Environmental changes in the Mediterranean Sea could facilitate the western expansion of loggerhead turtles

Climate change may affect life on Earth in multiple ways. Whereas some populations may encounter detrimental conditions that cause extirpations, those occupying cooler thermal limits of a range may benefit by expanding. For sea turtles, egg maturation in the female oviduct and nest incubation are temperature-dependent and vulnerable to climate change. Mediterranean loggerhead turtles Caretta caretta nest in the eastern basin although sporadic nesting occurs on the western side. To assess the likelihood of a climate-related expansion, we compared historical air and sea surface (SST) temperatures between locations near established eastern nesting areas and western areas where sporadic nesting is increasing (Palinuro, Italy) or just started (Balearic Islands, Spain). Our results suggest that summer air and water temperatures in western sites were suitable for nesting over the last 40-50 yr, at least in July-August, having (1) SSTs above suboptimal threshold temperature (22 degrees C) and (2) similar air temperatures to those of Greece, but among the lowest in the Mediterranean. There was a decreasing east-to-west gradient in SST. However, SSTs were similar around beaches of Zakynthos (Greece), Palinuro and Ibiza (Balearic Islands), where SST was above 22 degrees C for at least 60 d, potentially allowing turtles to lay multiple clutches. A warming trend was detected in air temperature and SST since the 1970s-1980s. Although conditions in the western Mediterranean currently seem suitable for nesting, lower air temperatures in May-June and higher precipitation in September could shrink the nesting window. If warming continues, conditions in the western basin could progressively become more favorable for nesting.