Estimated density of the population of bottlenose dolphins (Tursiops truncatus) in Slovenian territorial waters has been relatively stable since 2004, with an annual density estimate of 0.068-0.07 specimen per square km or around 70 dolphins per year on average.
Slovenian territorial waters regularly host the local dolphin population throughout the entire year. Due to their mobile lifestyle, however, dolphins can also be seen beyond Slovenian maritime borders, in Italian and Croatian waters.
The parameter indicates the status of the population of bottlenose dolphins (Tursiops truncatus) in Slovenia and, indirectly, the conservation status of their natural habitat.
The estimates regarding the status of the population of bottlenose dolphins are based on the number of specimen and the density of the population, and the overall trends of these two indicators. For example, a low number of specimen in the population or the decrease in its number indicates poor conditions of the marine environment. On the contrary, a population large and stable enough (or increasing even) bears prof of sufficiently beneficial environment for its living.
The size and status of the dolphin population in Slovenia may be affected by the following factors:
1. Natural (food provision, mating season)
2. Human (see below)
The following human factors may potentially affect the dolphin population in Slovenian waters:
1. Excessive fishing (leading to lack of food)
2. By-catch (unintended catch) and direct injury and death to dolphins which get entangled in fishing nets
3. Pollution (higher disease rate, higher mortality rate, lower reproductive capability)
4. Marine traffic (noise, stress, change in behavior, higher energy consumption, disruption of normal vital functions, loss of habitat).
Definition of favourable conservation status
The EU Directive on habitats defines a favourable conservation status to be such that population dynamics data on the species concerned indicate that it is maintaining itself on a long-term basis as a viable component of its natural habitats, and the natural range of the species is neither being reduced nor is likely to be reduced for the foreseeable future, and there is, and will probably continue to be, a sufficiently large habitat to maintain its populations on a long-term basis.
The problem occurs where the criteria to determine the conservation status of a population are limited to recent trends and development (covering the last 6 years or less), as this can somewhat distort the real situation in certain cases; if a population decreased slowly, yet persistently, in size in the past only to reach a certain low, although stable, number in the past years, this should not be deemed to constitute a favourable conservation status.
Given the fact that data on the status of the population of bottlenose dolphins in Slovenian waters (and the Adriatic as a whole) has only become available recently (Genov et al. 2008), whereas the number of dolphins in the Adriatic sea has spiked in the past 30 years (Bearzi et al., 2004), this data does not provide sufficient basis in determining the initial status, which would probably be assessed as favourable to the species or population in this area.
It is thus important to focus on the best available data and prevent further deterioration, until further research allows a more detailed definition of a favourable conservation status of the aforementioned population to be devised and helps us pinpoint the actual negative impacts on the population.
Research during 2002 through 2008 show that the population of bottlenose dolphins more or less regularly present in Slovenian waters includes at least 69 specimen, with the density of the population reaching above 0.069 (specimen per suqare km).
Morigenos – Marine mammal research and conservation society, 2009
|annual estimate of animals||number||38||68||108||68||69|
- One of the main objectives defined in the Resolution on National Environmental Action Plan 2005-2012 (NPVO) is the protection and preservation of natural systems, habitats, of wild flora and fauna, to prevent the loss of biodiversity and genetic diversity. The Resolution also aims to »preserve or attain favourable conservation status of endangered species and habitat types.«
-All types of cetaceans (including bottlenose dolphins - Tursiops truncatus) are included in Appendix I to the Decree on protected wild animal species, which is the executive act under the Nature Conservation Act, and are thereby protected. Pursuant to the aforementioned Decree, the status of species from Appendix I must be monitored, especially as regards the natural range of the animal species, the size and density of the population, and any activities, actions, interventions or phenomena, which have an adverse effect on the status of conservation of the species, including unintentional capture or killing of the animal.
- The Republic of Slovenia is a party to the international Agreement on the Conservation of Cetaceans of the Black Sea, the Mediterranean Sea and the Contigeous Atlantic Area (ACCOBAMS), and is thereunder engaged in monitoring of species of cetaceans and dolphins so as to ensure favourable conservation status.
- The red list of endangered species in the Republic of Slovenia lists bottlenose dolphins as endangered (E). Conservation measures are primarily focused on presumably extinct, endangered, vulnerable and rare species of flora and fauna.
- Bottlenose dolphins are also listed in Appendix II to the EU Directive on habitats, meaning its conservation is in the interest of the Community and special areas should be designated for their conservation.
The main objective to attain in this area is therefore to preserve and guarantee favourable conservation status for the population of bottlenose dolphins in Slovenian territorial waters so as to mitigate negative impact on the population.
This could also contribute to ensuring favourable conservation status for other species within the same ecosystem, thereby preserving biodiversity.
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):
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).
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)
Data available thusfar on the population of bottlenose dolphins (Tursiops truncatus) in Slovenian waters are presented in Genov et al. (2008) and are summarised below.
Bottlenose dolphins use Slovenian waters in all seasons and can be seen during nearly every month of the year. Due to Slovenian geographical and political attachment to Croatia and Italy, the animals also enter Italian and Croatian waters daily, meaning that a common approach is necessary to protect these animals populating a common area.
Natural marks on dorsal fins allowed for the identification of 101 specimen, and data show that approximately 75% of the population can be identified in this manner. Gender was determined for 18 females and 2 males, whereas it remains unknown for 81 specimen of the population. Some specimen rarely use the area, while others have been spotted every year and are therefore defined as residents.
The estimated size of the population for individual years is presented in NB10-1 Diagram and Table. 2005 and 2008 have been assessed as the most robust, making the estimates therein the most reliable (68 specimen in 2005 and 69 specimen in 2008). The density of the population was 0.068 specimen per square km in 2005, and 0.069 in 2008. Density and use of the area vary in time, which is normally the case with bottlenose dolphins. Nevertheless, the density in individual years was more or less stable (except in 2006, where a far wider area was included in the sample, and in 2007, where the density was lower because a lower number of specimen was spotted). A detailed explanation of the estimated density of the population in individual years is provided for in Genov et al. (2008). The aforementioned variations make it more difficult to project the future development of the phenomenon.
The size of groups ranges between 1 to 43 specimen, with 8 specimen in a group on average. Most (88.9%) of the groups include 15 specimen or less. Although groups constantly mix and change, some specimen can be traced to relatively stable groups. Offspring has been noticed in 53.3% of groups. Some newborn dolphins are reported of every year, and records show two instances of offspring mortality.
Dolphins use the area to feed, play, rest, and also to breed and raise calves.
Interactions between dolphins and the fishing sector are common, but by-catch of dolphins in fishing nets is a rare phenomenon in Slovenia.
The only other known and documented resident population of bottlenose dolphins in the Adriatic Sea is the one in the Lošinj–Cres archipelago (Croatia); it is estimated to include 100-130 specimen (Bearzi et al., 1997; Fortuna, 2006). Other areas in the Adriatic Sea have not yet been explored in this regard.
There are several populations of bottlenose dolphins documented in EU area but comparison is difficult because of different geographical features.