Loggerhead turtles are successfully adapting to warming temperatures by breeding earlier, but depleting ocean productivity is causing a major decline in their reproductive capacity.

The 17-year study was undertaken by researchers from Queen Mary University of London and conservationists from NGO Associação Projeto Biodiversidade. They observed the turtles – listed as vulnerable on the IUCN Red List – in Cabo Verde.

The researchers found that warmer sea surface temperatures cause turtles to arrive and nest earlier in the season. However, the decline of ocean productivity along their West African feeding grounds, has meant females are taking longer breaks between seasons.

The interval length between nesting has doubled over the course of the 17 years: from about two years to four years. When turtles do return they build fewer nests, and lay fewer eggs per nest.

Turtles are ‘capital breeders’ meaning they use the energy stored during their time feeding at sea to fuel reproduction. The study shows that declining ocean productivity, measured through satellite estimates of chlorophyll, is strongly linked to longer remigration intervals, and fewer nests and eggs being laid.

Conservationists have said the study highlights the need for a change of tack when looking to protect turtles: protecting nesting beaches remains essential, but it is no longer enough. 

“From the beach, everything is a conservation success – more nests, earlier nesting, lots of activity,” said 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,’ she added.

Researchers have said this information is crucial in informing future conservation efforts, and highlights the need for strategies which extend beyond the shoreline.

“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,” said Fairweather.

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