NSP2 route with V2 - Line A
7.10 Marine mammals
Marine mammals are an important component of the marine food chain and ecosystem in the Baltic Sea. Furthermore, a number of marine mammal species have protected status under national/in-ternational legislation and are therefore considered an important receptor.
This section describing marine mammals is an extract of a report prepared by DCE, Aarhus Univer-sity /210/.
Marine mammals in the Baltic Sea
Marine mammal species residing in the Baltic Sea include harbour porpoise (Phocoena phocoena), grey seal (Halichoerus grypus grypus), ringed seal (Pusa hispida baltica) and harbour seal (Phoca vitulina). Several other cetacean species, such as the minke whale (Balaenoptera acutorostrata), fin whale (Balaenoptera physalus), humpback whale (Megaptera novaeangliae), common dolphin (Delphinus delphis) and white-beaked dolphin (Lagenorhynchus albirostris) are sighted from time
to time, mainly in the southern part of the Baltic Sea, although these sightings are not considered frequent and these species are not native to Baltic waters /208/.
The following sections describe the biology, distribution and abundance of the three residential species in the Danish part of the Baltic Sea: harbour porpoise, harbour seal and grey seal.
Harbour porpoise
This section presents the Baltic Sea population of harbour porpoise, with information on popula-tion structure and size, distribupopula-tion, behaviour, reproducpopula-tion, echolocapopula-tion, hearing, and protec-tion.
7.10.2.1 Population structure and size
Several studies have sought to understand the population structure of harbour porpoises in the north-east Atlantic Ocean and particularly in the transition zone between the North Sea and the Baltic Sea. Studies on morphometric skull differences /211/ and genetics /212/ have found that three populations (or sub-populations) may exist in this area, namely (1) in the Baltic Proper (henceforth called the Baltic Sea population), (2) in the western Baltic Sea, the Belt Sea and the southern Kattegat (henceforth called the Belt Sea population) and (3) in the Skagerrak and North Sea.
These studies could not, however, determine the exact borders between the populations, perhaps due to some overlap in distribution in so-called transition zones. The proposed NSP2 route, the NSP2 route V1 and the NSP2 route V2 cross the Baltic Sea population management border and the majority of the route is located in the transition zone between the Baltic Sea and the Belt Sea populations. Individuals from both populations may therefore inhabit the area.
The Baltic Sea population has been studied by conducting visual surveys (albeit with low resolution in coverage) of population size in the Baltic Proper. 599 (95% confidence interval (CI): 200-3,300) individuals were observed in 1995 /213/, and 93 individuals (95% CI: 10-460) were observed in 2002 /214/. In 2016, the Static Acoustic Monitoring of Baltic Sea Harbour Porpoise (SAMBAH) project ended after having deployed 304 acoustic data loggers (C-PODs) for two years covering all Baltic EU countries. The project estimated the remaining number of porpoises in the Baltic Proper to be approximately 500 (95% CI: 80-1,100) /215/. The severe decline of the harbour porpoise population in the Baltic Sea makes it the smallest population of harbour porpoise in the world /216/.
The Belt Sea holds high densities of porpoises, especially in the Sound, Great Belt, Little Belt and Fehmarn Belt. Based on surveys performed in 1994, 2005, 2012 and 2016, the number of porpoises residing in this area was estimated to be 27,923 (95% CI: 11,916-65,432, 1994), 10,614 (95%
CI: 6,218 - 18,117, 2005), 18,495 (95% CI: 10,892 - 31,406, 2012), and 42,324 animals (95%
CI: 23,668 – 76,658, 2016), respectively /217//218/. Therefore, on the basis of the newest avail-able data from 2016, the Belt Sea population is assessed to be stavail-able.
For comparison, the total number of harbour porpoises in the continental shelf waters of the north-east Atlantic Ocean was estimated at 375,358 (95% CI: 256,304–549,713) /219/. This number includes all populations of porpoises in the North Sea as well as the majority of the spatial extent of the Belt Sea population.
7.10.2.2 Distribution
Harbour porpoises are widely, but unevenly, distributed throughout European waters. The distri-bution is presumably linked to the distridistri-bution of prey (e.g. /220/), which in turn is linked to pa-rameters such as hydrography and bathymetry /221/.
The porpoise detections from the SAMBAH project /215/ were analysed as porpoise positive sec-onds (PPS) per day and divided into two seasons: summer and winter (see Figure 7-42 and Figure 7-43, respectively).
In Figure 7-42, each acoustic station is indicated by a dot. If porpoises were detected, the dot is black and scaled in size to depict the density (PPS per day). If no porpoises were detected, the station is indicated by a white circle. In the summer period, the data points could be divided into the two population groups (i.e. east and west of the determined population border). Orange indi-cates the area inhabited by part of the Belt Sea porpoise population extending to the east, and blue is believed to contain the breeding distribution of the remaining Baltic Sea porpoise population.
During the breeding period in summer, porpoises in the Baltic Proper concentrate around the shal-low banks south of the Gotland and Öland islands (Figure 7-42). There is a clear drop in the density of harbour porpoises with distance from this area in all directions, which illustrates the isolation of this population. The highest density of the Baltic Sea harbour porpoise population is found around the Midsjö Banks, south of Gotland, in the summer. According to the results from the recently finished EU LIFE+ SAMBAH project, this area is considered a population hotspot and the most important area during the breeding season for this population of porpoises /215/.
Figure 7-42 Summer distribution of porpoises in the southern part of the Baltic Sea. Source: SAMBAH /215/.
During winter, porpoises are more widespread in the northern part of the Baltic Sea (see Figure 7-43). Each acoustic station is indicated by a dot. If porpoises were detected, the dot is black and
scaled in size to depict the density (PPS per day). If no porpoises were detected, the station is indicated by a white circle. In the winter, it is not possible to separate the two populations.
With respect to migration, there are no studies of current harbour porpoise migration routes in the Baltic Sea. However, in the adjacent waters of the Belt Seas, Kattegat, Skagerrak and the North Sea, satellite tracking of over 100 harbour porpoises have not indicated specific migration routes between sites or seasons.
Figure 7-43 Winter distribution of porpoises in the southern part of the Baltic Sea. Source: SAMBAH /215/.
7.10.2.3 Behaviour and reproduction
In the Baltic Sea, harbour porpoises have a maximum length of 1.8 m and a maximum weight of up to 90 kg. They are relatively short-lived, with a maximum recorded lifetime in the wild of 23 years. Harbour porpoises are opportunistic feeders, with a preference for herring and sprat.
The breeding period for Baltic harbour porpoises begins in mid-June and ends in late August. Ov-ulation and conception typically take place in late July and early August /222/, and the females give birth to a calf in early summer. Calves are sighted throughout their range and areas of high porpoise density may therefore also be considered to be important for reproduction /223//224/.
No specific breeding areas for harbour porpoises have been identified in the Danish sector of the Baltic Sea.
7.10.2.4 Echolocation and hearing
Harbour porpoises have good underwater hearing and use sound actively (i.e. echolocation) for navigation and prey capture. Harbour porpoises produce short ultrasonic clicks (130 kHz peak
frequency, 50-100 μs duration; /225//226/) and are able to orient and find prey in complete dark-ness. Data from porpoises tagged with acoustic data loggers indicate that they use echolocation almost continuously /227//228/. Their hearing sensitivity is extremely high and covers a vast fre-quency range (see Figure 7-44, /229//230//231//232/). The audiogram (see Figure 7-44) shows the hearing threshold; porpoises can only hear sound above the threshold for each frequency. Their best ability to detect sound is at frequencies with the lowest threshold (the highest sensitivity).
Marine mammals do not hear equally well over their entire range of hearing. For sound intensities close to the hearing threshold, the audiogram is a good approximation of the perceived sound levels (the loudness of the sound). In marine mammals, there is a great difference in sensitivity between the frequencies of best hearing and those close to the cut-off frequencies.
Figure 7-44 Audiograms for harbour porpoises modified from /232/ (green), /229/ (blue) and /230/
(red). The audiogram also shows the frequency range of harbour porpoise vocalisation (yellow).
7.10.2.5 Biodiversity status
In 2017, HELCOM assessed the integrated biodiversity status of the Baltic Sea /111/. Harbour porpoise is not assessed in this report.
7.10.2.6 Protection
A number of international treaties, agreements and laws have been enacted in order to protect the harbour porpoise. In northern European waters, the species has been listed in Annex II and Annex IV of the Habitats Directive 92/43/EEC, Annex II of the Bern Convention, Annex II of the Bonn Convention and Annex II of the Washington Convention. Furthermore, the harbour porpoise is covered by the terms of ASCOBANS, a regional agreement under the Bonn Convention and HELCOM. In Denmark, the species is furthermore protected under Administrative Order 867 of 27/06/2016 /233/.
In the regional assessment for Europe, the harbour porpoise is listed as “Vulnerable”, while the sub-population of the Baltic Sea harbour porpoise is listed as “Critically Endangered” on the IUCN Red List.
Protected areas for marine mammals are described in sections 7.12 and 7.13.
Harbour seal
This section presents the Baltic Sea population of harbour seal, with information on population structure and size, distribution, behaviour, reproduction, hearing and protection.
7.10.3.1 Population structure and size
Based on genetic data and satellite telemetry, harbour seals in the Baltic Sea region have been split into three management units or sub-populations, among which there is at least partial repro-ductive isolation: (1) Kalmarsund (between Öland and the Swedish mainland), (2) the south-west-ern Baltic (along the southsouth-west-ern Danish and Swedish coasts) and (3) the Kattegat /234//235/. The Kalmarsund population comprises approximately 1,000 individuals /236/, the south-western Baltic population comprises approximately 1,500 individuals, and the Kattegat population comprises ap-proximately 7,800 individuals /237/. The proposed NSP2 route, the NSP2 route V1 and the NSP2 route V2 are located in the transition zone between the Kalmarsund population and the south-western Baltic population.
7.10.3.2 Distribution
Harbour seals are found in temperate and arctic waters of the Northern Hemisphere. Haul-out sites (also called colonies) are land localities occupied by seals during periods of mating, giving birth and moulting. Haul-out sites for harbour seals are well-known and do not change between years.
Annual counts are conducted during the moult in August. Knowledge of seal abundance and density is extensive with respect to the locations of the haul-out sites, which are shown in Figure 7-45. A tagging study showed that 10 tagged harbour seals travelled with a mean travel range of below 25 km /242/ and the zone of regular occurrence is taken as the maximum distance from the tagging site.
In the Baltic Sea, harbour seals are only found in Kalmarsund between Öland and the mainland of Sweden and in the south-western Baltic Sea, concentrated around the Rødsand sandbar (7 km west of Gedser in Denmark) and Falsterbo and Saltholm in the Sound.
The Danish section of the proposed NSP2 route, the NSP2 route V1 and the NSP2 route V2 do not cross any areas with colonies or regular occurrence of harbour seals. However, foraging harbour seals may potentially be present at all depths within their range in the areas surrounding the proposed NSP2 route, the NSP2 route V1 and the NSP2 route V2.
Figure 7-45 Haul-out sites (colonies) in the Baltic Sea used by harbour seals for resting, breeding and moulting. Global positioning system (GPS) tracking of harbour seals in the Danish sector is indicated by blue dots. No satellite tracking has been undertaken in the Swedish colonies. Source: HELCOM Seal Data-base /243/.
7.10.3.3 Behaviour and reproduction
The harbour seal is a relatively small seal with an adult weight of approximately 65-140 kg. Harbour seals are opportunistic predators. They feed mainly on benthic fish but can catch and eat all fish species. Moulting occurs in August, when seals spend more time on land to develop their new fur.
Females are believed to give birth once a year on land between May and June, after a gestation period of 11 months. The pup suckles for about three to four weeks, after which it is left to feed on its own. Harbour seal pups shed their embryonic fur (lanugo) before birth and are thus born with adult fur. Pups are able to swim and dive just after birth. Mating occurs immediately after the end of suckling and takes place in the water. Little is known on the exact circumstances surrounding mating; however, as noted above, mating and periods of birthing are focused on haul out sites/col-onies (as shown in Figure 7-45).
7.10.3.4 Hearing
Seals have ears that are well-adapted to an aquatic life. These adaptations include cavernous tissue in the middle ear, which allows for balancing the increased pressure on the eardrum when the animal dives /244/.
Figure 7-46 shows an audiogram of harbour seals, demonstrating that they have good underwater hearing in the range from a few hundred Hz to approximately 50 kHz.
Figure 7-46 Audiograms of three harbour seals, showing the threshold of hearing under quiet conditions at frequencies in the range from 80 Hz to 150 kHz. The names Møhl, Terhune and Kastak in the legend refer to results from references /245/, /246/ and /247/, respectively.
7.10.3.5 Biodiversity status
In 2017, HELCOM assessed the integrated biodiversity status of the Baltic Sea /111/. The pro-posed NSP2 route, the NSP2 route V1 and the NSP2 route V2 are located in the zone between the Kalmarsund population and the south-western Baltic population. Under the most recent assess-ment of biodiversity under HELCOM, the Kalmarsund population falls below the threshold for
‘Good Status’, based on the low abundance, while the growth rate of the stock is satisfactory.
The south-western Baltic population falls below the threshold based on a positive growth rate lower than the threshold.
7.10.3.6 Protection
Harbour seals are protected under the EU Habitats Directive and the Bonn Convention. In addition, they are fully protected under national legislation. Furthermore, the Kalmarsund population is listed as endangered by the IUCN. The harbour seal is listed on the EU Habitats Directive Annex II, which means that it should be protected via the designation of special areas of conservation. For seals, these areas are primarily designated in connection with important haul-out areas on land. In Den-mark, the species is furthermore protected under Administrative Order 867 of 27/06/2016 /233/.
Protected areas for marine mammals are described in sections 7.12 and 7.13.
Grey seal
This section presents the Baltic Sea population of grey seal, with information on population struc-ture and size, distribution, behaviour, reproduction and protection.
7.10.4.1 Population structure and size
There are three separate populations of grey seal in the world. One of them is the Baltic grey seal, which is found in the Baltic Proper, in the Bothnian Sea and in the Gulf of Finland; the other two populations live in the north-east and north-west Atlantic Ocean, respectively.
One hundred years ago, the grey seal population in the Baltic Sea comprised 80,000-100,000 individuals, but by the 1970s it had decreased to approximately 4,000 due to hunting and repro-ductive disorders that have been connected to pollution by organochlorides /248/. Abundance based on photo-identification in 2000 revealed an estimate of 15,600 individuals, while an aerial survey in 2004 observed 17,640 grey seals on land /249/. Studies have counted 30,000 grey seals
Audiogram
in the Baltic Sea /236/, and it is estimated that the total population in the Baltic Sea was up to 40,000.
7.10.4.2 Distribution
Baltic grey seals are distributed from the northern-most part of the Bothnian Bay to the south-western waters of the Baltic Proper (Figure 7-47). Generally, during the breeding period, the seals dwell on drift ice in the Gulf of Riga, the Gulf of Finland, the northern Baltic Proper and the Bothnian Bay or on the rocks in the north-western Baltic Sea. As is the case with harbour seals, haul-out sites/colonies are land localities occupied by grey seals. The locations of these sites are shown in Figure 7-47.
Satellite tracking of grey seals has shown that this species moves over several hundreds of kilo-metres in the Baltic Sea. There are indications that seasonal migrations are closely related to spe-cies requirements for feeding and suitable breeding habitats /242/. Typically, however, the animals feed locally, foraging just offshore and adopting a regular pattern of travelling between local feed-ing sites and preferred haul-out sites /250//251/.
Figure 7-47 Haul-out sites (colonies) used by grey seals for resting, breeding and moulting. GPS tracking of grey seals is indicated by blue dots. Source: HELCOM Seal Database /243/.
In the Danish part of the Baltic Sea, the number of grey seals has increased drastically over the last decade (see Figure 7-48). The grey seal colony in closest vicinity to the proposed NSP2 route, the NSP2 route V1 and the NSP2 route V2 is at Christiansø (part of Ertholmene), north-east of Bornholm, more than 29 km from the combination of the proposed NSP2 route with V1, and more than 35 km from the combination of the NSP2 route with V2. This colony is, at present, the largest Danish grey seal colony. Up to 600 grey seals have been counted here, and in