locations, we could see that their foraging habitat was more than 30 meters deep.
More than 183,000 depth measurements transmitted from tags on three turtles showed that the average depth was between 35 and 40 meters. Most dives reached depths between 30 and 60 metres. This is much deeper than we expected.
Important for protection
Coral reefs, located at depths of 30 – 150 meters below the waves, are known as mesophotic (or low-light) ecosystems. Now knowing that these habitats are so important for critically endangered sea turtles suggests that marine life deep in the seabed is much richer than previously thought, with more nutritious food for turtles to eat.
We would expect to find an abundance of other invertebrate prey items, such as colorful sponges and soft corals, which form a large part of the hawksbill’s diet. Our finding adds to growing evidence that submerged banks at these mesophotic depths may host a diverse community of life, including sponges and seagrasses, which are also essential nutrients for green turtles that breed and forage in the western Indian Ocean.
Mesophotic ecosystems cover a large area, so they should be an important part of conservation considerations. We estimated that submerged shores (30 to 60 meters deep) in the western Indian Ocean extend over an area of over 55,000 km²; this is approximately three times the size of a small country like Wales.
Scientific understanding of mesophotic ecosystems is quite poor, in part because they are difficult to discover. They are often remote and far from land, and their depths are often beyond the limits of routine scientific scuba diving.
There is huge scope for more fascinating research to investigate the ecology of these misunderstood marine habitats. Recent research suggests that rich biodiversity and abundant fish, corals and sponges live at depths of over 30 metres.
reef refuges
Under pressure from 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 on Caribbean mesophotic reefs (30 to 40 meters depth) remained stable during hurricanes, bleaching, and disease events in 2017-2019, when coral cover decreased in shallow and intermediate water depths. This demonstrates the importance of these mesophotic reefs as breeding refuges for corals.
Our study findings highlight that submerged shores and mesophotic depths are important foraging areas for critically endangered marine animals such as turtles and can support a rich array of marine life. The mesophotic reefs used for foraging by hawksbills in our study are located in one of the largest marine protected areas in the world, with protection against industrial fishing, while there are ongoing discussions for the future conservation management of this area.
These submerged shores in the Chagos archipelago, and possibly other shores around the world, should be key areas of conservation focus. The resilience of marine ecosystems and everything that lives within them may depend on the health of these deeper, unexplored habitats, especially in the face of climate change.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
Nicole Esteban receives funding from the Bertarelli Foundation under the Bertarelli Program in Marine Sciences (grant numbers BPMS-2017-4 and 820633).