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:
The following human factors may potentially affect the dolphin population in Slovenian waters:
Definition of favourable conservation status
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).
Image NB10-1: Eastimated density of the dolphin population
Source: Morigenos – Marine mammal research and conservation society, 2009
- 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.
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.
Data for Slovenia
Source database or source: Morigenos – marine mammal research and conservation society
Other sources and literature: