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Key message

The population of bottlenose dolphins (Tursiops truncatus) in Slovenian territorial waters is part of a larger population living in the northern Adriatic Sea. The state of the Slovenian population is being monitored by the Morigenos  – the Slovenian Marine Mammal Society. Throughout the year, Slovenian waters regularly host 40–100 bottlenose dolphins. Despite considerable temporal variability in density and frequence of appearance in Slovenian waters, the population trend seems to be negative and the estimated number of animals has been slightly decreasing over the years. On the basis of regular sightings of the same individuals, it can be assumed that the area is of great importance at least for a part of the population. Consequently, preservation of habitat needs to be provided.

This indicator shows the state of the population of bottlenose dolphin (Tursiops truncatus) in Slovenia, which is part of a larger population inhabiting the northern Adriatic Sea (Genov et al., 2008, Genov, 2011). The bottlenose dolphin is the only representative of the family of oceanic dolphins (Delphinidae) and the order of whales (Cetacea) in the northern Adriatic and, thus, the Slovenian sea. Bottlenose dolphins appear in the Slovenian sea throughout the year, where they perform all life functions, including feeding, resting, reproducing and raising their offspring. Part of the population uses the area on a regular basis, while the other part uses it only occasionally. As the population inhabits Slovenian as well as Croatian and Italian waters, it should be dealt with holistically and internationally, as a single protection unit.

The bottlenose dolphin is protected by Slovenian as well as European and international legislation and conventions, including the Decree on protected wild animal species, the European Habitats Directive and the Agreement on the Conservation of Cetaceans of the Black Sea the Mediterranean Sea and Contiguous Atlantic Area (ACCOBAMS). It is on the red list of threatened species in the Republic of Slovenia as an endangered species (E). Based on the above mentioned acts, Slovenia is obliged to monitor the state of the species, ensure a favourable state of the population and designate special protection areas for the species.

The size of the dolphin population in the Slovenian sea can be affected by natural as well as human factors. Natural factors include e.g. seasons of the year, oceanographic conditions, prey distribution, intra-species interactions and population dynamics of the greater northern Adriatic population. Human factors that potentially affect the dolphin population in the Slovenian sea include, in particular a) overfishing, which depletes prey; b) bycatch (non-planned catch) or entanglement of dolphins in fishing nets, which is a direct cause of mortality; c) pollution, which causes an increased incidence of disease, increased mortality of adult and juveniles as well as reduced fertility; d) underwater noise, which disturbs dolphins, changes their behaviour, makes searching for prey and communication between dolphins more difficult, increases energy consumption, causes loss of habitat and potential hearing impairment; and e) collisions with fast vessels and resulting injuries and deaths.

The state of the dolphin population in Slovenia is mostly assessed on the basis of the number of animals and their density in the monitored area and, consequently, on the basis of trends within the aforementioned parameters. For instance, low numbers or a decreasing number of animals in the population indicate unfavourable conditions in the marine environment. Large and stable or even a growing population indicates favourable conditions in the environment. Other indicators of the state of the population are the range of the species, the use of habitat and causes of mortality.


Figure NB10-1: Eastimated density of the dolphin population

Morigenos – Marine mammal research and conservation society, 2009

Show data
2004 2005 2006 2007 2008 2009 2010 2011
density number/km2 0.064 0.095 0.06 0.047 0.074 0.07 0.038 0.031
number/km3 0.047-0.085 0.063-0.141 0.044-0.083 0.030-0.072 0.057-0.095 0.055-0.088 0.029-0.050 0.024-0.039
Figure NB10-2: Lokacije opažanj delfinov

Morigenos, 2012

Figure NB10-3: Zabeleženi premiki delfinov v letih 2008–2010

Morigenos, 2012


Long-term preservation and ensuring of a favourable state of the bottlenose dolphin population and its habitat in Slovenia. Achieving of this goal may indirectly contribute to ensuring a favourable state of other species in the same ecosystem and thus the preservation of biodiversity.

The bottlenose dolphin is included in the following strategic and nature protection goals of the Republic of Slovenia:

  • Resolution on the National Environmental Protection Programme 2005–2012 (NPVO) – one of the basic goals is to protect and preserve natural systems, habitats, and wild animal and plant species with the aim of halting the loss of biodiversity and genetic diversity. The same resolution defines one of the fundamental goals as the “preservation or attainment of a favourable state of threatened species and habitat types.”
  • All species of cetaceans, including the bottlenose dolphin, are listed in Annex I of the Decree on protected wild animal species, which is an implementing regulation of the Nature Conservation Act, and thus protected;
  • As a signatory of the international Agreement on the Conservation of Cetaceans of the Black Sea the Mediterranean Sea and Contiguous Atlantic Area (ACCOBAMS), the Republic of Slovenia has committed to monitor the state of cetacean species, designate protected areas for them and ensure a favourable state of their populations.
  • The bottlenose dolphin is classified as an endangered (E) species on the red list of threatened species in the Republic of Slovenia. Priority protection measures are being carried out accordingly.
  • The bottlenose dolphin is listed on Annex II of the European Habitats Directive, which means that special conservation areas must be designated for the species.


Globally, it is known that studying whales and dolphins is extremely difficult. The main reason is the fact that these animals tend to live large areas and in an environment largely inaccessible to humans (far from the coast and mostly underwater). Identifying trends in population size is even more complicated, as these animals are known to have a long life span and slow reproduction. In order to detect changes in population dynamics, many years of data gathering are required, as short-term research can lead to false conclusions.

The number of dolphins in Slovenia was assessed using the mark-recapture method, on the basis of photo identification of naturally marked individuals (without actual capturing). The period in which the amount of data allows the assessment of population size is 2004–2011. In the years 2002 and 2003, preliminary monitoring was carried out, while in 2012 there were too few sightings, making it impossible to make a reliable assessment of the population.

The most accurate assessment of the size of the bottlenose dolphin population that uses Slovenian waters is 74 animals (95% confidence interval = 57-95) or 0.074/km2. On an annual basis, 40–100 bottlenose dolphins use Slovenian waters (Genov, 2011). In the assessments presented above, confidence intervals need to be taken into account in addition to individual assessments. Temporal variability in the density and frequency of the use of the area is obvious, which is understandable as regards bottlenose dolphins. Nevertheless, a negative population trend, i.e. a slight decrease in the number of animals over the years, can be observed.

It is not easy to predict the population trend in the upcoming years, but it can be assumed that the state of the population is not entirely favourable.

Although the number and density of bottlenose dolphins in the monitoring area are relatively low, the area is not insignificant for the species. On the contrary, due to the regular appearance of the same identified individuals, we can assume that the area is of great importance at least for part of the population, which is why its habitat needs to be preserved. Until the 1970s, the short-beaked common dolphin (Delphinus delphis) was present in the northern Adriatic as well (Genov et al., 2012). Considering the fact that only one large dolphin species is present in the area and that its population density is relatively low, the situation raises a certain degree of concern.



Data for Slovenia

Source database or source: Morigenos – marine mammal research and conservation society
Data administrator: Morigenos – marine mammal research and conservation society, Jarška cesta 36/a, SI-1000 Ljubljana; contact: Tilen Genov
Data acquisition date for this indicator: 18 July 2009
Methodology and frequency of data collection for the indicator (Genov et al., 2008):
Land-based (between 67 and 82 times since 2004) and boat-based (between 24 and 63 times since 2004) surveys were carried out. Further information on the frequency of data collection are available in Genov et al. (2008).
Due to weather and logistic considerations, most observations were carried out during the summer. Boat-based surveying was carried out from boats at a constant speed (25 to 30 km/h). Land-based surveys were carried out at high coastal points with binoculars. The position of the boat and of the dolphins were established by GPS. "Sighting" was defined as an uninterrupted continuous observation of a dolphin focal group.
Survey conditions were considered good if a) the sea state of Beaufort scale was 2 or less; b) at least one experienced observer searched for dolphins (usually 2–5 other observers could participate in the search); c) visibility was not reduced by heavy fog or precipitation. If survey conditions did not match these criteria, no systematic search for dolphins was carried out. During each survey, navigation and environmental data (time, position, sea state, etc.) were collected every 15 minutes or whenever the direction or conditions of the search changed. When dolphins were found, the tracking of the group begins. The boat had to approach the group slowly and remain parallel to the group, and the steering of the craft must be in accordance with protocol, without upsetting the animals.
Standard photo identification was carried out upon each sighting (Würsig, Jefferson, 1990). Natural marks on the dorsal fin (nicks, notches and scars) were used to identify individual specimen. Photos of each animal record the presence of specimen.
Data on position, time, size of group, offspring, behaviour, breathing patterns, and interactions with fishing activities and marine traffic were recorded upon each observation. The size of groups was assessed on the spot and later supported by photo identification.
Data processing methodology (Genov et al., 2008):
Over 10,000 photographies were taken, analysed, marked and sorted during the research phase. New photos were optically scanned and compared to earlier photos. Two catalogues arose from this, one including all photographies in chronological order, and the other containing the best photos of each individual specimen. To avoid any bias in analysing the size and density of the population, only the well marked specimen on high-quality photographies were taken into account. Every specimen was given a name for further reference. Data on spotting identified specimen were used in the mark-recapture method on a closed population (Otis et al., 1978) to establish the size and density of the population. MARK 4.3 computer software was used in the process. Photo identification data in individual observations were catalogued in two groups of observations' history: one for 15 days, and the other 30 days. These two time frames were set as a good compromise between the need for sufficient data and the need to allow animals to mix during sampling. The most suitable model was chosen based on χ2 test (chi-square goodness-of-fit test), which is implemented within the framework of the MARK programme. The annual estimated size of the population, includng offspring, were calculated based on the assessed number of marked animals and the share of non-marked specimen within the population.
Information concerning data quality:
- Advantages of the indicator: the estimated annual size of the population is a suitable indicator, as it relies on unified methodology and reflects relatively well the real situation, provided field work is carried out regularly.
- Disadvantages of the indicator: natural variability in density and use of any given area (due to the species's social nature and tendency to adapt to the presence and accessibility of the prey) by the bottlenose dolphine can lead to some variations, which must be accounted for in the interpretation of the results.
- Overall assessment (1 = no major comments, 3 = data to be considered with reservation):
Relevance: 1
Monitoring and guaranteeing a favourable conservation status of the bottlenose dolphins population is required by the secondary acts issued on the basis of the Nature Conservation Act, in Directive on habitats, the Resolution on National Environmental Action Plan 2005-2012 (NPVO) and the international Agreement on the Conservation of Cetaceans of the Black Sea, the Mediterranean Sea and the Contigeous Atlantic Area (ACCOBAMS).
Accuracy: 1
Data collection and analysis are carried out through internationally accepted standard methodology.
Completeness over time: 2
Each data string is 6 years long; each additional year of research will contribute to higher comparability of data.
Completeness over space: 2
As Slovenian territorial waters cover but a small geographic area, while bottlenose dolphins often cross national borders, the population of the entire area (thus beyond Slovenian waters) must be considered as a whole.

Other sources and literature:
• Bearzi, G., D. Holcer & G. Notarbartolo di Sciara (2004): The role of historical dolphin takes and habitat degradation in shaping the present status of northern Adriatic cetaceans. Aquatic Conservation: Marine and Freshwater Ecosystems, 14, 363–379.
• Bearzi, G., G. Notarbartolo di Sciara, & E. Politi (1997): Social ecology of bottlenose dolphins in the Kvarneric (northern Adriatic Sea). Marine Mammal Science, 13, 650–668.
• Fortuna, C. M. (2006): Ecology and conservation of bottlenose dolphins (Tursiops truncatus) in the north-eastern Adriatic Sea. PhD thesis, University of St. Andrews, Scotland. 256 pp.
• Genov, T., Kotnjek, P., Lesjak, J., Hace, A. & C. M. Fortuna (2008). Bottlenose dolphins (Tursiops truncatus) in Slovenian and adjacent waters (northern Adriatic Sea). Annales, Series Historia Naturalis, 18(2), 227-244. (1,12 MB)
• Otis, D. L., K. P. Burnham, G. C. White & D. R. Anderson (1978): Statistical inference from capture data on closed animal populations. Wildlife Monographs 62. 137 pp.
• Würsig B. & T. A. Jefferson (1990): Methods of photo-identification for small cetaceans. Report of the International Whaling Commission, Special Issue, 12, 43–52.
• Council Directive 92/43/EEC of 21 May 1992 on the conservation of natural habitats and of wild fauna and flora
• Pravilnik o uvrstitvi ogroženih rastlinskih in živalskih vrst v rdeči seznam. Uradni list RS, št. 82/2002 (Rules on the inclusion of endangered plant and animal species in the Red List. Official Journal of the Republic of Slovenia, 82/02)
• Resolucija o Nacionalnem programu varstva okolja 2005-2012 (ReNPVO). Uradni list RS, št. 2/2006 (Resolution on National Environmental Action Plan 2005-2012 (ReNPVO. Official Journal of the Republic of Slovenia, 2/06)
• Uredba o zavarovanih prosto živečih živalskih vrstah. Uradni list RS, št. 46/04 (Decree on protected wild animal species. Official Journal of the Republic of Slovenia, 46/04)
• Zakon o ratifikaciji Sporazuma o ohranjanju kitov in delfinov Črnega morja, Sredozemskega morja in atlantskega območja ob njem (MSOKD). Uradni list. RS-MP, št. 16/06 (Act ratifying the Agreement on the Conservation of Cetaceans of the Black Sea, Mediterranean Sea and Contiguous Atlantic Area (ACCOBAMS). Official Journal of the Republic of Slovenia RS-MP, 16/06)

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