Tracking tropical turtles deep down to the seabed reveals their feeding habits

Hawksbill turtles are critically endangered, they’re present in each ocean and are probably the most tropical of sea turtles. Grownup hawksbills have lengthy been thought of to have an in depth affiliation with shallow (lower than 15 metres depth) seas the place coral reefs thrive.

However new analysis my colleagues and I performed reveals for the primary time that hawksbill turtles feed at reef websites a lot deeper than beforehand thought.

Younger hawksbills drift in currents throughout their pelagic (open water) section of their growth earlier than they transfer to benthic (sea mattress) habitats. Hawksbills are normally seen foraging in coral reefs the place their food regimen is predominantly sponges. Additionally they feed on a wide range of algae, corallimorphs (coral-like anemones), tunicates and extra.

To check their feeding habits in additional element, my workforce at Swansea College used high-accuracy international positioning system (GPS) satellite tv for pc tags to trace 22 grownup feminine hawksbills from their nesting website on Diego Garcia within the Chagos archipelago within the Indian Ocean to their foraging grounds.

Three of the tags included a stress transducer that was programmed to report depth each 5 minutes and relay the measurement to the satellite tv for pc system each time the turtle surfaced. This gave us details about the whereabouts of the turtles and the way deep they have been diving to feed as they swam.

We predicted that hawksbills tracked in our research would in all probability migrate to shallow coral reefs across the seven atolls of the Chagos archipelago. Many research have proven the pristine nature of those reefs and we’ve beforehand noticed hawksbills often foraging in reef habitats there.

However, surprisingly, all turtles migrated to deep, distant banks and submerged reefs within the archipelago, remaining at these deep websites for greater than 6,000 mixed days of monitoring. By nautical charts for the turtle places, we might see that the foraging habitat was positioned at greater than 30 metres depth.

Greater than 183,000 depth measurements relayed from the tags on three turtles confirmed that common depths have been between 35 and 40 metres. Most dives reached depths between 30 and 60 metres. That’s a lot deeper than we anticipated.

Essential for conservation

The coral reefs positioned at depths of between 30 – 150 metres beneath the waves are often called mesophotic (or low mild) ecosystems. Now, understanding that these habitats are so essential for critically endangered sea turtles means that the marine life deep down on the seabed is far richer – with extra nutritious meals for turtles to eat – than beforehand thought.

We’d anticipate finding an abundance of vibrant sponges and different invertebrate prey gadgets reminiscent of gentle corals that make up a giant portion of the hawksbills’ food regimen. Our discovering provides to the rising proof that submerged banks at these mesophotic depths could be residence to a various neighborhood of life, together with sponges and seagrass which might be key meals for inexperienced turtles that additionally breed and forage within the western Indian Ocean.

Mesophotic ecosystems cowl an unlimited space so they need to be a major a part of conservation issues. We estimated that submerged banks (at depths of 30 to 60 metres) within the western Indian Ocean prolong throughout over 55,000 km² – round 3 times the scale of a small nation reminiscent of Wales.

Scientific understanding of mesophotic ecosystems may be very poor, partly as a result of they’re troublesome to discover. They’re normally distant and much from land, plus the depths are sometimes past the restrict of routine scientific scuba diving.

There’s enormous scope for extra fascinating analysis to research the ecology of those misunderstood marine habitats. Current research have advised wealthy biodiversity and ample fish, corals and sponges dwell at depths over 30 metres.

Reef refuges

With the pressures of local weather change and warming seas, mesophotic reefs may very well be a refuge for corals and sponges that usually dwell in shallow coral reefs. For instance, coral cowl in Caribbean mesophotic reefs (30 to 40 metres depth) remained fixed throughout hurricanes, bleaching and illness occasions in 2017 to 2019 when coral cowl declined in shallow- and mid-water depths. That demonstrates the significance of those mesophotic reefs as a reproductive refuge for corals.

Our research findings spotlight that submerged banks and mesophotic depths are essential foraging grounds for critically endangered marine animals reminiscent of turtles and will assist a wealthy array of marine life. Whereas the mesophotic reefs utilized by foraging hawksbills in our research lie inside one of many world’s largest marine protected areas, with safety from industrial fishing, there are ongoing negotiations for future conservation administration of this area.

These submerged banks within the Chagos archipelago, and doubtless others world wide, needs to be key areas for conservation focus. The resilience of marine ecosystems, and all that lives inside them, could depend on the well being of those deeper, unchartered habitats, particularly within the face of local weather change.