The handle http://hdl.handle.net/1887/33217 holds various files of this Leiden University dissertation.
Author: Osinga, Nynke
Title: Comparative biology of common and grey seals along the Dutch coast : stranding, disease, rehabilitation and conservation
Issue Date: 2015-06-09
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Common seals (Phoca vitulina vitulina Linnaeus, 1758) - also known as harbour seals - and grey seals (Halichoerus grypus Fabricius, 1791) are coastal phocids which are sympatric over much of the North Atlantic range. Both species occur in the Wadden Sea in the northern part of the Netherlands and in the delta area in the southwestern part of the Netherlands.
In the 1970s the common seal population reached a historical low point and only a few hundred seals were left in Dutch waters. Grey seals had already disappeared from Dutch waters in preceding centuries. The great conservation concern which arose at that time, together with the high appreciation of the Dutch for their largest wildlife species, have made these seals key species for nature conservation in the Netherlands. In the latest decades, the common seal population has shown a recovery and the grey seals successfully recolonised Dutch waters.
Seals in Dutch waters live in the immediate vicinity of a densely inhabited coastline.
Thus, the seals in this region are exposed to a multitude of human activities. Studying seals not only provides valuable data on how seals are affected by human influences, but also on how human presence impacts vulnerable coastal ecosystems such as the Wadden Sea or the Southwest Delta.
Taxonomy
The common seal and the grey seal (Figure 1A and 1B) belong to the order of Carnivora and the family Phocidae (true seals) (Wilson & Reeder 1993). The common seal belongs to the genus Phoca and has five subspecies: Western Atlantic common seals (Phoca vitulina concolor), Pacific common seals (Phoca vitulina richardii), Eastern Atlantic common seals (Phoca vitulina vitulina), insular seals (Phoca vitulina stejnegeri), and freshwater seals (Phoca vitulina mellonae) (Rice 1998). The grey seal is the single species in the genus Halichoerus and occurs in temperate waters on both sides of the North Atlantic (Rice 1998). The grey seal has two subspecies: Atlantig grey seals (Halichoerus grypus grypus) and Baltic grey seals (Halichoerus grypus macrorhynchus) (Rice 1998).
Biology
Common seal adult males are ± 160 cm, nose to tail length and weigh ± 90 kg, whereas adult females are ± 150 cm and weigh ± 65 kg (Bigg 1981). Grey seal adult males are
± 210 cm nose to tail length and weigh ± 230 kg whereas adult females are ± 180 cm and weigh ± 155 kg (Bigg 1981). Common seals (Figure 1) have no sexual dimorphism.
In grey seals, males reach a substantially longer length, have a darker coat, and their skulls have a more ‘roman’ nose profile (Hewer 1974). In grey seals males are usually darker with light spots whereas females are lighter with dark spots (Lockley 1966). For both species, ages were recorded up to 30-40 years (Härkonen & Heide-Jørgensen 1990;
Hauksson 2007).
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In Dutch waters, pupping of the common seal occurs in May, June and July (Havinga 1933; Van Haaften 1981; Reijnders 1990). Common seal pups generally shed their fetal coat, also called lanugo, before birth and are immediately able to swim with the mother (Venables & Venables 1955). Pups must acquire adequate fat reserves during the four week lactation period (Venables & Venables 1955; Drescher 1979). Mating takes place after the pupping season; in July-September (Venables & Venables 1957; Van Haaften 1983).
The timing of the grey seal pupping season varies along different pupping sites of the United Kingdom (Hewer 1974). In Dutch waters, the majority of pups are born in December and January although occasionally pups are born before or after these months (SRRC unpublished stranding data). Grey seal pups are born with a lanugo coat of creamy white hair and start to moult when they are two to three weeks of age (Hewer 1974).
Grey seal pups are able to swim at once, although according to Hewer (1974) this is only done under compulsion; they usually do not come into contact with water during the first weeks of life. Grey seals pups wean when they are two to three weeks of age, after which a fasting period on land follows (Bennett et al. 2007). The mating period directly follows the nursing period (Van Haaften 1974).
Ecology and habitat use
The Wadden Sea is located in the southeastern part of the North Sea (Figure 2). It is an ecosystem of intertidal sand and mud flats that stretches from Den Helder in the northwest Netherlands to the Danish west coast. Common seals haul out on sandbanks throughout the Wadden Sea. Since grey seal pups do not swim the first weeks of life, they are dependent on high sandbanks that do not submerge at high tide. Such sandbanks are mainly present in the western Wadden Sea (Razende bol, Richel and Engelsche hoek;
indicated with A, B and C in figure 2) and it is here where most grey seals occur (SRRC aerial surveys). Another important region for seals is the Dutch Delta, which comprises several estuaries in the southwestern part of the Netherlands (Belgium border to Hoek
B A
Figure 1. The seals species native to Dutch coastal waters; 1A the common seal and 1B the grey seal (male behind and female in front).
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van Holland). After the devastating flood in 1953, large sea barriers, the Delta Works, were built resulting in the separation of several estuaries from the North Sea. Common and grey seals use the sandbanks in the estuaries, but also the sandbanks in The North Sea west of the Delta coastline (Strucker et al. 2012). There are no sandbanks along the remainder of the western Dutch coast (Hoek van Holland to Den Helder), although seals often forage or swim in these North Sea coastal waters.
The sandbanks are most important during the pupping and moulting season, which is the summer for common seals and the winter for grey seals. In winter, common seals use the North Sea to forage and they haul out less frequently on the sandbanks of the Wadden Sea (Van Haaften 1974). Similarly, grey seals are also seen in much lower numbers in the Wadden Sea outside the breeding season (Härkönen et al. 2007).
Figure 2. A map of the Netherlands showing the coastal areas in which seals occur. The Wadden Sea lies between the mainland coast and a range of Wadden Sea islands, it stretches from Den Helder in the northwest Netherlands to the Danish west coast. The Southwest Delta comprises several estuaries in the southwestern part of the Netherlands, located between the Belgium border and Hoek van Holland.
The Dollard is an important estuary for common seals and is located in the eastern Dutch Wadden Sea.
Sandbanks used by grey seals: A. Razende bol, B. Richel and C. Engelsche hoek. Harbour and industry areas: 1. Rotterdam, 2. Vlissingen, 3. Eemshaven and 4. Delfzijl.
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Both seal species are primarily fish eaters, predating on a wide range of species.
However, they also occasionally feed on invertebrates, molluscs or crustaceans; shrimp is an especially important component of the diet of weaned common seal pups (Havinga 1933; Sergeant 1951). Even though seals of both species haul out in the Wadden Sea and the Southwest Delta, there is only a partial overlap in foraging areas. Grey seals make more extensive movements than common seals (Thompson et al. 1996).
Population dynamics of common seals
Goodman (1998) studied genetic patterns of common seals in Europe (Phoca vitulina vitulina) and identified six distinct population units, the Wadden Sea being one of them. This population consists of seals from the Dutch, German and Danish areas of the Wadden Sea.
The genetic origin of the common seals of the Southwest Delta has not been studied, however frequent migration to the Wadden Sea occurs and it is assumed that they are part of the same population. Several studies have linked the lower genetic variation of European common seals compared to North American subspecies, to a severe bottleneck during the last ice age or even to recolonisation from ice age refugia (Stanley et al. 1996; Kappe et al. 1997).
The population size in previous centuries is unknown; however common seals must have been very abundant considering that they could sustain a high hunting pressure for many centuries. For the Southwest Delta alone, several hundreds of seals were caught annually (data from 1591-1810 and 1822-1856; ‘t Hart 2007). Estimates are available for the number of common seals in Dutch waters in 1900; these are 16,000 for the Dutch part of the Wadden Sea (Dankers et al. 1990) and 11,500 for the Delta area (Reijnders 1994). The common seal population reached a historical low point in the 1970s due to centuries of intensive hunting, followed by exposure to pollution (Koeman et al. 1973;
Van Haaften 1974, 1978; Reijnders 1980). In 1977, only 430 common seals were left in the Dutch Wadden Sea (Van Haaften 1978).
In recent decades, there has been an increase in the Wadden Sea common seal population (TSEG 2013). Also, the number of common seals in the Southwest Delta has increased (CBS et al. 2013). This is despite two outbreaks of the phocine distemper virus in 1988 and 2002, which resulted in the mortality of half of the number of common seals in Dutch waters (Osterhaus & Vedder 1988; Rijks et al. 2005). In 2012, 6,529 common seals were counted in the Dutch part of the Wadden Sea and 500 common seals were counted in the Southwest Delta (CBS et al. 2013).
Population dynamics of grey seals
Grey seals in Dutch waters are part of the Northeast Atlantic grey seal population. This population is centred around the British Isles, but ranges from Iceland eastward along the coast of France and north to Norway and the Kola Peninsula (Bonner 1981; Haug et al.
1994; Boskovic et al. 1996).
No grey seals were present in Dutch waters in the 19th century and it is unknown in which century grey seals disappeared from Dutch waters. Remains of grey seals have been
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found in archaeological studies of dwelling mounds distributed along the coast of the Wadden Sea (Clason 1988) and it is therefore assumed that grey seals were once abundant in Dutch waters, but disappeared in previous centuries due to intensive hunting (Van Bree et al. 1992).
Grey seals occasionally visited the Wadden Sea, but they only began to settle again from the 1980s onward and in 1985 a grey seal pup, which was born in the Dutch Wadden Sea was observed (‘t Hart et al. 1988). They originated from the coasts of England and Scotland, most likely from the Farne islands, where seals were culled and harassed in the 1960s, 1970s and early 1980s (Hickling 1962; Van Haaften 1974; Thompson & Duck 2010).
Since the 1980s, there has been an increase in the number of grey seals in the Dutch and German Wadden Sea (TSEG 2013). In the Southwest Delta, grey seals have been observed during aerial surveys since 2003 (CBS et al. 2013). In 2012, 3,059 grey seals were counted in the Dutch part of the Wadden Sea and 835 grey seals were counted in the Southwest Delta (CBS et al. 2013).
Human influence
Previous investigations have shown that human activities had a severe impact on seals in the 20th century and even before that time. In particular, the effects of hunting and pollution have been extensively studied. From 1591 until 1942 (with short interruptions) incentives to hunt seals were given in the form of bounties (‘t Hart 2007). After the Second World War, seal hunting was resumed with renewed intensity because of the value of seal pups pelts, leading to a drastic reduction of the common seal population until the abolishment of hunting in 1963 (‘t Hart 2007). Intensive hunting is also thougth to be the cause of the disappearance of grey seals in previous centuries (Van Bree et al. 1992).
The role of pollution in the mid-20th century decline of the common seal population was first revealed by Koeman and Van Haaften (Koeman et al. 1973; Van Haaften 1974). The low birth rate of seals was linked to pollution (Van Haaften 1978; Reijnders 1980). Later, the suppression of the immune system of seals was also linked to pollution (Ross 1995; de Swart 1995).
Human activities still have a significant effect on seal populations in Dutch waters.
The Dutch coastline is densely populated and Dutch waters are heavily used for a variety of human activities. There are several large harbour and industry areas in the Netherlands, such as Rotterdam, Vlissingen, Eemshaven and Delfzijl (indicated with 1, 2, 3 and 4 in Figure 2). Also in recent years, several windmill parks have been constructed in Dutch coastal waters. Furthermore, industrial pollutants from the discharge of several large rivers draining the European hinterland flow into Dutch coastal waters. Another anthropogenic impact on seals in the Wadden Sea is the disturbance of seals by boats, either for professional or recreational purposes (Doornbos 1980; Reijnders 1981; Van Wieren 1981). Direct human-induced injury or mortality were described for seals that swallowed fishhooks (Osinga & ‘t Hart 2006) or that were entangled in fishing gear (Van Liere et al. 2012).
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The Seal Rehabilitation and Research Centre
Rehabilitation of wildlife has become an activity that is anchored in the present day society of many countries. The first rehabilitation of seals in the Netherlands took place in 1960. The seals were taken care of by Mr. R. Wentzel, a municipal official and a keen hunter. After his retirement, this initiative was continued with the establishment of the Seal Rehabilitation and Research Centre (SRRC), which was founded in Pieterburen in 1971 by Mrs. L. ‘t Hart. The SRRC’s stranding network (EHBZ: Eerste Hulp Bij Zeehonden or First Aid for Seals) is trained to provide first aid to the seal, but also collects basic data of live and dead-stranded seals and other marine mammals. Post-mortem examinations on seals that stranded dead along the Dutch coast have been done by the SRRC since 1979 (Van der Kamp 1987, 1994). The SRRC stranding network covers the entire Dutch coast, with the exception of the island of Texel and a stretch of the mainland adjacent to Texel, which is mainly covered by the seal rehabilitation centre Ecomare at Texel.
The data collected by the SRRC provides a wealth of information on the health status of seals in Dutch waters such as stranding data, veterinary diagnoses of live-stranded seals, and information from post-mortem examination of dead-stranded seals or those that died in rehabilitation. The SRRC also has an extensive marine mammal blood and tissue bank. The long duration of data and sample collection (over 40 years) is unique for marine mammals and allows retrospective studies covering several decades. The only two marine mammal rehabilitation centres that have been conducting marine mammal research for multiple decades are the SRRC and the Marine Mammal Center in Sausalito (USA) (see for instance Greig 2011). The present dissertation is based on research conducted at the Seal Rehabilitation and Research Centre (Pieterburen, the Netherlands) during the period 2006-2013.
Outline of this thesis
The Netherlands has one of the most densely inhabited coastlines of the world. This makes the Dutch seal populations ideal for the study of various human influences on seals.
In addition, the long data series available at the SRRC on stranding and disease allow retrospective study over multiple decades. Another interesting aspect of studying seals in Dutch waters is the occurrence of the two sympatric seal species. Common and grey seals differ in physiology, behaviour and the use of the Wadden Sea and North Sea ecosystems.
It is, therefore, interesting to compare these two species with regard to population dynamics, disease and the level at which they are impacted by human presence.
In this thesis, I studied the patterns and trends in stranding for both seal species (section A), followed by a study of their current genetic status (section B). Based on the analysis of stranding data, parasitic pneumonia (common seals only) and orphanage (both species) were found to be the main causes of stranding. Therefore, the occurrence of parasitic infections and the breeding biology of seals were studied in section C and section D respectively.
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Section A. Patterns and trends in stranding
Results of clinical and postmortem examination may not only assist in identifying the cause of death in affected individuals, but can point to health problems in free-living populations. Analyses of such data provides a minimum estimate of the level of mortality as well as an inventory of causes of disease and mortality. The SRRC dataset has collected stranding and postmortem data since 1971. However, the dynamics of seals strandings along the Dutch coast had not been studied before and post-mortem data had not been analysed since 1991 (Van der Kamp 1994), except for the phocine distemper outbreak in 2002 (Rijks et al. 2005; Rijks 2008). Two sources of data were available, data of live- stranded seals and data of dead-stranded seals. These were separately analysed as to compare the causes of strandings between these two groups.
The Southwest Delta is the only coastline in the world where seals occur in estuaries that are (semi) enclosed by dams and a stormsurge barrier. To find out how the building of the Deltaworks (1970s/1980s) affected the occurrence of seals, we analysed seal stranding data for both open and enclosed estuaries of the Southwest Delta. The other marine mammal species that occurs in the estuaries is the harbour porpoise (Phocoena phocoena). Data of this species is included, since comparing stranding patterns among marine mammal species can provide insight into how different marine species are able to cope with rigorously altered habitats.
The following research questions were formulated for this section:
1. What are the main factors associated with the stranding of live common and grey seals over the recent decades?
2. What are the main factors associated with the stranding of dead common and grey seals over the recent decades?
3. What are the patterns and trends of seal strandings in the estuaries of the Southwest Delta that are (semi) enclosed by the Deltaworks? Are the stranding patterns of seals different to those of porpoises?
In Section A of this thesis, Chapter 1 includes an analysis of live-stranded seals and Chapter 2 includes and analysis of dead-stranded seals. Stranding patterns of seals and porpoises in the Southwest Delta were studied in Chapter 3.
Section B. Genetic variation
Studies of the level of genetic variation in common seals of the Wadden Sea that were conducted in the early 1990’s concluded that the Wadden Sea common seal population is depauperate of genetic variation (Swart et al. 1996; Kappe et al. 1995; Kappe et al. 1997;
Kappe 1998). However, the Wadden Sea common seal population has shown a recovery in recent decades. The genetic status of the grey seals that recolonized in the 1980s had not been surveyed yet. Because of these recent population recoveries, there was a need to re-assess the current level of genetic variation for both seal species. This is important since
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past population contractions may have led to a decreased level of genetic variation, which ultimately may have affected the species’ vulnerability to disease.
Small populations may have a high or low frequency of particular alleles (Hedrick 2005). This was shown in a study of Antarctic fur seals, where a relatively high number of leucistic animals were found in newly settled minor populations (Bonner 1968). The strong population contractions of common and grey seals in Dutch waters may also have led to an increased occurrence of pigment disorders. The SRRC dataset enabled the study of the occurrence of pigment disorders in a large dataset of stranded seals collected over multiple decades.
The following research questions were formulated for this section:
1. What is the current level of genetic variation of the two seal species?
2. Is there an increased occurrence of pigment disorders in seals of Dutch waters?
In Section B of this thesis, the genetic status of common seals and grey seals were studied in Chapter 4. In Chapter 5, the occurrence of the colour aberrations albinism and melanism in seals was studied.
Section C. Parasitic infections
Parasitic bronchopneumonia is currently the primary cause of disease in common seals of the Dutch Wadden Sea. The causative parasites in this population have been documented as two lungworm species of the superfamily Metastrongyloidae; Otostrongylus circumlitus (Railliet 1899) of the family Crenosomatidae and Parafilaroides gymnurus (Railliet 1899) of the family Filaroididae (Borgsteede et al. 1991; Dailey 2006). Gosselin and Measures (1997) observed that the P. gymnurus from eastern Atlantic common seals in France as described by Railliet (1899) were larger in size than their specimens from western Atlantic common seals in Canada. This observation had also been made by staff at the Seal Rehabilitation and Research Centre (SRRC) in Pieterburen, the Netherlands. The parasite species involved may be a new species of Parafilaroides or a variant of P. gymnurus. The identification of the parasite species and the study of its morphology is important since they may have affected the pathogenity of these parasites.
Seals become infected after weaning when they feed on fish that harbour larvae (Measures 2001). The diet of the host is known to be a principle factor determining helminth infection patterns (Lagrue et al. 2011). In contrast to the common seal, the grey seal rarely suffers from parasitic bronchopneumonia. This difference in levels of parasite infestation between the two seal species may have its source in a different dietary intake of lungworm larvae. Similarly, a change in diet of young common seals may underlie the recent increase of parasitic pneumonia in this species. It is, therefore, essential to find out which fish species carry lungworm larvae and may thus infect seals.
In recent years, very young pups admitted for rehabilitation have been diagnosed with verminous pneumonia. A number of these seals appeared to have stranded before
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weaning; i.e. before they have foraged on fish. Although it is generally believed that the transmission of seal lungworms is horizontal via the food chain (Measures 2001), the possibility of vertical transmission - that is from mother to pup - has not yet been investigated.
The following research questions were formulated for this section:
1. Are the lungworms that occur in common seals of the Wadden Sea a variant of P.
gymnurus or are they a new species of Parafilaroides?
2. Which fish species in the diet of seals carry lungworm larvae and may thus serve as an intermediate host of the seal lungworms?
3. Are there any indications of vertical transmission of lungworm larvae in seals?
In Section C of this thesis, the morphology of the Parafilaroides sp. from Wadden Sea common seals was studied in Chapter 6. The presence of lungwormlarvae in fish was studied in Chapter 7 and the presence of these larvae in maternal tissues was studied in Chapter 8.
Section D. Breeding biology
Next to parasitic pneumonia in common seals, orphanage is a frequent cause of stranding in both seal species. In both the Wadden Sea and the Southwest Delta, seals haul out on sandbanks close to the mainland. The Dollard estuary (eastern Wadden Sea) is one of the core breeding areas for common seals. Seals in this estuary haul out on sand ridges which are accessible for people. It is known from other seal colonies with open access, that most disturbances are caused by pedestrians who unintentionally disturb the seals (Lewis
& Mathews 2000; Kovacs & Innes 1990). A high stranding rate of orphaned pups was found for the Dollard, but it was unknown whether seals were disturbed by agricultural activities, recreational activities or by boats in the water. Observations were needed to find out which disturbance factors affect seals in this estuary.
Analysis of stranding dates of orphaned common seal pups revealed that the birth season had advanced in time. Phenology can provide particularly sensitive indicators of changes in climate (Brakefield 1987; Roy & Sparks 2000; Crick et al, 1997). We studied the timing of the Wadden Sea common seals’ birth season since 1974 and explored possible underlying causes of the shift such as changes in food availability and changes in climate. We also studied whether the advance of birth had affected the birth weight of the pups.
The following research questions were formulated for this section:
1. What is the impact of different disturbance factors on a colony of seals exposed to nearby recreational and agricultural activities?
2. What are the underlying causes of the advance in the birth season of common seals?
Did the advance in birth season affect the weight of the pups?
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In Section D of this thesis, the disturbance effect that human activities have on the seal colony were studied in Chapter 9. The underlying causes of the shift in phenology were studied in Chapter 10.
The chapters of this thesis are followed by a synthesis, in which the key results are given and the findings are discussed in the context of seals biology and the effects of human activities. Also recommendations are given for further research, conservation and management. Finally, the English summary and the Dutch summary are included.
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