Comparative distribution of Common Dolphin (Delphinus delphis) and Striped Dolphin (Stenella coeruleoalba) in western Mediterranean Sea

The summer distribution and habitat of the Striped Dolphin Stenella coeruleoalba and Common Dolphin Delphinus delphis in the western Mediterranean Sea were investigated with small boat survey data collected from 1988 to 2012. An effective survey effort of 57 912 km enabled to collect 2300 sightings of Striped Dolphin and 46 of Common Dolphin, in most regions of the western basin. High Striped Dolphin relative abundance indices (RAI) were obtained in the northernmost regions and the Alboran Sea, about one dolphin per km of effort, and very low Common Dolphin RAI were obtained off the French continental coast. Common dolphin relative abundance was at the highest in the Alboran Sea (about 0.38 individual/km), moderate in the western Sardinia regions, and significant in other southern regions. Striped and Common Dolphins did not share the same topographic habitat, the former favouring slope and oceanic waters (mean bottom depth 1839 m) and the latter being more frequent in the shelf and slope domains (mean bottom depth 649 m). However, Striped Dolphin was frequently observed in the three habitat types, when Common Dolphin was almost not recorded in the oceanic domain. French stranding statistics were in agreement with our survey results and suggested that Striped Dolphin frequency increased during the 20th century, when Common Dolphin strandings decreased, in particular on French continental shores. However, both species were present in records since the 19th century. While some scientific accounts mentioned Common Dolphin as 'common' and Striped Dolphin as 'rare', erroneous identifications and dubious mentions were identified in the literature. The overall picture suggests that the Striped Dolphin is, and was, a common species in oceanic waters of the western Mediterranean Sea, while the Common Dolphin occupies and occupied a more specific habitat in shallower waters, certainly in relation to its preferred diet. Common Dolphins might have disappeared from regions where their preferred habitat has been overexploited. This study brings new elements to understand Common Dolphin current conservation issues in the Mediterranean Sea.