Distribution and abundance of common bottlenose dolphins (Tursiops truncatus) in Istria region, Croatia

Abstract

Bottlenose dolphins are considered endangered in Croatia. Rough estimates based on aerial surveys suggest that there are around 5 700 individuals in the Adriatic, with North Adriatic being recognized as a significant area for this species (estimates of around 2 600 individuals). Data on the status of local communities in Croatia exist only in the Kvarner area, Northern Dalmatia and in the waters of the island of Vis. However, in other areas, research has only recently been carried out occasionally. West side of Istrian peninsula is a Natura 2000 protected area for bottlenose dolphins, and it covers more than 700 km² of marine area. In the broader area of the Istrian region occasional surveys were done in 2014, 2017 and 2020. Preliminary analysis suggests that there are more than 200 individuals present in the area, and all life stages of this species were recorded. Using the photo-identification data and mark-recapture analysis, the aim of this study is to determine how many individuals inhabits the Istrian region, group size and composition, and encounter rate. The abundance estimate is based on data collected in 2020 in two sub-areas. The analysis, carried out in two sub-areas, showed a population of about 560 individuals present in the coastal part and about 890 individuals present in the offshore area. These are the first abundance estimates for this region. Long-term monitoring is needed to better understand the population dynamics of bottlenose dolphins in Istrian region. It will also allow to take more appropriate conservation measures, as the threats in the Adriatic Sea are undeniable.