Harbour porpoises (Phocoena phocoena, Linnaeus 1758) are distributed widely throughout continental shelf waters of the Northern Hemisphere and are present in the eastern North Atlantic Ocean and Black Sea (Gaskin 1984; Read 1999; Hammond et al. 2013). The historical presence of harbour porpoises in the Mediterranean Sea has been debated in the past (Frantzis et al. 2001, 2003; Notarbartolo di Sciara 2002; Rosel et al. 2003; Viaud-Martínez et al. 2007; Fontaine et al. 2012). Prior to this study there were 21 published records of harbour porpoises documented in the Mediterranean Sea dating back to 1981 (Frantzis et al. 2001; Bellido et al. 2006; Frantzis 2009; Tonay et al. 2009; Notarbartolo di Sciara & Birkun 2010; Tonay & Dede 2013) with all but two reported from the Aegean Sea; the remaining two records were from the Strait of Gibraltar and the waters of Tunisia. These previous records are mostly intermittent strandings, although in 1993 there was a confirmed report of free-swimming porpoises followed by three strandings in the north of the Thracian Sea, including a live-stranded animal (Frantzis et al. 2001).

It is likely that the increase in harbour porpoise records in the Aegean Sea during the last two decades is due to an increased effort in reporting of stranded animals from coordinated stranding networks and improved public awareness of marine mammals (Rosel et al. 2003), although a genuine increase in the number of individuals in the region cannot be excluded. In Greece, the port-police have been responsible for collating stranding records from the public since 1991. Since 2000 it has been mandatory to report stranded cetaceans; this has increased the number of cetacean strandings reported from 50 to 75 each year to around 100. Since 2010, the Pelagos Cetacean Research Institute has systematically recorded Greek cetacean strandings through a national stranding network involving port-police authorities and volunteers, with an average of 125 events reported per year since this date. Although Turkey does not have a systematic stranding scheme, Istanbul University/Turkish Marine Research Foundation collates stranding records opportunistically. With the development of the internet and smart phones over the last 10 years, there has been an increase in the public awareness of the importance of reporting strandings in Turkish waters.

Results from a molecular genetic study indicate that the Black Sea population of harbour porpoises is distinct from the Atlantic population, having been isolated by the Mediterranean Sea over 5000 years ago (Fontaine et al. 2010). Due to the very limited geographic area within the Mediterranean Sea where sightings and strandings have been recorded in the past (Aegean Sea and close to the Strait of Gibraltar), the presence of a continuous population of harbour porpoises linking the Atlantic and Black Sea populations is considered very unlikely (Frantzis et al. 2001). From genetic analyses of stranded specimens, harbour porpoises in the Aegean Sea have been found to share common haplotypes with harbour porpoises in the Black Sea as well as in the Turkish Straits System (Viaud-Martínez et al. 2007; Tonay et al. 2014). It is therefore thought that these Aegean porpoises are migrants from the Black Sea population (Rosel et al. 2003; Viaud-Martínez et al. 2007; Tonay et al. 2012, 2014). The possibility still exists however, that Black Sea and Aegean Sea harbour porpoises could form a single random-mating (panmictic) population (Fontaine et al. 2012), or that harbour porpoises sighted in the northern Aegean Sea are from a small isolated Mediterranean population (Rosel et al. 2003; Notarbartolo di Sciara & Birkun 2010). Indeed, five harbour porpoises found stranded in the Marmara Sea, between the Black Sea and the Aegean Sea, exhibit a unique haplotype, suggesting that these individuals may comprise a distinct and isolated population (Viaud-Martínez et al. 2007; Tonay et al. 2012, 2014). However, with sample sizes as small as those reported from the region, caution must be exhibited in interpreting the genetic results.

The Black Sea harbour porpoise (P. phocoena relicta, Abel 1905) subspecies has recently been listed on the IUCN Red List as Endangered (Birkun & Frantzis 2008), both genetically and morphologically distinct from the eastern North Atlantic population (Rosel et al. 1995, Rosel et al. 2003; Fontaine et al. 2007, 2010; Viaud-Martínez et al. 2007; Galatius & Gol’din 2011; Tonay et al. 2014). The Agreement on the Conservation of Cetaceans in the Black Sea, Mediterranean Sea, and Contiguous Atlantic Area (ACCOBAMS) noted four “priority” species in the Mediterranean and Black Seas (Notarbartolo di Sciara 2002) which were at greatest risk of declining or disappearing. The harbour porpoise was among them. Over the last 50 years the Black Sea harbour porpoise population is thought to have been reduced to just 10 % of its former size (Fontaine et al. 2010, study based on genetics). This has been attributed to a commercial dolphin fishery which continued until 1983, in addition to high mortality from bycatch in bottom set gillnets, habitat degradation (Birkun & Frantzis 2008) and pollution (Notarbartolo di Sciara 2002; Ozturk 2013). This sub-species clearly requires urgent protection from anthropogenic impacts throughout its range, including the Aegean Sea.

In order to document the presence and distribution of harbour porpoises in the Thracian Sea, a simultaneous visual and acoustic survey was conducted in the northern Aegean Sea. The results from this first systematic survey for harbour porpoises in the Greek and Turkish waters of the Thracian Sea are presented here, together with an update on stranding records.

This study represents the first dedicated survey for harbour porpoises conducted in the Mediterranean Sea. It confirms the presence of free-swimming harbour porpoises in Greek and Turkish waters of the northern Aegean Sea in summer 2013. The inconspicuous nature of harbour porpoises severely reduces the probability of seeing individuals in sea conditions above sea state two (Teilmann 2003) and reduces significantly above sea state three (Palka 1996). Due to these challenges, harbour porpoises are not easy to observe in anything but perfect sea conditions. Although the mode sea state during this survey was two, during many parts of the survey the sea state exceeded three, thus compromising sighting conditions and highlighting the value of passive acoustic detection techniques in surveys for inconspicuous, vocally-active species. It is worth noting that all harbour porpoise sightings in this study were made when the sea-state was between zero and one and porpoises were never visually detected in higher sea states, even in blocks where several acoustic detections were made.

There were a total of 21 distinct acoustic or visual detections of harbour porpoises throughout the survey (visual n = 5, acoustic n = 12 and combined n = 4) grouped in three discrete blocks: north of the Greek Island of Thasos; southwest of Alexandroupolis; and Saros Bay, Turkey where the highest encounter rate (acoustic and/or visual) was recorded. Similar to porpoise populations in the Atlantic (Hammond et al. 2008) and Black Sea (Birkun & Frantzis 2008), the harbour porpoises were all encountered in shallow (<150 m) coastal habitat (within <6 nm of land). The sea-surface temperature in Saros Bay, where the highest density of porpoises occurred, was up to 25 °C, above the previously postulated “limiting thermocline” for the species (Tolley & Rosel 2006) and to the authors’ knowledge, may be the warmest sea surface temperatures in which harbour porpoises have been observed. Although no previous systematic survey for harbour porpoises had been conducted in the Thracian Sea, parts of this area had previously been surveyed for cetaceans. During spring and summer visual surveys of the Turkish waters of the Aegean Sea between 2005 and 2008, including Saros Bay, no harbour porpoises were observed (Dede & Öztürk 2007; Öztürk et al. 2009; Altuğ et al. 2011); possibly due to poor sea state or disturbance from the motor research vessel. Previous strandings data indicated the presence of porpoises in the Aegean Sea was largely confined to the northern Thracian Sea coast with several strandings northwest of Thasos Island and close to Alexandroupolis (Birkun & Frantzis 2008). The results of this survey are consistent with the distribution of strandings in the region. However, this study highlights Saros Bay as an additional potentially significant area for porpoises, with the highest relative acoustic encounter rate in a location with just one previously reported stranding (Tonay et al. 2009; Tonay et al. 2012). This demonstrates the importance of using both strandings and systematic surveys to inform our understanding of harbour porpoise distribution. The recent increase in the number of strandings in the more southerly latitudes of the Aegean Sea suggest a possible increase in the range of harbour porpoise in this area; however the presence of high parasite loads in three of the six southerly strandings (Tonay & Dede 2013 and Pelagos Cetacean Research Institute unpublished data) may indicate difficulties in survival for porpoises in these southerly latitudes. The multiple sightings, and the presence of a calf in one of the groups, indicate that porpoises in the northern Aegean may be breeding there. That a pregnant porpoise was found stranded north-west of Thasos Island further supports this hypothesis. With the Black Sea population of harbour porpoises estimated from genetic analysis to be just 10 % of its former size (Fontaine et al. 2010), and considering the unknown status of harbour porpoises in the Mediterranean Sea, signs of a possible breeding group of porpoises in the Aegean Sea are highly significant. Considering that porpoises occur in the northern Aegean Sea in low relative abundances, occasionally with calves and exhibit an apparent preference for coastal waters, management measures towards their protection are warranted. The porpoises within the Black and Aegean Seas face many anthropogenic threats including pollution, climate change, fisheries bycatch and habitat degradation (Notarbartolo di Sciara 2002). As an example, a fisheries survey in the 1990s revealed 996 licensed fishing vessels with 964 set nets and longlines, 29 purse seines, two trawls, and one beach seine operating in Turkish waters of the northern Aegean Sea alone (Kara & Gurbet 1999). Since this study was published the number of registered fishing vessels has increased, Ayaz et al. (2008) reported that there were about 250 fishermen using gillnets and trammel nets in Saros Bay. One of the animals listed as stranded in this study demonstrated evidence of entanglement in fishing gear (Fig. 5). As information on the levels of harbour porpoise bycatch in fisheries within the Mediterranean Sea is currently lacking, efforts are needed to quantify and reduce bycatch in all fisheries, especially in the Thracian Sea. Additionally, more information on the distribution of harbour porpoises in the Aegean Sea is required, especially seasonally and through the Turkish Straits System.

In small discrete porpoise populations, such as that in the northern Aegean Sea, monitoring trends in abundance is essential to maintaining their conservation status. The Baltic Sea and north west African coastal waters are both areas considered to have discrete harbour porpoise subpopulations (Huggenberger et al. 2002; Tolley & Rosel 2006) with the Baltic Proper population being critically endangered due to its low density (Hammond et al. 2008). The results from this first dedicated northern Aegean Sea survey reveal acoustic encounter rates which are comparable to that of the Baltic Proper. Scientists studying small populations of phocoenids, such as harbour porpoises in the Baltic Sea and vaquita in the Gulf of California, have, following initial vessel and aerial surveys, established large arrays of autonomous acoustic recording devices to facilitate longer term monitoring of certain areas (Rojas-Bracho et al. 2010; Benke et al. 2014). Now that harbour porpoise presence has been confirmed in the Aegean Sea from this study, and certain areas noted as potentially significant, fixed acoustic devices have the potential to be used here to monitor presence and detect changes in abundance of porpoises more rapidly and over extended periods of time. As such it is recommended that relevant agencies, such as ACCOBAMS and the Turkish and Greek governments, consider these methods to monitor this small population in order to target further protection and assess the efficacy of conservation measures to reduce anthropogenic threats. Zones for marine protected area (MPA) designation have already been suggested in the high seas of the Aegean Sea (Öztürk 2009). In certain areas, such as the coastline off Alexandroupolis, and north of Thasos Island, the need for designation of MPAs has been noted; the information presented here supports the case for the creation of marine Natura 2000 sites with harbour porpoises as a qualifying interest (PCRI 2009). In recognition of the high biodiversity in Saros Bay, the Bay was declared an MPA in January 2011 and limitations have been set on the large-scale trawling, purse-seining and bivalve dredging fisheries in the area. This is a positive development in terms of conservation but, at present there are no limitations on gillnet fisheries, which have been described as “the single most important threat to porpoises” in other areas (Jefferson & Curry 1994) and their prevalence in the region is of serious concern. In light of new evidence presented here on the presence and distribution of harbour porpoises in the northern Aegean Sea in both Greek and Turkish waters, a sharp focus on the protection of this small, coastal and possible breeding group is urgently required.

ACCOBAMS, the agreement on the conservation of Cetaceans in the Black Sea, Mediterranean Sea, and contiguous Atlantic area; IUCN, International Union for Conservation of Nature; MPA, marine protected area; SD, standard deviation; SE, standard error; SOTW, song of the Whale (the survey research vessel)