Baltic Proper IMMA

Size in Square Kilometres

109,324 km2

Qualifying Species and Criteria

Harbour porpoise – Phocoena phocoena

Criterion A, B (1) D (1)

Harbour seal – Phoca vitulina

Criterion B (1), D (1)


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This IMMA encompasses the portion of the Baltic Sea (Baltic Proper) that encompasses the main distribution of the genetically and morphometrically distinct Baltic Proper harbour porpoise (Phocoena phocoena) population, as well as the full distribution range of the geographically isolated and genetically distinct Kalmarsund population of harbour seals (Phoca vitulina). The harbour porpoise is the only cetacean species resident in the Baltic. With an estimated population size of a few hundred individuals (95% CI 71-1105 individuals, point estimate 491), the Baltic Proper population is listed as Critically Endangered by IUCN. Breeding is thought to take place on and around the offshore banks in the central Baltic Proper. The IMMA also includes the main winter distribution areas of the population. The Kalmarsund harbour seal population is a distinct population of the Atlantic harbour seal which has been isolated in the Baltic Proper for approximately 8000 years. This population went through a bottleneck in the 1970s but has since then increased to just over 2000 individuals.

Description of Qualifying Criteria

Criterion A – Species or Population Vulnerability

The harbour porpoise (Phocoena phocoena phocoena) in the Baltic Proper is considered a separate sub-population by IUCN and is listed in the IUCN red list as critically endangered (Carlström et al. 2023). It is also listed as Critically Endangered by HELCOM (HELCOM 2013) ( There are several threats to the population, the main ones likely being bycatch in static fishing gear and environmental contaminants, but it is likely that prey quantity and quality as well as disturbance from various sources of underwater noise also has an impact on the population status.

Criterion B: Distribution and Abundance

Sub-criterion B1: Small and Resident Populations

The Baltic Proper harbour porpoise population is estimated at 71-1,105 individuals (95% CI, point estimate 491) (Amundin et al. 2022) within the range in the Baltic Proper defined by Carlén et al. 2018. The main range of the Baltic Proper harbour porpoise population is clearly separate from that of the neighbouring Belt Sea population which resides in the southwestern Baltic, the Sound, the Belt Sea and the southern Kattegat Sea (Sveegaard et al. 2015). This spatial separation is especially clear during the summer breeding period (Carlén et al. 2018), when the Baltic Proper population gathers around the offshore banks in the middle of the Baltic Proper, south of the island of Gotland. The Baltic Proper population is genetically divergent from the Belt Sea population and the North Sea populations of harbour porpoises (Lah et al. 2016).

Harbour porpoises in the Baltic Sea region give birth in the summer, with the main peak in June-August (Siebert et al. 2006; Sørensen and Kinze 1994; Börjesson and Read 2003). Mating takes place at the same time of year as calves are born. Harbour porpoises seem to be philopatric (Kinze 1990), meaning they return to their place of birth to breed, and based on the seasonal distribution of animals in the Baltic Proper (Carlén et al. 2018), the main breeding area of the Baltic Proper population is situated around the offshore banks Hoburgs bank and the Northern and Southern Midsea banks in the central Baltic Proper, all areas within the candidate IMMA.

Harbour porpoises need to eat very regularly to sustain their high metabolic rate and as a result spend a considerable amount of their time foraging (Wisniewska et al. 2016) and (Kastelein, Helder-Hoek, and Jennings 2018; Koopman 1998; Read 1990; Rojano-Doñate et al. 2018). This means that they forage wherever they are, and that any area frequented by harbour porpoises is, in effect, a feeding area.

The Kalmarsund harbour seal (Phoca vitulina vitulina) is an isolated population of the Atlantic harbour seal (Härkönen et al. 2005; Goodman 1998). This population is resident around the southern tip of Öland island and in the narrow sound between Öland and the Swedish mainland in the Baltic Proper. Harbour seals generally have small home ranges (Dietz et al. 2013; 2015), which is also true for the Kalmarsund population. The population has increased by approximately 9% per year since around 1975, and was estimated at just over 2000 individuals in 2021 (ICES 2022).

Criterion D: Special Attributes

Sub-criterion D1: Distinctiveness

Harbour porpoises entered what is now the Baltic Sea approximately 9000 years ago, and the Baltic Proper population is now genetically (Lah et al. 2016; Wiemann et al. 2010) and morphometrically (Galatius, Kinze, and Teilmann 2012) differentiated from the neighbouring population in the Belt Sea, which is supported by results on spatiotemporal distribution (Carlén et al. 2018). There is limited interbreeding with the neighbouring Belt Sea population.

The Kalmarsund harbour seal (Phoca vitulina vitulina) is an isolated population of the Atlantic harbour seal (Härkönen et al. 2005; Goodman 1998). The population was founded approximately 8000 years ago, and adjacent populations were very small or went extinct, which meant the Kalmarsund population has been isolated ever since. Genetic diversity is very low due to a severe bottleneck with only approximately 30 individuals counted in the 1970’s (Härkönen et al. 2005). The usual small home range of harbour seals (Dietz et al. 2013; 2015) means that they still have very little genetic exchange with the neighbouring population in the southwestern Baltic Sea and the Belt Sea.

Supporting Information

Amundin, Mats, Julia Carlström, Len Thomas, Ida Carlén, Jonas Teilmann, Jakob Tougaard, Olli Loisa, et al. 2022. ‘Estimating the Abundance of the Critically Endangered Baltic Proper Harbour Porpoise (Phocoena Phocoena) Population Using Passive Acoustic Monitoring’. Ecology and Evolution 12 (2): e8554.

Börjesson, Patrik, and Andrew J. Read. 2003. ‘Variation in Timing of Conception between Populations of the Harbor Porpoise’. Journal of Mammalogy 84 (3): 948–55.

Carlén, Ida, Len Thomas, Julia Carlström, Mats Amundin, Jonas Teilmann, Nick Tregenza, Jakob Tougaard, et al. 2018. ‘Basin-Scale Distribution of Harbour Porpoises in the Baltic Sea Provides Basis for Effective Conservation Actions’. Biological Conservation 226 (October): 42–53.

Carlström, Julia, and Ida Carlén. 2016. ‘Skyddsvärda områden för tumlare i svenska vatten’. AquaBiota Report 2016:04. Stockholm, Sweden: AquaBiota Water Research.

Carlström, J., Carlén, I., Dähne, M., Hammond, P.S., Koschinski, S., Owen, K., Sveegaard, S. & Tiedemann, R. 2023. Phocoena phocoena (Baltic Sea subpopulation). The IUCN Red List of Threatened Species 2023: e.T17031A50370773. Accessed on 22 December 2023.

Conley, Daniel J., Svante Björck, Erik Bonsdorff, Jacob Carstensen, Georgia Destouni, Bo G. Gustafsson, Susanna Hietanen, et al. 2009. ‘Hypoxia-Related Processes in the Baltic Sea’. Environmental Science & Technology 43 (10): 3412–20.

Dietz, Rune, Anders Galatius, Lonnie Mikkelsen, Jacob Nabe-Nielsen, Frank F. Rigét, Henriette Schack, Henrik Skov, Signe Sveegaard, Jonas Teilmann, and Frank Thomsen. 2015. ‘Kriegers Flak Offshore Wind Farm, Marine Mammals EIA – Technical Report’. Niras, Aarhus university, DHI.

Dietz, Rune, Jonas Teilmann, Signe M. Andersen, Frank Rigét, and Morten T. Olsen. 2013. ‘Movements and Site Fidelity of Harbour Seals (Phoca Vitulina) in Kattegat, Denmark, with Implications for the Epidemiology of the Phocine Distemper Virus’. ICES Journal of Marine Science 70 (1): 186–95.

Galatius, Anders, Carl Christian Kinze, and Jonas Teilmann. 2012. ‘Population Structure of Harbour Porpoises in the Baltic Region: Evidence of Separation Based on Geometric Morphometric Comparisons’. Journal of the Marine Biological Association of the United Kingdom 92 (Special Issue 08): 1669–76.

Goodman, S J. 1998. ‘Patterns of Extensive Genetic Differentiation and Variation among European Harbor Seals (Phoca Vitulina Vitulina) Revealed Using Microsatellite DNA Polymorphisms.’ Molecular Biology and Evolution 15 (2): 104–18.

Hammond, P.S., G. Bearzi, A. Bjørge, K. A. Forney, L. Karczmarski, T. Kasuya, W. Perrin, et al. 2008. ‘Phocoena Phocoena (Baltic Sea Subpopulation)’. The IUCN Red List of Threatened Species 2008. International Union for Conservation of Nature.

Härkönen, Tero, Karin C. Harding, Simon J. Goodman, and Kerstin Johannesson. 2005. ‘Colonization History of the Baltic Harbor Seals: Integrating Archaeological, Behavioral, and Genetic Data’. Marine Mammal Science 21 (4): 695–716.

HELCOM. 2013. ‘HELCOM Red List of Baltic Sea Species in Danger of Becoming Extinct’. Baltic Sea Environment Proceedings No 140.

ICES. 2020. ‘ICES Special Request Advice on Emergency Measures to Prevent Bycatch of Common Dolphin (Delphinus Delphis) and Baltic Proper Harbour Porpoise (Phocoena Phocoena) in the Northeast Atlantic’.

———. 2022. ‘Working Group on Marine Mammal Ecology (WGMME)’. 4:61. ICES Scientific Reports.

Kastelein, Ronald A., Lean Helder-Hoek, and Nancy Jennings. 2018. ‘Seasonal Changes in Food Consumption, Respiration Rate, and Body Condition of a Male Harbor Porpoise (Phocoena Phocoena)’. Aquatic Mammals 44 (1): 76–91.

Kinze, Carl Christian. 1990. ‘The Harbour Porpoise (Phocoena Phocoena (L.)): Stock Identification and Migration Patterns in Danish and Adjacent Waters’. Ph.D., University of Copenhagen.

Koopman, Heather N. 1998. ‘Topographical Distribution of the Blubber of Harbor Porpoises (Phocoena Phocoena)’. Journal of Mammalogy, 260–70.

Lah, Ljerka, Daronja Trense, Harald Benke, Per Berggren, Þorvaldur Gunnlaugsson, Christina Lockyer, Ayaka Öztürk, et al. 2016. ‘Spatially Explicit Analysis of Genome-Wide SNPs Detects Subtle Population Structure in a Mobile Marine Mammal, the Harbor Porpoise’. Edited by Roberta IMMAruta. PLOS ONE 11 (10): e0162792.

Lass, Hans-Ulrich, and Wolfgang Matthäus. 2008. ‘General Oceanography of the Baltic Sea’. In State and Evolution of the Baltic Sea, 1952-2005: A Detailed 50-Year Survey of Meteorology and Climate, Physics, Chemistry, Biology, and Marine Environment, 5–43. John Wiley & Sons, Inc.

Naturvårdsverket. 2006. ‘Inventering Av Marina Naturtyper På Utsjöbankar’. 5576. Stockholm, Sweden: Naturvårdsverket.

Read, Andrew J. 1990. ‘Age at Sexual Maturity and Pregnancy Rates of Harbour Porpoises Phocoena Phocoena from the Bay of Fundy’. Canadian Journal of Fisheries and Aquatic Sciences 47 (3): 561–65.

Rojano-Doñate, Laia, Birgitte I. McDonald, Danuta M. Wisniewska, Mark Johnson, Jonas Teilmann, Magnus Wahlberg, Jakob Højer-Kristensen, and Peter T. Madsen. 2018. ‘High Field Metabolic Rates of Wild Harbour Porpoises’. Journal of Experimental Biology 221 (23): jeb185827.

Siebert, Ursula, Anita Gilles, Klaus Lucke, Martje Ludwig, Harald Benke, Karl-Hermann Kock, and Meike Scheidat. 2006. ‘A Decade of Harbour Porpoise Occurrence in German Waters – Analyses of Aerial Surveys, Incidental Sightings and Strandings’. Journal of Sea Research 56 (1): 65–80.

Sørensen, Thomas Buus, and Carl Christian Kinze. 1994. ‘Reproduction and Reproductive Seasonality in Danish Harbour Porpoises, Phocoena Phocoena’. Ophelia 39 (3): 159–76.

Sveegaard, Signe, Ida Carlén, Julia Carlström, Michael Dähne, Anita Gilles, Olli Loisa, Kylie Owen, and Iwona Pawliczka. 2022. ‘HOLAS-III Harbour Porpoise Importance Map. Methodology’. Technical Report No. 240. Aarhus, Denmark: Aarhus University, DCE – Danish Centre for Environment and Energy.

Sveegaard, Signe, Anders Galatius, Rune Dietz, Line Kyhn, Jens C. Koblitz, Mats Amundin, Jacob Nabe-Nielsen, Mikkel-Holger S. Sinding, Liselotte W. Andersen, and Jonas Teilmann. 2015. ‘Defining Management Units for Cetaceans by Combining Genetics, Morphology, Acoustics and Satellite Tracking’. Global Ecology and Conservation 3: 839–50.

Voss, Rüdiger, Hans-Harald Hinrichsen, and Mike ST John. 1999. ‘Variations in the Drift of Larval Cod (Gadus Morhua L.) in the Baltic Sea: Combining Field Observations and Modelling’. Fisheries Oceanography 8 (3): 199–211.

Wiemann, Annika, Liselotte Andersen, Per Berggren, Ursula Siebert, Harald Benke, Jonas Teilmann, Christina Lockyer, et al. 2010. ‘Mitochondrial Control Region and Microsatellite Analyses on Harbour Porpoise (Phocoena Phocoena) Unravel Population Differentiation in the Baltic Sea and Adjacent Waters’. Conservation Genetics 11 (1): 195–211.

Wisniewska, Danuta Maria, Mark Johnson, Jonas Teilmann, Laia Rojano-Doñate, Jeanne Shearer, Signe Sveegaard, Lee A. Miller, Ursula Siebert, and Peter Teglberg Madsen. 2016. ‘Ultra-High Foraging Rates of Harbor Porpoises Make Them Vulnerable to Anthropogenic Disturbance’. Current Biology 0 (0).


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