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 .

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.

Distribution of the harbour porpoise in the North Sea

North Sea

In general, harbour porpoises are found in deeper waters close to the coast (Edrén et al., 2010 241468, Gilles et al., 2011 260955, Gilles et al., 2016 260957). Gilles et al.( 2016 260957) developed habitat-based density models on harbour porpoises in Belgium, the Netherlands, Germany, UK and Denmark. Therefore, observations of harbour porpoises by aerial surveys were used and multiple predictors (depth, distance to sandeel grounds, ditance to the coastline, sea surface temperature, proxies for fronts and daylength) were considered. These models produced seasonal maps of the predicted densities of harbour porpoises in the North Sea. Figure 4 shows the predicted harbour porpoise distribution during spring.

The SCANS II project in 2005 (small cetaceans abundance in the North Sea and adjacent waters) was the second large-scale survey in the North Sea and adjacent waters to assess the population of harbour porpoises, after the first SCANS project in 1994. This large-scale survey was conducted with the aim to determine the extent of measures that need to be taken to reduce levels of bycatch and to recover populations or maintain a favourable conservation status. The North Sea is an important source of animal protein both for marine animals and for the human population (sea fishing). Unfortunately, bycatch of small cetaceans during fishing activities is a major threat to their conservation. In European Atlantic waters and in the North Sea, the harbour porpoise is one of the species with highest bycatch levels in bottom set gillnet fisheries (SCANS II, 2008 210295). Aerial and shipboard surveys were carried out to estimate the abundance for each survey block, see table 2 and 3 for the results.

Densities estimated from the SCANS II survey were similar for most survey blocks and ranged between 0.274 and 0.394 ind/ km². Lowest abundance were found in offshore regions near west of Scotland and Ireland (block Q) and near coasts of SW France, Spain and Portugal (block W). Highest estimated abundances were in the south/central North Sea (block U) and off the west coast of Denmark (block L). Although the overall abundance was consistent for both SCANS surveys, the model-based density surfaces showed a notable difference in harbour porpoise distribution between 1994 and 2005, possibly caused by bycatch on harbour porpoise in some parts of the study area (Hammond et al., 2013 241480). The difference in distribution between 1994 and 2005, may be simply due to interannual variation. However, the recent increases in sightings of harbour porpoises off the Dutch coast and harbour porpoise strandings and bycatches in the Southern North Sea strongly suggest that the difference reflects a trend. Perhaps the most likely reason for the changes in harbour porpoise distribution is a change in the distribution and/or availability of key prey species. Harbour porpoises range widely and, although their diet is varied, they feed primarily on species that are widely distributed, such as sandeel, whiting and herring. Research has shown that porpoises left areas rich in sandeels (Ammodytes sp.) and moved to an area with abundant leaner gobies Pomatoschistus sp. and whiting Merlangius merlangus (Hesse et al., 2014 234824). Furthermore, several studies on habitat preferences of harbour porpoises in the North Sea, often links them to areas with strong currents and steep slopes that are associated with small-scale upwelling zones and as a consequence with areas of prey aggregation (Gilles et al, 2011 260955). Heath, 2005 241467 and Christensen and Richardson, 2008 241478 found that during the last decades the structure of the food web in the North Sea has changed markedly because of large scale fisheries removals and the influence of decadal scale oceanic changes. Appropriate mitigation steps are required to sustain the apparent recovery of porpoises in the Southern North Sea (Marine Conservation Research International, 2012 241473). Extensive research on the harbour porpoise is done by Germany in the Baltic Sea as it is the only cetacean species resident there. See list below:

• MINOS: http://books.google.be/books?id=wM4_asdEi4QC&pg=PA243&lpg=PA243&dq=MINOS+future+programme+%28ZIP%29&source=bl&ots=_xxRVSajDR&sig=cdu21njDURQu0_KFt5bXkXpPxUE&hl=nl&sa=X&ei=FmsIVIjmCOu_ygPr3oDABg&ved=0CCQQ6AEwAA#v=onepage&q=MINOS%20future%20programme%20%28ZIP%29&f=false

• BioConsult SH: http://bioconsult-sh.de/en/projekte

• SAMBAH: http://www.sambah.org/

• ASCOBANS: http://www.ascobans.org/en

According to the EC Habitat Directive (92/43/EEC 1992), EU Member states –specifically those with significant populations in their marine waters – have the obligation to protect the harbour porpoise (listed on Annex II) by designating MPAs, referred to as Special Areas of Conservation (SAC). Following article 17 of the Habitat Directive, each member state has to report about the conservation status of these areas every six years. A next assessment is to be completed by 2018. The harbour porpoise is also protected under the Agreement on the conservation of Small Cetaceans in the Baltic, North East Atlantic, Irish and North Seas (ASCOBANS). To this purpose, there is a need for accurate information about the abundance, spatial and temporal (seasonal) distribution, ecology, key habitat, and behaviour of the species.

Abundance and distribution of the harbour porpoise in the Belgian part of the North Sea

The harbour porpoise is a common species in the Belgian part of the North Sea (BPNS) and is reported since at least the 14th century (Verhoeven, 1781 226436; Egmond et al., 2003 58682; De Baets, 2013 247289). Harbour porpoises, called ‘meerzwijnen’ in Dutch during the Middle Ages, were exploited (De Baets, 2013 247289). By the mid of the 20th century, this species was less abundant in Belgian waters, but recently (since the end of the 20th century) it occurs in higher abundance in the BPNS (Haelters, 2007 118040). Because of this increase in abundance, it is important to know where the key habitat(s) of the harbour porpoise are situated, as well as seasonal shifts in its distribution patterns. Haelters et al. (2013) 231904 described a general spatial and temporal pattern with a relative high abundance in the BPNS from late winter to early spring with a density up to 2.7 ind/ km². During the other seasons, a lower number of porpoises was found in the BPNS (0.05 ind/ km²). According to Haelters et al. (2010 199753), P. phocoena tends to inhabit more offshore and northerly waters in the BPNS. The few measurements which were conducted by porpoise detectors (PoDs, see below in Research on the harbour porpoise) (Haelters et al., 2011 210068), also showed that throughout the year higher densities of harbour porpoises occur in the offshore areas within the BPNS. In contrast, in late winter and early spring, they are found regularly close to shore (Haelters et al., 2011 210068).

Degraer et al (2013) 231864 found that the harbour porpoise ‘population’ in the BPNS shifts from the north towards the southwest and west in late winter – early spring. Later in spring the highest densities occur in the western part. A higher density is observed in the western part near shore, compared to the eastern part. Off the central part of the Belgian coast, and up to 30 km offshore, densities are lower.

Prey

According to Haelters et al. (2011 210068) the observed patterns described above, can be explained by migration, random movement, dispersal or avoidance of areas with temporarily poorer feeding conditions or availability of prey. Due to the limited area of the Belgian waters (3454 km²) compared to the larger distribution range of the harbour porpoise (750.000 km²), the pattern of its distribution in the BPNS is variable. Also Degraer et al (2013) 231864 explained the species’ distribution throughout the year by the seasonal movement linked to the availability of food. The primary energy storage of porpoises is found in the inner blubber layer of the thorax, which has a higher temperature, compared to the outer layer. Therefore, lipids in the inner layer are more easily mobilised (Koopman et al., 2002 241476). Because of the small size of the harbour porpoise, it cannot store as much energy as other cetaceans, so it has to eat regularly to keep warm and to stay fit. The average food consumption per day is between 2.5 and 5 kg (Lockyer, 2007 117563). Therefore, the harbour porpoise always stays close to its food source or follows mobile prey. So food availability is a strong drive for the movements of the porpoises. For further reading on distribution patterns of the harbour porpoise in the wider North Sea, click here (Hammond et al., 1995; Hammond et al., 2013; Gilles et al., 2016 260957).

For further reading on feeding biology: Christensen and Richardson, 2008 241478

Noise

Just like humans, animals can suffer hearing damage or mental disturbances due to elevated noise levels, even complete deafness can occur. Underwater sound travels at a speed of 1500 m/s, and can travel up to considerable distances. The Marine Strategy Framework Directive (MSFD) of the European Commission (EC) describes goals of good environmental status related to underwater noise. The level of impulsive sounds induced by humans should be lower than 185 dB re 1 μPa at a distance of 750 m from the source and there should be no positive trend in the noise level between the 1/3 octave bands of 63 Hz and 125 Hz (Directive 2008/56/EC, article 10).

A good environmental status relate to underwater noise is reached (Directive 2008/56/EC, article 9):

• When low frequency impulsive sounds do not have an adverse impact on marine organisms.
• When loud low and mid frequency impulsive sounds and continuous low frequency sounds from human activities do not have an adverse impact on marine ecosystems.

Shipping is an important source of noise affecting marine mammals, especially in areas with dense shipping traffic such as the North Sea (Dyndo et al., 2015272773). Another important source of acoustic impact is the construction of windfarms, in particular during the construction phase when mono- or pin piles are driven in the seafloor. For pile driving, sound pressure levels (peak to peak, SPL ) of up to 200 dB re 1 pPa at a distance of 750 m from the noise source have been measured or estimated (Madsen et al., 2006 241636; Norro et al., 2010 199744; 2012218684). The species suffers a significant noise disturbance up to a distance of 8 km for pin piling and 16 km for mono-piling (Haelters et al., 2012 218683, Norro et al., 2012 218684). Research on the estimated average densities of harbour porpoises in the Belgian waters during March and April of 2011 showed abundance density of respectively 2.5 and 1.3 animals/ km² before and during piling (Haelters et al., 2012 218683). Average density halved during piling probably due to a combination of disturbance by pile driving over a large part of this area and by the onset of a general seasonal movement out of Belgian waters. Both aerial surveys and the application of the model to a reference situation during pile driving indicated a disturbance up to at least 20 km from the piling location in Belgian waters (Haelters et al., 2015 244118). Porpoises were encountered again at the construction sites only 5.9 h to 7.5 h after completion of the pile driving phases (Brandt et al., 2012 241635).

Wind farms

During bird monitoring surveys in the operational offshore wind farm parks, a total of 35 harbour porpoises were observed between Sept 2010 - Dec 2012 on the Bligh Bank, a relatively high sighting rate compared to observations in the control area and before the construction of the wind farm. However, this difference was not significant (Vanermen et al., 2013 230583). The apparent attraction of the porpoises to operational wind farms is explained by the increased abundance of fish who in their turn are attracted by the new habitats formed by the hard substrate of the foundations of the wind turbines (reef effect). Another plausible explanation for the increased abundance of porpoises in the vicinity of the wind farms is thetotal prohibition of fishing and shipping (shelter effect). As present wind farm areas cover a relatively small area compared to the foraging area for the porpoises, differences in prey density within and outside the wind parks possibly do not as yet affect the local distribution patterns of porpoises on a small temporal and spatial scale. However, once all planned wind farms will be operational, these differences may become significant (Degraer et al., 2013 231864). Such an attraction effect was already confirmed In Dutch offshore wind farms (Scheidat et al., 2011 241094). Long-term monitoring is important to follow the effect on harbour porpoise distribution and feeding ecology (Degraer et al., 2013 231864). Underwater noise data is required to investigate the hearing sensitivities of harbour porpoises and to compare with data collected through passive acoustic monitoring (PAM) (Degraer et al., 2013 231864).

According to the research of Reubens et al. (2013 232101), cod (Gadus morhua) is attracted towards the offshore windfarms by the reef effect. Contrastingly, Debusschere et al. (2016 251068) found that sea bass (Dicentrarchus labrax) is experiencing acoustic stress because of the pile driving. Ook het onderzoek checken op basis van de referenties van het artikel van haelters et al 2012 en 2015, Vanermen 2013 en degraer 2013…

Strandings

Throughout the wide distribution area of the harbour porpoise, accidental catches in fisheries and strandings occur (Haelters et al., 2012 220389). Data on the annual number of stranded animals collected from Belgian beaches between 1970 to the first decade of the 21st century, showed a tenfold increase. This increase started specifically during the last years of the 20th century, and suggest a recent increase in harbour porpoise numbers in Belgian and adjacent waters (Haelters et al., 2011 210068; Haelters et al., 2012 220389). This apparent increase in abundance is however related to a shift in the distribution of the population, rather than an increase in the population size (Camphuysen, 2004 241631; SCANS II, 2008 210295, see also above). Further, a clear seasonal pattern was shown by Haelters et al. (2012 220389). Two peaks of strandings were observed in late winter – early spring and in summer, probably because of the higher abundance of harbour porpoises in the BPNS during that period. In June and in autumn and early winter less animals were found on the beach. Strandings of harbour porpoises are mainly caused by bycatch in fisheries. Additional causes are collisions with ships and natural mortality by starvation, disease or predation (by seals). Also, some juvenile animals and pregnant females strand because of mortality at birth or during giving birth (Haelters et al., 2012 220389). Strandings on Belgian beaches are recorded in the Marine Mammals Strandings Database of RBINS