Each year, female sea turtles travel across vast areas of ocean to reach the sandy shores where their lives began. Following years spent feeding far offshore, they haul themselves onto land, excavate a nest and deposit eggs that will eventually produce tiny hatchlings.
This may seem like an uncomplicated natural cycle. Yet warmer seas and dwindling food resources are subtly altering both the frequency of reproduction and its success.
A 17-year investigation of loggerhead turtles in Cabo Verde shows how climate change is altering reproduction within one of the world’s largest nesting populations.
Researchers documented more than 178,000 nests and tagged over 14,000 females on Sal Island.
Warmer oceans shift turtle nesting
Sea turtles are highly dependent on temperature. Higher sea-surface temperatures in feeding grounds during March and April were associated with females arriving earlier at nesting beaches.
Warmer waters around the beaches between June and September were likewise associated with an earlier nesting peak and a sooner end to the season.
“Sea turtles are adjusting their timing to warmer temperatures, which shows a remarkable capacity for flexibility,” said 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.”
Rather than identifying the first and final nests alone, the team used proportions to define the nesting season. Its beginning was set at the point when 5 percent of all nests had been laid, its midpoint at 50 percent, and its conclusion at 95 percent.
During warmer years, all of these markers came sooner, demonstrating a distinct change in nesting timing.
Increased temperatures also reduced the interval between clutches within a single season. Eggs develop more rapidly under warm conditions. Bigger females had still shorter intervals in warm water, as their larger bodies retain heat for longer.
Food supply controls energy
Temperature does not, however, account for the full pattern. Loggerheads are capital breeders, storing the energy needed to produce eggs over long periods of feeding at sea.
The scientists assessed ocean productivity through chlorophyll concentrations, which indicate the amount of plant life growing in seawater.
Their data showed a continuing fall in chlorophyll concentrations across the feeding grounds throughout the study. Reduced productivity means fewer prey items and less energy available for storage.
The remigration interval-the period between nesting seasons-varied from one to eight years, with an average of just over three years. This interval grew over time, meaning females now spend longer in feeding areas before returning to reproduce.
“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.”
Females of smaller size were more strongly affected by limited food availability. When ocean productivity rose, larger females replenished their energy reserves more rapidly.
Fewer clutches and fewer eggs
Females produced an average of roughly 2.3 clutches each season, but this figure fell during the 17-year period. Greater chlorophyll levels in feeding grounds were linked with a higher clutch frequency.
A clutch contained about 79 eggs on average, and clutch size also decreased over time. Larger females produced more eggs, in line with long-standing biological patterns.
In exceptionally warm years, however, the relationship between female size and clutch size became weaker. Hot conditions may constrain the energy that can be directed towards reproduction.
Egg totals also declined slightly as the season went on. This reduction was quicker in smaller females, probably because they exhausted stored energy sooner.
The study’s senior author was Christophe Eizaguirre, a professor of evolutionary and conservation genetics at Queen Mary University of London.
“Temperature alone doesn’t tell the full story,” said Eizaguirre. “You must protect sea turtles on the nesting beaches, but not only.”
“What happens thousands of kilometers away, in their feeding grounds, directly determines how many eggs they can produce and therefore the next generation of turtles.”
Body size is changing
Female turtles measured between 60 and 111.8 cm, with an average length of around 79.6 cm. Average body size declined over time as a greater number of first-time nesters entered the population. These new nesters were smaller than experienced remigrants.
Body size had a strong effect on reproductive performance. Larger females showed shorter intervals between nests, shorter remigration intervals and larger clutches.
A continued reduction in average size could further lower overall reproductive output.
A complex future for loggerheads
On Sal Island, nest totals reached their highest levels in July, August and September. Estimates indicate that about 9,000 to 17,000 females nest during peak activity.
On the surface, these figures appear to signal success. More detailed examination, however, identifies an underlying cost.
Warmer water advances the timing of nesting, while a diminishing food supply reduces egg production and lengthens the time between breeding seasons.
Climate change operates through several routes. Temperature determines timing, ocean productivity determines energy storage, and body size affects each individual’s response. Together, these influences decide how many hatchlings enter the ocean annually.
The outlook for loggerheads in Cabo Verde relies not just on secure beaches, but on healthy feeding grounds far out at sea.
Safeguarding marine ecosystems will be vital if future generations of sea turtles are to endure in a warming world.
The study appears in the journal Animals.
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