Oceans cover 71% of the Earth’s surface area and they routinely provide food for approximately 1 billion people while supporting the livelihoods of a further 200 million people. Despite their importance in sustaining human life, our knowledge of them as functioning ecosystems lags far behind that of terrestrial ones. Why?

At their deepest, some oceans can present a seawater column of >10 km depth which means that they are often inaccessible to even the keenest marine biologist. However, time is running out and we cannot hide behind logistical difficulties as an excuse not to conduct fundamental research into how marine ecosystems function to sustain both biodiversity and us.

Anthropogenic impacts on oceans are severe and include pollution from petrochemical spillages and plastics, overfishing, habitat loss and ocean warming to name but four. Their adverse effects pervade whole ecosystems resulting in 90% of all large fish species having now been lost, 15 of the 17 largest fisheries being close to collapse and, if United Nations’ estimates are correct, a predicted total collapse of the world’s fisheries by 2048.

Such consequences are dire for us but they are potentially catastrophic for species that cannot change their chosen route for resource exploitation. One such group are seabirds that among all birds have perhaps suffered most at the hands of man in recent times. That said, they may offer the most potential as sentinels to monitor the health of our oceans and study marine ecosystem function. After all, they travel over vast expanses of ocean in search of fish, whose numbers and distributions are reliant on balanced marine food webs, and they breed in large numbers at traditional land-based nesting locations where they can be readily studied. Therefore, they are far better equipped than us to undertake widespread monitoring of entire oceans. 

Sooty terns, the most numerous tropical seabird species, have been studied over the long-term by the Army Ornithological Society (AOS) who have worked on Ascension Island since 1990. Ascension Island is a remote UK Overseas Territory (UKOT) in the tropical Atlantic halfway between Brazil and West Africa, and it has the largest breeding population of sooty terns in the whole Atlantic. Sooty terns are long-lived (38 years or more), breed in vast numbers and rely almost entirely on large predators to feed because large fish and whales drive small fish, their preferred food, to the ocean surface where sooty terns feed on them.

Like seabirds generally, sooty terns have undergone population declines throughout their range in recent decades. A research team based at the University of Birmingham in collaboration with the AOS have called for a change in conservation status of sooty terns on Ascension Island from ‘Least Concern’ to ‘Critically Endangered’ based upon a 84% population collapse from more than 2 million birds in 1942 to 350,000 birds in 1990. What caused such a precipitous decline in tern numbers on the island?

Ascension’s sooty terns may be threatened by many anthropogenic impacts and the Birmingham team along with the AOS and collaborators in the Ascension Island Government Conservation Department (AIGCD), and at the Universities of Exeter (UK) and Coimbra (Portugal), set out to investigate. Researchers’ interests turned to the diet of the birds following fieldwork in 2012 when handled birds often regurgitated low quality food such as squid, marine snails and tiny crabs but rarely small fish, as expected. Dietary reconstruction from feathers of skins of Ascension sooty terns from museum collections from 1890 to the present day revealed a marked shift in diet of birds from one rich in fish before, to one dominated by squid after their population collapse. What caused this dietary shift?

Their population decline coincided with increases in reported extractions by fishing boats of surface-schooling tuna species with which hungry sooty terns predominantly associate in the tropical Atlantic. However, this dietary shift also coincided with ocean warming, perhaps indicating that tuna and their preferred prey are moving further away from the island. Birds can respond by either foraging over greater distances for small fish or switching prey; both responses are predicted to have negative population-level outcomes.

What are the wider implications of such research? The UK Government is committed to designation of a marine reserve – the Ascension Island Ocean Sanctuary – covering 440,000 km2 of the tropical Atlantic in 2019. This is sure to relieve some pressure on species such as sooty terns. However, marine ecosystems are finely balanced and the disrupted feeding ecology of this sentinel species that ranges widely over the ocean has revealed that we are still a long way from understanding how marine ecosystems function and thus from effective whole marine ecosystem management.

Dr Jim Reynolds, Lecturer in Ornithology and Animal Conservation, University of Birmingham.