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Turtle on the beach facing the sea under a cloudy sky
A new 17-year study of loggerhead sea turtles nesting in Cabo Verde reveals exactly this tension. Researchers from Queen Mary University of London and conservationists from NGO Associação Projeto Biodiversidade report that warming oceans are triggering earlier nesting in one of the world's most important loggerhead turtle populations. Yet at the same time, declining ocean productivity is reducing how often females reproduce and how many eggs they lay.
The study, published in Animals, shows that climate change impacts sea turtle reproduction through multiple, interacting pathways.
"Sea turtles are adjusting their timing to warmer temperatures, which shows a remarkable capacity for flexibility," says Fitra Nugraha, the study's lead author at Queen Mary University of London. "But at the same time, the part of the Atlantic oceans they depend on for food are becoming less productive - and that is quietly eroding their reproductive output."
Earlier nesting, shorter breaks - but longer waits between breeding seasons
The researchers found that warmer sea surface temperatures cause turtles to arrive and nest earlier in the season. Higher temperatures also shorten the interval between successive nests, likely because warmth speeds up egg development.
However, the story changes once the turtles leave the beaches. As ocean productivity declines along their West African feeding grounds, females now take longer breaks between breeding seasons: over 17 years, renesting intervals has increased from about 2 years to 4 years. When they do return, they lay fewer clutches and fewer eggs per nest.
"From the beach, everything is a conservation success - more nests, earlier nesting, lots of activity," says Kirsten Fairweather, co-lead author and scientific coordinator at Associação Projeto Biodiversidade. "But when you follow individual turtles over many years, a more complex picture emerges. The turtles are working harder for less return."
Why food matters as much as temperature
Sea turtles are "capital breeders": they rely on energy stored during years spent feeding at sea to fuel reproduction. The study shows that declining ocean productivity - measured using satellite estimates of chlorophyll - is strongly linked to longer remigration intervals, smaller clutches and fewer nesting events.
This means climate change iaffects turtles through multiple pathways at once: warming alters timing, while changes to marine food webs reduce reproductive capacity.
"Temperature alone doesn't tell the full story," says Christophe Eizaguirre, senior author of the study and Professor of Evolutionary and Conservation Genetics at Queen Mary University of London. "You must protect sea turtles on the nesting beaches, but not only. What happens thousands of kilometres away, in their feeding grounds, directly determines how many eggs they can produce and therefore the next generation of turtles."
Implications for conservation
Cabo Verde hosts tens of thousands of nesting loggerhead females each year, making it a population of global importance. The study highlights the value of long-term, NGO-led monitoring efforts, which make it possible to detect subtle but consequential biological changes that short-term studies would miss.
For conservationists, the message is clear: protecting nesting beaches remains essential, but it is no longer enough.
"To safeguard sea turtles in a warming world, we need conservation strategies that extend beyond the shoreline," says Fairweather. "That includes protecting feeding habitats, reducing pressures on marine ecosystems, and recognising that climate change can undermine reproduction even in populations that appear to be thriving."
As oceans continue to warm and productivity shifts, the study suggests that the future of sea turtles will depend not just on their ability to adapt - but on how quickly conservation can adapt with them.
Read the paper, Warming and change in ocean productivity alter phenology of an expanding loggerhead population in Cabo Verde.
