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)
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Summary
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
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