| University of Basle, Switzerland Faculty of Natural Sciences, Institute of Zoology |
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| Doctorate in Zoology |
Studies on the ecology and conservation status of short-beaked common dolphins (Delphinus delphis) and common bottlenose dolphins (Tursiops truncatus) in the Mediterranean Sea List of articles and interpretation of results
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1 In the first section of this thesis I have included a series of four articles, either published or submitted, aimed at providing an overview on the past and present status of the cetacean fauna living in the northern Adriatic Sea. 1.1. 1.2. The first two articles zoom in the Kvarneric, a relatively small area where bottlenose dolphins have been studied since 1987. Evidence provided here sheds light on the ecology and behaviour of a resident community composed of approximately 100 individuals. Most animals showed high levels of year-round site fidelity and spent the majority of their time engaging in activities that appeared to be feeding-related. These included long dives, apparently to look for demersal prey, and opportunistic feeding behind trawlers. The articles describe several aspects of the natural history of this bottlenose dolphin community, including reproductive and social patterns, daily, seasonal and yearly activity trends, and potential threats affecting the animals, of which prey depletion was suggested to be the most important. 1.3. This article is intended to provide quantitative evidence of low dolphin densities in the waters off Venice and the Po river, and to document the almost exclusive presence of bottlenose dolphins in these waters. Although this article is based on opportunistic sightings reports and on limited survey data, this information is important as it represents a first attempt to describe the current status of dolphins living in a large portion of the basin, including offshore waters where the scarcity of animals reduces statistical power and makes cetacean studies quite difficult to perform. 1.4. This article is my favourite one. Based on a comprehensive overview of historical and recent Adriatic literature that encompasses almost two centuries, it sketches what was likely to be the status of cetaceans in the past, what kind of changes have occurred, and what is our current understanding on the status and problems of cetaceans living in the region. Following an overview of the local cetacean fauna, the article focuses on the reasons that may have caused the recent disappearance of once-abundant common dolphins. High levels of culling and deliberate takes having occurred until the early 1960s, followed by decades of severe habitat degradation, are suggested to represent primary causes for dolphin decline. Bottlenose dolphins remain the only regular cetaceans living in the northern Adriatic, but it appears that their current numbers are now much reduced. The article stresses the importance of looking back in time and evaluating the impact of current threats in light of historical trends. Section
2 In the second section I have included a selection of four contributions that provide an overview on some aspects of the ecology of common dolphins and bottlenose dolphins in the eastern Ionian Sea. This work should be regarded as preliminary as we are currently in the process of analysing the material gathered across ten years of intensive data collection. Still, I thought that the information presented here can be interesting for comparative purposes, and it offers a glance at the kind of research approach that has been used to study dolphins in Greece, and on some of the most interesting results. 2.1. The first article focuses on the abundance and site fidelity of common dolphins and bottlenose dolphins living in eastern Ionian Sea coastal waters. Since 1997, we recorded a negative trend in the sighting frequency and size of common dolphin groups using the area, indicative of relatively quick changes that may relate to either dispersion or population decline. While the status of common dolphins in the area represents a source of concern, much remains to be done to document the reasons behind the observed negative trends. Conversely, bottlenose dolphins - no matter their small community size and degree of emaciation (see below) - did not show signs of decline over the past decade. 2.2. 2.3. 2.4. These three contributions have been presented for inclusion in the proceedings of European Cetacean Society annual conferences. They represent preliminary work that has been included here to show the diversity of methods used to investigate the ecology of common and bottlenose dolphins in the study area. In the first contribution my intern student Stefano Agazzi has analysed fish scales collected during surface feeding events by common dolphins under my supervision, to document prey preferences by the animals. The results showed that during surface feeding about 50% of common dolphin prey is represented by sardines (either Sardina pilchardus or Sardinella aurita), and 50% by European anchovies (Engraulis encrasicholus). In the second contribution we described an apparent increase in the time spent by local bottlenose dolphins around fish farm cages, and we suggested that the animals may have learned to feed in the proximity of the cages, possibly as an opportunistic response to prey scarcity. In the last article, my colleagues and I reported that a remarkably high percentage (about 40%) of individual bottlenose dolphins photo-identified around Kalamos showed signs of emaciation that are suggestive of insufficient prey availability. Section
3 The third section includes a single, long article that reviews information on Mediterranean common dolphins and ranks the threats that are most likely to have determined their decline in the region. Based on the qualitative assessments included in this article, a proposal was presented to the IUCN Cetacean Specialist Group to list the Mediterranean common dolphin subpopulation as “Endangered” in the IUCN Red List of Threatened Species. The proposal, submitted by the candidate and reviewed by Randall R. Reeves, passed the scrutiny by the Cetacean Specialist Group and was eventually approved by the World Conservation Union. 3.1.
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