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) ( ). Until the beginning of the 20th century harbour porpoises were exploited for their oil and flesh in the North Sea (De Baets, 2013 247289).

In the last years the frequency of sightings of harbour porpoises in the Southern North Sea has increased, a trend that is mainly explained by a southward shift in their distribution area in the North Sea. This shift is in line with other findings such as the shift in distribution of prey fish, which are becoming more abundant in the Southern North Sea and Belgian part of the North Sea (BPNS) (Hammond et al., 2013 241480). 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. Hence, it is 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.

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.

1. Introduction
2. Morphology and physiology

• • Blubber
  • Biosonar and acoustics
    • Physiology of acoustics
    • Echolocation behaviour

1. Distribution of the harbour porpoise in the North Sea
2. Research on the harbour porpoise

Blubber

To cope in cold waters, harbour porpoises are adapted with a blubber layer consisting mainly of fatty acids. This layer plays a role in thermoregulation, short-term energy storage, buoyancy and streamlining of the body (Koopman, 1998 260959). Because of their small body size and therefore a large surface-volume ratio, there is much heat loss. Therefore, harbour porpoises allocate a larger amount of their body mass to blubber, compared to other marine mammals (McLellan et al., 2002 260956). The thickness of the blubber layer of harbour porpoises varies amongst age classes, especially in the thoracic-abdominal region of the body. Blubber in the posterior region probably plays a role in the locomotion, as little variation is found between reproductive classes. The thoracic-abdominal blubber layer is thickest in calves (ca 23 mm), because of its important role in insulation and energy storage to enhance the survival chances during the first year (in case of food shortage and lack of good foraging abilities). Insulation by blubber is very important in calves, because there is a larger amount of heat loss due the larger surface-volume ratio in young animals compared to adults. Because of their small size, the surface-volume ratio of calves and mature harbour porpoises is larger compared to other marine mammals. In mature males and non-lactating females the blubber has an intermediate thickness (ca 17 mm). Pregnant and lactating females have the thinnest blubber layer (ca 14 mm). In contrast to other marine mammals, the blubber layer of harbour porpoises is an enantiomeric property, meaning that blubber thickness decreases with increasing body size (Koopman et al., 1998 260959).

Further reading on thermoregulation in small cetaceans: Read et al. (1997) 23195.

Biosonar and acoustics

Toothed wales (Odontoceti) use biosonar for foraging and orientation, in order to determine the position of potential prey or obstacles. This adaptation called echolocation is necessary to live in water where light barely penetrates or where food is distributed at higher depth or in unpredictable patterns (Kastelein et al., 2002 33971). The harbour porpoise has special physiological adaptations of the body to emit acoustic signals and receive the returned echoes. The acoustic signals (clicks) are unique to each species (see frequency) and therefore allow to discriminate the harbour porpoise from other Odontoceti. However, this adaptation does not imply that odontocetes do no use their vision at all, as a study with blindfolded porpoises showed a reduction in swimming speed (Verfuß et al., 2009 260953). Further research is required on the topic of vision.

Physiology of acoustics

Kastelein et al. (1997a 23210) investigated the pathway of sound in the harbour porpoise and, like many previous researchers (Au Withlau,….), found that underwater echolocation signals are especially received via the pan-bone (acoustic window formed by an oval-shaped opening in the interior face of the mandibular bone). When blocking off the external auditory meatus by placing ear cups on the meatal orifices, the hearing sensitivity is not significantly affected. Also, this does not lead to damage to the tympanic membrane, confirming earlier findings that the external auditory meatus of porpoises are partly blocked with mucous substances, lipids or fibrous tissue (Kastelein et al., 1997a 23210).

Harbour porpoises have an alternative way to receive acoustic signals by detecting bone conductor signals at various places on the body. Like in other odontocetes, its underwater hearing covers a wide frequency range and it has an efficient directional hearing capacity. The tympanic bulla (enclosing the middle and inner ear) is connected to the skull by cartilage, connective tissue and fat (instead of a bony connection). This connection improves directional hearing under water. Being able to detect bone conductor signals suggests that sound is well conducted from the auditory meatus, or the surrounding tissues, to the inner ear of harbour porpoises. The detection of bone conductor signals presented at various locations on the harbour porpoise’s body is frequency-dependent (Kastelein et al., 1997 23212).

Cranford et al. (1996 260941) found strong evidence for nasal sound production in echolocation in odontocetes. Although the exact mechanism is unclear, Cranford et al. (1996 260941) identified the monkey lips/ dorsal bursae complex (MLDB) as the location of sound production (see figure 6). A pair of elliptical fat bodies (dorsal bursae) in the monkey lip on each side of the head, are located just above the nasal plugs in the airways. Sound vibrations are generated in the fat tissue of the dorsal bursae by forcing air and fluid over the edges of the nasal plugs by means of a pneumatic mechanism. The vibrations are then transferred to the forehead, probably aided by the form of the skull, connective tissues, and the nasal air sacs Finally, the vibrations are transferred to the surrounding water via the melon - a body of ‘acoustic fat’ in the bulbous forehead of the porpoise – which helps to focus and orient the sound (Huggenberger et al., 2009 241471).

Further reading on anatomy and physiology of the biosonar and acoustics in odontocetes: Read et al. (1997) 23195 and Huggenberg et al., 2009 241471.

Echolocation behaviour

The acoustic signals emitted by harbour porpoises, cover a very broad frequency range, from 40 Hz to 140 kHz (Verboom and Kastelein, 1995 260949). Optimal frequencies are situated into a range between 100 and 140 kHz (Kastelein et al., 2002 33971). The signals consist of 4 components: 1) low-frequency (LF) components of high amplitude (1.4 kHz – 2.5 kHz), 2) mid-frequency (MF) components of low amplitude (between 30 kHz and 60 kHz), which may be ‘lower harmonics’ of high-frequency (HF) components, 3) broadband mid-frequency components (13 – 100 kHz) and 4) HF components (110 kHz – 140 kHz). The high frequency components are especially used for detection of objects. Continuous sine signals (40 – 600 Hz), or whistles, which have variable frequencies, have also been recorded; these are believed to be social signals (Verboom and Kastelein, 1995 260949, 1997 23216).

Harbour porpoises (emit high-frequency, narrowband ‘clicks’ or acoustic signals for echolocation purposes. The clicks have an average duration of 100 microseconds (µs), an inter-click interval of on average 60 milliseconds (ms), a frequency of 130 kHz and a maximum intensity level of 172 dB (re 1 µPa pp @ 1 m). These characteristics of echolocation signals were recorded for harbour porpoises in captivity (Dubrovskij et al., 1971; Møhl and Andersen, 1973; Akamatsu et al., 1994; and Teilmann et al., 2002. Harbour porpoises are expected to have a significantly shorter detection range in echolocation compared to that of larger odontocetes, because the signal intensity is lower (Villadsgaard et al., 2007 241630). In general, the size of the sound production organ, which depends on the size of the animal, is inversely proportional to the transmission beam size and directly proportional to the directivity. This implicates that the small harbour porpoises have a broad transmission beam and thus a lower directivity (Au et al., 1999 260944). However, the small size of the harbour porpoise may be relevant for a high-frequency use of echolocation signals, enhancing directivity (Kastelein et al., 2002 33971). A series of clicks emitted by the harbour porpoises are called ‘click trains’. It is likely that the click repetition rate depends on the animal’s vigilance. Regular click trains are produced when the animal is navigating at ease. The signal repetition frequency increases considerably to above 500 Hz, when the porpoise’s attention is drawn to an object. Both LF- and HF components are recorded in the click trains, mid-frequency range appears in high-amplitude signals (Verboom and Kastelein et al., 1997 2 3216).

Villadsgaard et al. (2007) 241630 wanted to know at what distance the harbour porpoise ‘sees’ objects in the water (vessels, fishing nets, other porpoises,... ). This information is important for the development of acoustic methods to reduce bycatch and for passive acoustic monitoring (PAM) as they depend on the biosonar performance of harbour porpoises. In table 1 the data from the comparison of the echolocation clicks of wild harbour porpoises and captive specimens are shown (modified from Villadsgaard et al. (2007) 241630). In Figure 7 (A), a typical signal envelope of a harbour porpoise is shown (dotted line), with an outline of the extremes in amplitude and (B) the accumulated energy content (%) in the click over time.

Further reading on echolocation behaviour by the harbour porpoise: Villadsgaard et al. (2007) 241630 and Read et al. (1997) 23195.