Evaluating viability of sea turtle foraging populations at high latitudes: age and growth of juveniles along the French Atlantic coast

Despite observations extending back several centuries, the presence of hard-shelled sea turtles in the eastern North Atlantic is often considered anomalous, as these species more typically inhabit lower latitudes. However, in recent years the occurrence of small juvenile loggerhead Caretta caretta, Kemp’s ridley Lepidochelys kempii, and green sea turtles Chelonia mydas along the French Atlantic coast has increased, primarily in the Bay of Biscay. Concurrent increases in water temperature and northward range expansion for sub-tropical and temperate fish and invertebrate species in this area raise the possibility that sea turtles might inhabit French Atlantic waters as part of their normal range. To facilitate investigation of the nature of sea turtle occurrence in the eastern North Atlantic, we estimated for the first time length-at-age relationships and somatic growth rates for juveniles of these 3 sea turtle species through analysis of skeletal growth marks in humerus bones (skeletochronology) of individuals found along the French Atlantic coast. These results were compared with age and growth data yielded by studies of juveniles inhabiting other, more typical foraging areas, but likely originating from some of the same reproductive populations. Age and growth of juvenile loggerhead and green sea turtles in the French Atlantic were comparable to other juvenile foraging populations, indicating that the turtles could have been occupying suitable foraging habitat prior to stranding. In contrast, size-at-age and growth rates for Kemp’s ridleys in the area were lower, suggesting that environmental conditions in the French Atlantic may be sub-optimal for this species.