Wadden Sea IMMA

Size in Square Kilometres

27,514 km2

Qualifying Species and Criteria

Harbour seal – Phoca vitulina

Criterion B (2), C (1,2)


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The Wadden Sea is a very large intertidal area hosting one of the two largest harbour seal (Phoca vitulina vitulina) populations in the Northeast Atlantic. The area is a UNESCO World Heritage site, consisting of vast bare, intertidal sand and mud flats, and is highly productive. Harbour seals, heavily hunted until the second half of the 20th century and later stricken by the Phocine Distemper Virus, have shown exceptional recovery. The growth of the population has levelled off, and is now receding despite good pup production. The genetically distinct population of probably 30-40,000 harbour seals is a flagship species for the area. The sand and mudflats of the Wadden Sea are used for resting and breeding, while the Wadden Sea and the adjacent North Sea is used for foraging.

Description of Qualifying Criteria

Criterion B: Distribution and Abundance

Sub-criterion B2: Aggregations

The latest count conducted in 2022 during the harbour seal (Phoca vitulina) moult in the Wadden Sea resulted in 23,654 seals counted (Galatius et al., 2022). This is one of the largest populations of harbour seals in the Northeast Atlantic, after Scotland (26,846 in 2016-2021, SCOS 2021).  The harbour seals in the Wadden Sea are part of a metapopulation (Carroll et al. 2022). However, this count from 2022 constitutes a decrease of 12% relative to 2021 and is the lowest since 2011. The drop continued in 2023 when the counts dropped another 4%. The Wadden Sea harbour seals are identified as separate genetic units from the Skagerrak, the western Baltic and Central Limfjord populations (Olsen et al., 2014). Two Phocine Distemper Virus (PDV) epizootics led to high seal mortalities in 1988 and 2002 (Härkönen et al, 2006). The population later quickly recovered, and started to stabilise from 2012-2013, despite a continuous increase in pup production until 2022 (Galatius et al., 2023). It is unclear if this indicates the population is approaching carrying capacity. Also, in 2022 the number of pups born in the area dropped by over 20% compared to the year before, indicating that there could be a change in breeding or in recruitment of pups into the breeding population.

In the south, the population extends to the Dutch Delta area, where numbers counted grew from 14 in 1990 to a moult count of 1162 in 2021 (Hoekstein et al 2023). However, these are mostly visitors from the Wadden Sea as too few pups are born to explain the growth and tracked females have been observed to swim back to the Wadden Sea to breed (Brasseur 2017).

Criterion C: Key Life Cycle Activities

Sub-criterion C1: Reproductive Areas

The Wadden Sea is an important breeding area for harbour seals in the Northeast Atlantic (Unger et al., 2022). Most pups are born on the tidal flats of the Wadden Sea. In 2023, a total of 9,334 pups were counted (Galatius et al., 2023). While this number had increased over recent decades, the accounts showed a decrease of 22% in 2022 relative to the 2021 count of 10,903 pups (Galatius et al. 2022). Pup production is not often estimated in other harbour seal populations, so these figures cannot be compared to other colonies in the region; however, according to the total number of adult seals counted, this must be one of the main harbour seal breeding colonies in the North East Atlantic (ICES 2022).

Sub-criterion C2: Feeding Areas

Numerous telemetry studies have been conducted to assess harbour seals’ movements and foraging areas from the haul-out sites in the Wadden Sea (Tougaard et al., 2008; Liebsch et al., 2010; Rojano Doñate, 2014; Wilson et al., 2015; Aarts et al, 2016; Vance et al., 2021; Nachtsheim et al. 2023). They show that like in other areas, harbour seals are benthic feeders, favouring areas over 30m deep with low mud content. Their feeding grounds are usually in the North Sea (Liebsch et al., 2006; Tougaard et al., 2008; Nachtsheim et al., 2023) and they travel to offshore areas located tens of kilometres, sometimes more before returning back to the same haul-out site. Although some offshore feeding hotspots are apparent from tracking data, the seals’ foraging distribution includes most of the coastal zone (<50 km offshore), (Rojano Doñate, 2014; Vance et al., 2021; Aarts et al. 2016, Brasseur 2017).

Supporting Information

Aarts, G., Brasseur, S., Poos, J. J., Schop, J., Kirkwood, R., Van Kooten, T., Mul, E., Reijnders, P., Rijnsdorp, A.D. and Tulp, I.. 2019. ‘Top-down pressure on a coastal ecosystem by harbor seals’. Ecosphere 10:e02538.

Aarts, G., Cremer, J., Kirkwood, R., Van Der Wal,J.T., Matthiopoulos, J. and Brasseur,S. 2016. Spatial distribution and habitat preference of harbour seals (Phoca vitulina) in the Dutch North Sea. Wageningen University. 43 pp.

Brasseur, S. 2017. Seals in motion – How movements drive population development of harbour seals and grey seals in the North Sea. PhD thesis. Wageningen University, Wageningen, the Netherlands, 176 pp.

Brasseur, S., van Polanen Petel, T., Gerrodette, T., Meesters, E., Reijnders, P. and Aarts, G. 2015. ‘Rapid Recovery of Dutch Gray Seal Colonies Fuelled by Immigration’. Marine Mammal Science 31(2):405–26.

Brasseur, S., Reijnders, P., Cremer, J., Meesters, E., Kirkwood, R., Jensen, L., Jeβ, A., Galaius, A., Teilmann, J. and Aarts, G. 2017. ‘Echoes from the past: Regional variations in recovery within a harbour seal population’. Plos One 13:e0189674.

de la Vega, C., Lebreton, B., Siebert, U., Guillou, G., Das, K., Asmus, R. and Asmus, H. 2016. ‘Seasonal variation of harbor seal’s diet from the Wadden sea in relation to prey availability’. Plos One 11:e0155727.

Galatius A., Brasseur S., Hamm T., Jeß A., Meise K., Meyer J., Schop J., Siebert U., Stejskal O., Teilmann J., Thøstesen C. B. (2023) Survey Results of Harbour Seals in the Wadden Sea in 2023. Common Wadden Sea Secretariat, Wilhelmshaven, Germany.

Galatius A., Brasseur S., Carius F., Jeß A., Meise K., Meyer J., Schop J., Siebert U., Stejskal O., Teilmann J., Thøstesen C.B. 2022. Survey Results of Harbour Seals in the Wadden Sea in 2022. Common Wadden Sea Secretariat, Wilhelmshaven, Germany.

Galatius, A. Abel, C. Brackmann, J., Brasseur, S. Jeß, A., Meise, K., Meyer, J., Schop, J., Siebert, U., Teilmann, J. and Bie Thøstesen, C. 2021. Harbour seal surveys in the Wadden Sea and Helgoland 2021. Common Wadden Sea Secretariat.

Härkönen, T., Dietz, R., Reijnders, P., Teilmann, J., Harding, K., Hall, A., Brasseur, S., Siebert, U., Goodman, S., Jepson, P., Dau Rasmussen, T. and Thompson, P.. 2006. ‘A review of the 1988 and 2002 phocine distemper virus epidemics in European harbour seals’. Diseases of Aquatic Organisms 68:115-130.

Hoekstein, M.S.J., W. Janse, M. Sluijter & K.D. van Straalen, 2023. Watervogels en zeehonden in de Zoute Delta in 2021/2022. Rijkswaterstaat, Centrale informatievoorziening Rapport BM 23.02. Deltamilieu Projecten Rapportnr. 2023-01. Deltamilieu Projecten, Vlissingen.

ICES. 2022. Working Group on Marine Mammal Ecology (WGMME). ICES Scientific Reports. 4:61. 151 pp. http://doi.org/10.17895/ices.pub.20448942

Liebsch, N., Wilson, R. and Adelung, D.. 2006. Utilisation of time and space by harbour seals (Phoca vitulina vitulina) determined by new remote-sensing methods. Pages 179-188 in H. Von Nordheim, D. Boedeker and J. Krause eds. Progress in Marine Conservation in Europe.

Lotze, H. K. 2007. Rise and fall of fishing and marine resource use in the Wadden Sea, southern North Sea. Fisheries Research 87:208-218.

Nachtsheim, D. A., Johnson, M., Schaffeld, T., Van Neer, A., Madsen, P.T., Findlay,C.R., Rojano-Doñate, L., Teilmann, J., Mikkelsen, L., Baltzer, J., Ruser, A., Siebert, U. and Schnitzler, J.G.. 2023. ‘Vessel noise exposures of harbour seals from the Wadden Sea’. Scientific Reports 13:6187.

Olsen, M., Andersen, L., Dietz, R., Teilmann, J., Härkönen, T. and Siegismund, H. 2014. ‘Integrating genetic data and population viability analyses for the identification of harbour seal (Phoca vitulina) populations and management units. Molecular Ecology 23:815-831.

Reijnders, P., Brasseur, S., Tougaard, S., Siebert, U., Borchardt, T. and Stede, M.. 2010. Population development and status of harbour seals (Phoca vitulina) in the Wadden Sea. NAMMCO Scientific Publications 8:95-106.

Reise, K., Baptist, M., Burbridge, P., Dankers, N., Fischer, L., Flemming, B., Oost,A.P., Smit, O., 2010. The Wadden Sea – A Universally Outstanding Tidal Wetland. Wadden Sea Ecosystem No. 29. Common Wadden Sea Secretariat, Wilhelmshaven, Germany, page 7 – 24.

Rojano Doñate, L. 2014. Spatial distribution and foraging behaviour of Harbour Seals (Phoca vitulina) in the Wadden Sea. Master of Science, Universitat Politecnica de Catalunya, Barcelona 85 pp.

Scheidat, M, Vrooman, J., J. Teilmann, J. Baltzer, C. Bie Thøstesen, B. Diederichs, R. Dietz, S.C.V. Geelhoed, A. Gilles, L. L. IJsseldijk, G. O. Keijl, J. Nabe-Nielsen, A. Ruser, J. Schnitzler, S. Sveegaard, U. Siebert (in review 2023). Status of harbour porpoise in the Wadden Sea World Heritage Site and requirements for trilateral monitoring. Marine Biodiversity.

Schop, J., Abel, C., Brasseur, S., Galatius, A., Jess, A., Meise, K., Meyer, J., van Neer, A., Stejskal, O., Siebert, U., 2022. Grey Seal Numbers in the Wadden Sea and on Helgoland in 2021-2022, Common Wadden Sea Secretariat.

Scos. 2021. Scientific advice on matters related to the management of seal populations: 2021. 266 pp.

Tougaard, J., Teilmann, J. and Tougaard, S.. 2008. ‘Harbour seal spatial distribution estimated from Argos satellite telemetry: overcoming positioning errors’. Endangered Species Research 4:113-122.

Unger B., Baltzer J., Brackmann J., Brasseur S., Brügmann M., Diederichs B., Galatius A., Geelhoed S.C.V, Huus Petersen H., IJsseldijk L.L., Jensen T. K., Jess A., Nachtsheim D., Philipp C., Scheidat M., Schop J., Siebert U., Teilmann J., Thøstesen C.B. & van Neer A. 2022. Marine mammals. In: Wadden Sea Quality Status Report. Eds.: Kloepper S. et al., Common Wadden Sea Secretariat, Wilhelmshaven, Germany. Last updated: 06.09.2022. Downloaded 24.05.2023. https://qsr.waddensea-worldheritage.org/reports/marine-mammals

Van Der Veer, H. W., Tulp, I., Witte, J.I.J., Poiesz, S.S.H. and Bolle,L.J.. 2022. ‘Changes in functioning of the largest coastal North Sea flatfish nursery, the Wadden Sea, over the past half century’. Marine Ecology Progress Series 693:183-201.

Vance, H.M., Hooker, S.K., Mikkelsen,L., Van Neer, A., Teilmann, J., Siebert, U. and Johnson, M.. 2021. ‘Drivers and constraints on offshore foraging in harbour seals’. Scientific Reports 11:6514.

Vermeersen, B.L.A., Slangen, A.B.A., Gerkema, T., Baart, F., Cohen,K.M., Dangendorf, S., Duran-Matute, M., Frederikse, T., Grinsted, A., Hijma, M.P., Jevrejeva, S., Kiden,P., Kleinherenbrink, M., Meijles, E.W., Palmer, M.D., Rietbroek, R., Riva, R.E.M., Schulz, E., Slobbe, D.C., Simpson, M.J.R., Sterlini, P., Stocchi, P., Van De Wal, R.S.W. and Van Der Wegen, M.. 2018. ‘Sea-level change in the Dutch Wadden Sea’. Netherlands Journal of Geosciences 97:79-127.

Wilson, R., Liebsch, N., Gomez-Laich, A., Kay, W., Bone, A., Hobson, V. and Siebers, D. 2015. ‘Options for modulating intra-specific competition in colonial pinnipeds: the case of harbour seals (Phoca vitulina) in the Wadden Sea’. PeerJ 3:e957.

Wolff, W. J. 2013. ‘Ecology of the Wadden Sea: Research in the past and challenges for the future’. Journal of Sea Research 82:3-9.


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