Turkish Straits System and Prebosphoric IMMA

Criterion A – Species or Population Vulnerability

In the Turkish Straits System and Prebosphoric IMMA, both Mediterranean and Black Sea subspecies of bottlenose dolphins, common dolphins and harbour porpoises are found. The Black Sea subspecies of harbour porpoise and bottlenose dolphin are listed as Endangered (EN), and Black Sea common dolphins as Vulnerable (VU) on the IUCN Red list (Birkun 2008; Birkun and Frantzis 2008; Birkun 2012). Common dolphins and bottlenose dolphins in the Mediterranean Sea are listed as Endangered (EN) and Vulnerable (VU) respectively (Bearzi 2003; Bearzi et al. 2012). Further, the Turkish Straits System also holds the northern extent of Mediterranean Monk Seals, a species which has shown critical decline in its population, although with a recent rise in their sightings. Mediterranean Monk Seals were recently downgraded from Critically Endangered (CR) to Endangered (EN) on the IUCN Red list (Karamanlidis et al. 2019). The Turkish Straits System acts as the only connecting corridor between the Mediterranean and Black Sea populations of cetaceans, all of which hold threatened population status. Moreover, this is the only location in which the Black Sea subspecies of delphinids mix with the Mediterranean subpopulations.

Criterion B: Distribution and Abundance

Sub-criterion B1: Small and Resident Populations

Bottlenose dolphins are commonly observed within the Istanbul Strait and its neighbouring waters, with high site fidelity between and within years and seasons (Tezel 1958; Dede et al. 2016; Akkaya Baş et al. 2019). Among 87 bottlenose dolphin individuals photo-identified within the Istanbul Strait, 40 show regular presence within and between years, indicating a resident population with a likely home range that extends beyond the boundaries of the strait (Akkaya Baş et al. 2019). Additionally, common dolphins and harbour porpoises are frequently reported within the Istanbul Strait with year-round presence. While bottlenose dolphins represent the species most frequently  encountered in the strait composing over 50% of sightings, harbour porpoises followed by common dolphins form 29% and 20% of the sightings, respectively (Bas et al. 2014). Although each species was present year-round in the strait, bottlenose dolphins were recorded most frequently in spring months, while it was winter for harbour porpoises and summer for common dolphins (Bas et al. 2014). Due to the lack of individual identification of common dolphins or harbour porpoises in the Strait, it is unclear if the year-round sightings belong to the same individuals, thus indicating a resident population, or that the species use the area as a corridor between the Black Sea and Mediterranean Sea or a combination of both.

Regarding Marmara Sea, all the cetacean species were frequently sighted throughout the year with highest encounters recorded in autumn followed by spring, with 0.8 and 0.5 sightings per 10nm, respectively (Dede and Öztürk 2007, Öztürk et al. 2009; Altuğ et al. 2011). However, due to the lack of photo-identification study in the Marmara Sea, their residency pattern still needs to be assessed.

Regarding their population genetics, a recent study revealed that common dolphins showed no significant genetic differences between the Turkish Straits System and the Mediterranean Sea (Tonay et al. 2020) and the existence of an isolated subpopulation of the harbour porpoise in the Turkish Straits System has been proposed based on mtDNA and ddRAD-seq analyses (Tonay et al. 2017; Uzun et al. 2018).  Whilst anthropogenic disturbances mean that the Istanbul Strait itself is unlikely to be a reproductive area, the high presence of calves and juveniles of bottlenose dolphin and harbour porpoises suggest that TSS may hold breeding grounds.

Sub-criterion B2: Aggregations

Bottlenose dolphins, common dolphins and harbour porpoises show year-round presence in the Istanbul Strait and are regularly sighted in Marmara and Çanakkale Strait (Dede et al. 2016; Akkaya et al. 2017). Each species shows spatial-temporal aggregations within the Turkish Straits System IMMA. In general, common dolphins are sighted in spring and autumn, harbour porpoises mostly between March and July, while bottlenose dolphins are sighted throughout the year in the Istanbul Strait (Dede et al. 2016). Bottlenose dolphins aggregate year-round within the southern and northern entries of the Strait, with increased group size in winter seasons.  Harbour porpoises and common dolphins aggregate in the northern entry of the Istanbul Strait, with group sizes reaching on average 9 and 12 individuals, respectively (Dede et al. 2016, Akkaya Bas et al. 2014; 2019). In general, cetacean sightings peaked in between May-June and October- November during the pelagic fish migration (Dede et al. 2016).

The Istanbul Strait is relatively well studied regarding the encounter rate of species. The average encounter rate of cetaceans was reported as 0.76 sightings per 10 km in the Istanbul Strait in 2009 (Öztürk et al. 2009). Another study estimated an average encounter rate of bottlenose dolphins of 4 groups (22 individuals) per 10 km, with a peak encounter rate of 11 groups (75 individuals) per 10 km during spring during surveys conducted between 2010 and 2014 (Akkaya et al. 2019). A year-round encounter rate of 0.33 groups (2.39 individuals) per 10km was estimated for common dolphins which peaked in summer with an encounter rate of 0.73 groups (4.97 individuals) per 10km (DMAD, unpublished data). In summary, during the pelagic fish migration, in spring and autumn, delphinid aggregation in the Istanbul Strait increases. It is predicted that the majority of these are Black Sea subspecies, but it should also be considered that they are Mediterranean subpopulations (for Tursiops and Delphinus) that arrived at this time.

Seasonal line transect surveys on cetacean populations in the Turkish Straits System estimated  669 bottlenose dolphin individuals (189-2372 95% CI) and 1192 (468-2592 95 %CI) common dolphins in April 1999 (Dede et al 2016). In a more recent study the abundance of bottlenose dolphins was estimated as 1978 individuals (95% CI: 781 – 5011), common dolphins as 702 individuals (95% CI: 245 – 2012) and harbour porpoise as 1940 individuals (95% CI: 632 – 5960) in July 2019 (TUDAV 2020 unpublished data).

Criterion C: Key Life Cycle Activities

Sub-criterion C2: Feeding Areas

The Istanbul Strait and its neighbouring waters to the Black Sea holds one of the most important and intensive fishing grounds in Turkey as a result of seasonal fish migration between the Mediterranean and Black Sea. It is in fact one of the top three fishing grounds of Turkey. Each of the three cetacean species show regular foraging behaviour within the Istanbul Strait. During the season of pelagic fish migration in spring and autumn, cetaceans use the Straits as a natural trap for feeding on migratory pelagic fishes. Passive acoustic monitoring also suggests that delphinids were mainly feeding or socializing in spring seasons in the Istanbul Strait (Dede et al. 2014; Kameyama et al. 2014). A behavioural study revealed that bottlenose dolphins and common dolphins, engage in foraging behaviour in spring and summer seasons at the northern entry, while bottlenose dolphins also forage throughout the year in the southern entry of the Strait and its neighbouring Marmara Sea. By contrast, in winter and summer seasons porpoises were concentrated in relatively smaller locations than bottlenose dolphins at the northern and southern entry of the strait (Akkaya Bas 2014).

Sub-criterion C3: Migration Routes

The Turkish Straits System IMMA is the only migration route for cetaceans between the Mediterranean and the Black Seas (Öztürk and Öztürk 1996). Recent studies on population genetics revealed that common dolphins are genetically similar in the Black Sea and Mediterranean Sea, which indicates the importance of the Turkish Straits System to their connectivity. Further, Black Sea harbour porpoises are sighted within the Northern Aegean Sea which is a species that is absent in the rest of the eastern Mediterranean Sea. Genetic evidence suggests that the Northern Aegean Sea populations of harbour porpoises are highly likely to be dispersed from the Black Sea through the Turkish Straits System (Tonay et al. 2017). A comparison between photo-identification catalogues revealed the presence of the same individual bottlenose dolphins both in the Istanbul Strait and Black Sea (unpublished data). Especially for common dolphins, which have high dispersal potential, the protection of narrow seaways like the Turkish Straits System to enhance connectivity may be crucial (Tonay et al. 2020).