Eastern North Sea and Southern Slopes of the Norwegian Trench Complex IMMA

Size in Square Kilometres

42,648 km2

Qualifying Species and Criteria

Harbour porpoise – Phocoena phocoena

Criterion B (2), C (2)



The Eastern North Sea and southern slopes of the Norwegian Trench complex IMMA is characterised by sandbanks, reef structures, and varied bathymetry including the slopes of the deep Norwegian Trench. Due to food-rich fronts and upwelling this is a productive area providing prey for harbour porpoises (Phocoena phocoena). Two decades of dedicated visual surveys and satellite tracking have provided clear evidence that the IMMA represents important habitats with high harbour porpoise densities throughout the year. The boundary was drawn to include the harbour porpoise high-density areas based on predictions from different species distribution models and kernel density from satellite tracking conducted over the last decades.

Description of Qualifying Criteria

Criterion B: Distribution and Abundance

Sub-criterion B2: Aggregations

More than 20 years of dedicated visual surveys and habitat-based spatial modelling provided clear evidence that the IMMA includes important habitat for harbour porpoises (Phocoena phocoena) throughout the year (Gilles et al. 2009, 2011, 2016; Waggitt et al. 2020; Nachtsheim et al. 2021; Hansen et al. 2021, 2022; Geelhoed et al. 2022, Lacey et al. 2022).  Harbour porpoises occur in the areas at higher density than in surrounding waters and aggregations have been observed consistently during all study years. A relatively high mean harbour porpoise density of 1.4 ind. /km2 (Gilles et al. 2016) is recorded in the southern IMMA polygon, in Sylt Outer Reef and Horns Rev. Further, high density areas for harbour porpoises were shown in the northern polygon, by means of satellite tracking, with aggregations extending from the northern Kattegat, southern Skagerrak along the southern slope of the Norwegian Trench (Sveegaard et al. 2011, 2018). Satellite tagging of six harbour porpoises in the northern part of the Wadden Sea inside this IMMA showed unexpectedly local and relatively small home ranges as well as very little overlap in distribution with porpoises tagged in Skagerrak. Albeit the small sample size, this may indicate a local population of harbour porpoises residing in this part of the Wadden Sea (van Beest et al. 2018;

Criterion C: Key Life Cycle Activities

Sub-criterion C2: Feeding Areas

The high porpoise densities and aggregations recorded in this IMMA are indicative of an abundance of prey fish species, nourished by food-rich fronts and upwelling areas. The areas host fish species characteristically found on sandbanks (such as many flatfish species), but also sandeels (Ammodytes spp.) and reef dwellers, for example cod (Gadus morhua), and pelagic species such as sprat (Sprattus sprattus) and herring (Clupea harengus). All these species are described as prey species for harbour porpoises (Benke et al. 1998; Gilles 2008; Leopold et al. 2015; Andreasen et al. 2017). Furthermore, porpoises have been connected with the high presence of herring in the Skagerrak (Sveegaard et al. 2012). Harbour porpoises have been shown to feed nearly continuously (Wisniewska et al. 2016, Rojana-Donate et al. 2018). Their aggregations shown by telemetry (Sveegaard et al. 2018) indicate a strong association to feeding grounds.

Supporting Information

Andreasen, H., Ross, S. D., Siebert, U., Andersen, N. G., Ronnenberg, K., Gilles, A. (2017). Diet composition and food consumption rate of harbor porpoises (Phocoena phocoena) in the western Baltic Sea. Mar. Mammal. Sci. 33, 1053–1079. doi: 10.1111/mms.12421

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Geelhoed, S., Authier, M., Pigeault, R., Gilles, A. 2022. Abundance and Distribution of Cetaceans. In: OSPAR, 2023: The 2023 Quality Status Report for the North-east Atlantic. OSPAR Commission, London. Available at: https://oap.ospar.org/en/ospar-assessments/quality-status-reports/qsr-2023/indicator-assessments/abundance-distribution-cetaceans/

Gilles, A. (2008). Chapter V, Feeding ecology of harbour porpoises. In: Characterisation of harbour porpoise (Phocoena phocoena) habitat in German waters. Ph.D. thesis, Christian-Albrechts-Universität zu Kiel, Kiel. 151 pp. Available online at: https://macau.uni-kiel.de/receive/diss_ mods_00003429

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Lacey, C., Gilles, A., Börjesson, P., Herr, H., Macleod, K., Ridoux, V., Santos, M.B., Scheidat, M., Teilmann, J., Vingada, J., Viquerat, S., Øien, N., Hammond, P. (2022). Modelled density surfaces of cetaceans in European Atlan-tic waters in summer 2016 from the SCANS-III aerial and shipboard surveys. Final report. available from https://scans3.wp.st-andrews.ac.uk/files/2022/08/SCANS-III_density_surface_modelling_report_final_20220815.pdf

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Nachtsheim, D., Viquerat, S., Ramirez-Martinez, N.C., Unger, B., Siebert, U., Gilles, A. (2021). Small cetaceans in a human high-use area: Trends in harbour por-poise abundance in the North Sea over two decades. Frontiers in Marine Sci-ence. Frontiers in Marine Science 7:606609. doi: 10.3389/fmars.2020.606609

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Sveegaard, S., Nabe-Nielsen, J., Stæhr, K-J., Jensen, T. F., Mouritsen, K. N., & Teilmann, J. (2012). Spatial interactions between marine predators and their prey: herring abundance as a driver for the distributions of mackerel and harbour porpoise. Marine Ecology – Progress Series, 468, 245-253. https://doi.org/10.3354/meps09959

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Wisniewska, D. M. M., Johnson, M., Teilmann, J., Rojano-Doñate, L., Shearer, J., Sveegaard, S., et al. (2016). Ultra-high foraging rates of harbor porpoises make them vulnerable to anthropogenic disturbance. Curr Biol 26, 1441–1446. doi: 10.1016/j.cub.2016.03.069


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