Long-term passive acoustic monitoring revealed seasonal and diel patterns of cetacean presence in the Istanbul Strait

The Istanbul Strait (Bosphorus) is a part of the Turkish Straits System, connecting the Aegean Sea and the Black Sea. There are three cetacean species in the Strait, namely the harbour porpoise (Phocoena phocoena), the common dolphin (Delphinus delphis), and the bottlenose dolphin (Tursiops truncatus). To monitor the presence of the cetaceans, a fixed stereo passive acoustic monitoring system (A-tag) was deployed in the middle of the Strait from July 2009 to September 2010. In total 26,814 click trains were detected. Presence, direction and inter-click intervals of phonating cetaceans were measured. Most click trains were detected during the night time. Diel presence pattern was prominent in March and April. In spring, the cetaceans were concentrated in one specific direction from the fixed monitoring system. In contrast, they were found in all directions for the rest of the year. Short range sonar (inter-click intervals (ICIs) less than 50 ms) was commonly detected in spring. During the rest of the year ICIs could reach up to 150 ms. All these findings suggest that they were feeding or socializing in spring and mostly travelling in the other seasons. It is well known that pelagic fish such as sprat and bluefish start their migration from the Aegean Sea to the Black Sea in spring. This study suggests that the cetaceans use the middle part of the Strait for feeding on the pelagic fish in spring when the fish migration has just started.