Acoustic monitoring of bottlenose dolphins (Tursiops truncatus) in Kornati national park

The bottlenose dolphin (Tursiops truncatus) is an iconic species of the Adriatic Sea and plays a vital role in the marine ecosystem of the whole Mediterranean Sea. Bottlenose dolphins (Fig. 1) have a vast distribution and are found in a variety of habitats around the world. Many of the worldwide stocks are healthy but the Mediterranean stock has undergone a severe decline within the last 60 years. The problems of this opportunistic species in the Mediterranean Sea feature: habitat degradation, pollution with xenobiotic chemicals, overfishing (loss of food), by-catch, direct takes for the captivity industry and acoustic disturbance.

In this project the universities of Zagreb (Croatia) and Hamburg (Germany) are collaborating with their NGO partners VAL – The Association of Environmental Protection (Croatia), GRD – Society for Dolphin Conservation (Germany) and the consultancy bureau BioConsult SH (Germany) in order to investigate the habitat use of bottlenose dolphins in Kornati national park. Earlier studies revealed that there are approximately 40 individual dolphins that can frequently be seen in the waters of the national park. The home range of these animals is likely to be greater than the national park itself but the waters play an important role for these animals. Increasing boat traffic and resulting noise seem to have an adverse effect on the dolphins. There is evidence that the dolphins avoid the inner waters of the national park during the busy and loud tourist season and spend the daytime in offshore waters. During the night they seem to return to the then much more quiet inner waters of the national park.

The studies that are carried out at the moment intend to clarify this proposed displacement effect and shall shed light on the characteristics of bottlenose dolphin habitat use. Therefore, CPODs (Porpoise Detectors) have been deployed at different areas in and around the national park (Fig. 2). These devices are hydrophones (underwater microphones) with autonomous data logging units inside. The PODs will record the sounds that dolphins emit when they use their biological sonar (echolocation) to find fish and other prey. As the sea is very dark below 40 meters and even in the upper water column visibility is often poor, dolphins and other toothed whales use hearing as their primary sense to forage and communicate. The emitted sounds for echolocation are called ‘clicks’ and can be recorded and analyzed with hydrophones (=PODs).

From the recorded data it cannot only be inferred when and where dolphins were present but also if they were foraging. When dolphins try to locate escaping fish they emit ‘click trains’ which are fast sequences of individual clicks. These click trains can be seen and analysed in the recorded hydrophone data. 

For this project the Najada diving base generously supports the scientist with diving equipment and logistic help.