Save Our Seas Blogs

The ongoing research of Rainer von Brandis into the impacts of hawksbill turtle foraging behaviour in the Seychelles has been revealing the startling importance of their role in maintaining balanced coral reef ecosystems. With another season’s field work drawing to a close, it’s a good opportunity to reflect on Rainer’s discoveries thus far.

Hawksbill turtles have been severely depleted by humans in recent centuries and their absence from modern reefs has almost certainly influenced ecosystem dynamics. Prior to their depletion, hawksbill turtles had substantial influence on their marine habitats through their many roles as consumers, prey, and competitors; as hosts for parasites and pathogens; as substrates for epibionts; as nutrient transporters; and as modifiers of the landscape.

At D’Arros Island, Rainer’s underwater observations indicate that the roles of foraging hawksbills as both consumers and modifiers of the landscape are fundamental to the sustained functioning of the platform reef ecosystem.

Hawksbill turtle extracting a sponge from the reef. Photo by Rainer von Brandis.

Hawksbills as consumers of sponges: Sponges are competitively superior in space acquisition to hard and soft corals and, such that when unchallenged by spongivores, are known to overgrow and kill corals. Consequently, at reefs where sponges are ubiquitous and space is limited, spongivores play an important role in maintaining biodiversity by reducing their competitive edge to the point where coral colonies can successfully establish.

On average, an immature hawksbill at D’Arros consumes 110-215 grams of sponge material each day. Considering that approximately 50 turtles regularly forage on coral reefs around the island, this means between two and four tonnes of sponge biomass is consumed each year by hawksbills alone! In the absence of predation, these fast growing sponges would probably dominate the surface of reef.

Thus, in the absence of hawksbills, hard corals and other benthos might be choked out by sponges resulting in a decrease in reef biodiversity. Given recent coral bleaching events, in which typically 50-90% of corals perished locally, the role of hawksbills in controlling sponges has become even more important.

Hawksbill turtle exposing food for reef fish. Photo by Rainer von Brandis.

Hawksbills as modifiers of the reef landscape: Hawksbills at D’Arros typically move between several feeding sites whilst foraging. Food hidden within the consolidated rubble is located by smell and then extracted by means of physical excavation techniques that result in structural alterations to the reef. Since food items are diminutive and evenly distributed, the average weight of food extracted per feeding site is a meagre 10.9 grams. Considering that approximately two to four tonnes of food are consumed annually, in excess of 183,000 feeding sites are exploited every year.

Turtles are able to displace considerable portions of reef substrate (biotic and abiotic) when gouging out food at feeding sites. Despite occasional damage to reef benthos, the ecological benefits of this foraging behaviour are considerable:

i) otherwise cryptic food is exposed for fish to eat;

ii) perforations and crevices created in the reef substrate provide microhabitats and shelter for juvenile fish and other vulnerable organisms;

iii) a diverse and patchy habitat mosaic is created, and

iv) increased topographical relief provides shelter for a variety of larger species.

Despite having submitted his Doctoral thesis, Rainer aims to continue working with hawksbill turtles in the Seychelles, expanding into the realm of acoustic and satellite tags to help determine both their local and large scale movements.