Harbour porpoise in the Belgian part of the North Sea

After many decades of near absence or low number of reported sightings rates, the frequency of sightings of harbour porpoises (Phocoena phocoena) in the Southern North Sea has increased in the last years. This trend is most likely explained by a southward shift in their distribution area in the North Sea and possibly related to a change in distribution and/or abundance of prey items in the Southern North Sea and Belgian part of the North Sea (BPNS). The harbour porpoise is included on Annex II of the Habitat Directive and is key or indicator species in a number of legal instruments oriented towards an improved environmental status (e.g. Marine Strategy Framework Directive (MSFD) and the Habitat Directive of the European Commission; the OSPAR Ecological Quality Objective (EcoQO), the Agreement on the Conservation of Small Cetaceans in the Baltic, North East Atlantic, Irish and North Seas (ASCOBANS)). This information is therefore highly relevant in the context of conservation, monitoring and evaluation of harbour porpoise populations that frequent the BPNS. It is therefore important for managers, policy- and decision-makers and professionals who work in the marine environment to rely on the best available scientific information about the distribution, biology and ecology of the harbour porpoise in the BPNS and adjacent areas. Although the general information on the harbour porpoise is very exhaustive for its global distribution area, specific information for the BPNS is less abundant and often scattered.

The present document attempts to gather the scientific information on the harbour porpoise (Phocoena phocoena) and its distribution in the Belgian waters and the Southern North Sea and provides a structured overview of research with a main focus on the Belgian part of the North Sea. More detailed scientific (full-text) sources are included as further reading for the interested user.

Introduction

Ecomare (Foto: Credits)

The harbour porpoise (Phocoena phocoena) is one of the smallest species of the cetacean family. This species is part of the group of the toothed whales (Odontoceti), which forms the order of the cetaceans together with the baleen whales (Mysticeti). The harbour porpoise belongs to the family of the porpoises (Phocoenidae), which are distributed worldwide in cold and temperate waters. Harbour porpoises are distributed in the Northern Hemisphere where they feed on sandeels and whiting, which are found on the seabed mostly in areas of strong tidal currents (see below in Distribution patterns; see also distribution in EMODNET-Biology ). The North Atlantic harbour porpoise (P. phocoena phocoena) is one of the three subspecies of the harbour porpoise. The other two subspecies are the North Pacific harbour porpoise (P. phocoena vomerina) and the Black Sea harbour porpoise (P. phocoena relicta). They are mostly spotted alone or in mother-calf pairs. Despite being a top predator itself, the harbour porpoise is reportedly scavenged by seals and other cetacean species and actively predated by the grey seal (Halichoerus grypus) (Haelters et al, 2012 240684; van Bleijswijk et al., 2014 265788; Bouveroux et al., 2014 279005; Jauniaux et al., 2014 271626; Leopold et al., 2015 267365). Until the beginning of the 20th century harbour porpoises were exploited for their oil and flesh in the North Sea (De Baets, 2013 247289).

Morphology and physiology

The harbour porpoise is a small cetacean species, with a body mass between 47 and 65 kg for mature animals (McLellan et al., 2002 260956). It has a low, rounded triangular fin and a non-distinctive beak and forehead. The colour of this species merges from dark to lighter grey. Despite its small size compared to the larger cetacean species, the harbour porpoise body shape is adapted in such a way that it can also resist the cold waters by its robust, chunky form and thick blubber layer. As is the case for all toothed whales (Odontoceti), the harbour porpoise has the ability of echolocation for orientation and foraging. Further reading on general morphology and physiology of the harbour porpoise: Read et al. (1997) 23195 and Huggenberg et al. (2009) 241741.