Combining Data Sources to Understand the Fine- Scale Distribution and Encounter Rates of Common Dolphins Delphius delphis in one of the busiest waterways in the world, the Istanbul Strait

Common dolphins in the Mediterranean have seen declines in recent decades, with increased fragmentation and patchiness, generally attributed to a range of anthropogenic threats. Despite this, few attempts have been made to identify critical areas of dolphins. One such area with heavy overlap of human threats and cetaceans is the Istanbul Strait. The current study presents the findings of a long-term, continuous survey effort that provides the first fine-scale, seasonal distribution and encounter rates of common dolphins for the Istanbul Strait. Land- and Boat-based surveys were conducted systematically between 2011 and 2013 with opportunistic boats surveys completed in 2020. A custom Python script divided the Strait into 500m
²
grid cells, before compiling the total hours of survey effort in each cell to enable calculation of seasonal encounter rate. Dolphins were distributed throughout the Strait with distribution varying seasonally. The majority of the strait was used by dolphins in the summer, with only the north used in winter. Encounter rate also varied by season, with the highest mean encounter rate across grid cells of 1.9 individuals/hour in the summer and just 0.07 individuals/hour in winter. The results provide baseline seasonal data for the area and identify the northern area of the Strait as important across all seasons. As previous studies on cetaceans have identified vessel type, vessel speed, intensity of marine traffic, and distance from the dolphin group having an effect on cetaceans, protection is recommended in the north with seasonal management suggested in the central region in spring and summer.