Loggerhead turtle territories defined by water characteristics in the Western Mediterranean

The Franco-Spanish research group started to tag more than 1,500 immature loggerhead turtles (Caretta caretta) in 1993, of which 36 were progressively recaptured until 2005 at an average interval of 390 to 462 days. The majority of the turtles were recaptured in the same region where they were tagged.

The results of this tagging exercise where recently published in Scientia Marina, and reveal only limited exchanges between the immature turtles as a result of a “barrier” dividing the species and their territories, with the Atlantic ones remaining in the south of the Western Mediterranean and the Eastern Mediterranean ones staying in the north of the Western Mediterranean.

Mónica Revelles, lead author of the study and a researcher in the Animal Biology Department at the University of Barcelona (UB), tells SINC that “before, we did not know the Atlantic turtles stayed in the southern Mediterranean, and the Mediterranean ones in the northern part, but the genetic data and satellite tracking led us to suspect this.”

The study shows the immature individuals are oceanic (unlike the adults, which remain close to the coast), but that they do not stray beyond the areas they are used to. For this reason, the experts believe that water salinity could play a significant role.

Maps of ocean currents and salinity show that salinity is lower in water masses moving around the southern area than in those circulating around the northern part of the Western Mediterranean.

The fact that exchanges between the turtles of Atlantic and Mediterranean origin are limited is “important”, because projects to protect immature turtles in the Western Mediterranean will have to be divided into at least two groups. Revelles says “the studies carried out in the northern and southern areas will relate to the Mediterranean and Atlantic populations, respectively”.

Limited exchanges between the two groups, and human activities such as long line fishing, could impact on those turtles travelling through the southern Mediterranean and nesting along the Atlantic coasts. This is why scientists insist upon the need for measures to protect this migratory species.

Same species, different genes

In terms of origin and place of birth, the Atlantic turtles tend to stay within the southern Mediterranean, while the Mediterranean ones establish themselves in the northern area, although the occasional individual does move from one area to the other, with some even travelling as far as the Caribbean. “This limited exchange between the north and the south means the populations do not interbreed,” says Revelles.

The researchers say the different origins of the loggerhead turtles mean those from the south and north of the Mediterranean exhibit “slightly” different behaviours. They are the same species, but with genetic and morphological differences between the Atlantic and Eastern Mediterranean populations, with the Atlantic animals being larger, while the Mediterranean ones grow less but become adult earlier.

The Western Mediterranean, where the immature animals from both species are found, is the feeding habitat for juvenile turtles. They return to the area where they were born in order to reproduce – the east of the Mediterranean for those from the north of the Western Mediterranean, and the Atlantic for those from the south of the Western Mediterranean.

In addition to the UB, the Spanish Institute of Oceanography (IEO), the Marine Animal Rescue Centre (CRAM), the University of Valencia, the University of Montpellier, France, and the University of Perpignan, also in France, took part in the study.