Activity at one end of a migration affects fitness at the other

Patel, S. H.; Panagopoulou, A.; Morreale, S. J., Kilham, S. S.; Karakassis, I.; Riggall, T.; Margaritoulis, D.; Spotila, J. R. Differences in size and reproductive output of loggerhead turtles Caretta caretta nesting in the eastern Mediterranean Sea are linked to foraging site. Marine Ecology Progress Series 535: 231-241, 2015. doi: 10.3354/meps11433

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Why we care

Migrations are common in the animal kingdom. And they’re fascinating. The Arctic tern has the longest migration of any animal; it migrates from the North to the South Pole. Whales migrate from the equator to the tropics. Even ephemeral insects like dragonflies migrate from India to islands in the middle of the Indian Ocean to Africa and back again. But no single dragonfly makes the entire journey- it takes 4 generations to make the loop.

Understanding migrations is crucial to understanding the biology and ecology of many organisms that are commercially and culturally important. For example, monarch butterflies are an umbrella species for conservationists: protecting these butterflies protects numerous other critical pollinators as well, since they face the same threats and use the same habitat. Monarchs migrate from the United States and Canada to Mexico every year. They pick up diseases en route and may be devastated by habitat destruction in Mexico. Monitoring the butterflies in their northern locations is not enough to know why populations are declining. Scientists must keep up with what happens to a migrating species along its entire route if they want to understand conservation threats and solutions.

Sea turtles have the longest migration of any reptile. They move between breeding/nesting sites and foraging grounds, though how and where a turtle eats depends on its species. Loggerhead sea turtles in the Mediterranean Sea exhibit differences in overall fitness: some turtles are smaller and produce fewer hatchlings than other, larger turtles.

The authors combined field surveys from the literature with satellite tracking to explore whether these fitness differences seen during reproductive periods are caused by food quality discrepancies among turtle foraging areas.


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Colored dots show the locations of foraging grounds for turtles tagged while nesting on the beach denoted by a white star.

Turtles were tagged in Greece after laying eggs and if an ultrasound showed that this would be their final nest for the season. Satellite tags were attached using a turtle-centric procedure, which you should definitely read about in the original article (it’s tough not to disturb an animal’s normal behavior when you attach something to it, but this method sounds like it does an extremely good job of not impacting the turtle). Fitness was measured using carapace (shell) length and clutch size (number of hatched and unhatched eggs). Foraging areas were evaluated using previously published data (see the paper for sources and details).


Tracking data was received from 19 of the 20 transmitters deployed. These turtles followed 3 general migration routes: west to northern Africa (9 turtles), north to the Aegean Sea (6 turtles), and very small movements to eat along the coast of Crete itself (4 turtles).

Carapace lengths were significantly different for turtles foraging in these 3 different areas. Aegean Sea turtles were largest, turtles that ate locally around Crete were the smallest, and African turtles fell between these two groups. Clutch sizes were only statistically placed into two categories: those from Aegean Sea turtles were larger than those from African and Cretian turtles. Body size was not a good indicator of clutch size.


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Measures of sea turtle fitness differed significantly according to foraging location.

Turtles foraging around Crete were the smallest, likely due to a lack of prey items there. Compared to the Aegean Sea, where the fittest turtles foraged, prey items in Crete were nearly 40% scarcer.

The lower fitness of turtles foraging in Northern Africa is likely due to human disturbance there. The Tunisian coast has a history of poor marine resource management. This has led to declines in fish production and substantial nutrient pollution, creating non-ideal foraging conditions for the turtles. Prey items were more than 80% less abundant there than in the Aegean Sea, although the total number of species that turtles could eat was higher.

In addition to these factors negatively impacting turtles foraging in Crete and Africa, the Aegean Sea is known as a very productive area of the eastern Mediterranean. Cold, low-salinity, and nutrient-rich water from the Black Sea constantly enters the Aegean Sea, supporting a variety of organisms in higher trophic levels and promoting high turtle fitness.

These results indicate that turtles that forage in the Aegean Sea may warrant special protection in order to encourage sea turtle population health, especially since clutch size and total nests per season is declining on multiple Grecian nesting beaches. The turtles that forage in the Gulf of Gabès region off of northern Africa would benefit most from a rejuvenation of their hunting grounds- no small feat since that would likely require international cooperation using data from Greece and Tunisia. But that’s what you get when you work with migratory animals.

Have your say

What animals migrate past the places where you work and play? Would you like help figuring out the answer to this question? Let me know in the comments!

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