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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
Chapter
Stranding of harbour porpoises, common seals and grey seals in the Southwest Delta of the Netherlands, 1993-2008
Wouter Wietses1,2, Jaap van der Hiele1, Marten Geerse1, Theo Meijer2, Helias A. Udo de Haes3, Paul M. Brakefield3,4, Nynke Osinga1,3
1. Seal Rehabilitation and Research Centre (SRRC), Pieterburen, The Netherlands, 2. Van Hall Larenstein, University of Applied Sciences, Leeuwarden, The Netherlands, 3. Institute of Environmental Sciences (CML) and Institute of Biology Leiden (IBL), Leiden University, The Netherlands, 4. Department of Zoology, University of Cambridge, Cambridge, United Kingdom
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Abstract
The Dutch Delta comprises several estuaries in the southwestern part of the Netherlands.
It is characteristic for the Dutch Delta that marine mammals occur in several estuaries in the immediate vicinity of a densely inhabited coastline. Since 1993, data on marine mammal strandings were systematically collected by a team of the stranding network of the Seal Rehabilitation and Research Centre. Stranding records (n=1,078; 1993-2008) were studied for the three marine mammal species indigenous to Dutch waters: the harbour porpoise (Phocoena phocoena), the common seal (Phoca vitulina), and the grey seal (Halichoerus grypus). For all three species, the number of strandings has shown a distinct increase over time up to 2006 and remained stable since then. In recent years, annual stranding rates were approximately 100 harbour porpoises, 35 common seals and 20 grey seals. Only a small number of harbour porpoises stranded alive. Live strandings comprised a third of common seal and half of grey seal strandings. All three species had a peak in strandings of juveniles during and after the birth season. Harbour porpoises and seals regularly stranded in the Oosterschelde, an estuary enclosed by a storm surge barrier. Seals also stranded in the estuaries that can only be reached through sluices.
Fortunately, the population numbers for these species in Dutch waters have recovered from lows in the 1970s, although they are thought to remain below natural historical levels. Careful monitoring of stranded marine mammals, provides crucial information on the state of the marine environment and is key to establishing adequate management actions for the conservation of marine mammals.
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Introduction
The Dutch Delta comprises several estuaries in the southwestern part of the Netherlands.
After a devastating flood in 1953, large sea barriers, the Delta Works, were built resulting in the separation of several estuaries from the North Sea. The estuaries of the Southwest Delta are an important habitat for three marine mammal species, namely the harbour porpoise (Phocoena phocoena), the common seal (Phoca vitulina) and the grey seal (Halichoerus grypus). It is characteristic for the Dutch Delta that marine mammals occur in several estuaries in the immediate vicinity of a densely inhabited coastline. This implies that humans may have an impact on the populations in various ways. In the 20th century, harbour porpoise and common seal populations had severely declined in Dutch waters (Smeenk 1987; Van Haaften 1963), while grey seals had completely disappeared (Van Bree et al. 1992). In recent decades, there has been a recovery in abundance for all three species in the Southwest Delta (Meininger et al. 2003; Strucker et al. 2007). We studied the stranding dynamics of harbour porpoises and the two seal species during the period of their recovery in this delta area.
Records on harbour porpoises stranded on the Dutch coast have been collected since the beginning of the 20th century and were published by Van Deinse (e.g. Van Deinse 1946);
Husson and van Bree (e.g. Husson & van Bree 1972); Van Bree and Smeenk (Van Bree
& Smeenk 1978); Smeenk (e.g. Smeenk 1986); and recently by Camphuysen et al. (2008).
Since 1971, strandings of marine mammals along the Dutch coast have been recorded by the Seal Rehabilitation and Research Centre (SRRC) in Pieterburen, the Netherlands. The SRRC has a stranding network that consists of trained volunteers covering the Dutch coast to provide first aid to live-stranded marine mammals. Live-stranded seals are admitted for rehabilitation to the SRRC and live-stranded porpoises are admitted for rehabilitation to SOS Dolfijn in Harderwijk. The SRRC stranding network also collects the carcasses of dead-stranded marine mammals for research purposes.
The study of marine mammal strandings reveals essential information on annual stranding numbers, seasonal patterns and geographical distribution. In this paper, we provide an overview of harbour porpoise, common seal and grey seal strandings in the Dutch Delta over the past 16 years (1993-2008).
Materials and methods
The Dutch Delta area stretches from the Nieuwe Waterweg estuary in the north, to the Belgian border in the south (Figure 1). The length of the Southwest Delta coastline is 488 km, which was measured by tracing the North Sea coastline and the estuary coastlines with Google Earth. The northern estuaries of Haringvliet and Grevelingenmeer, have been enclosed by dams since 1970 and 1971 respectively. The Oosterschelde has been separated from the North Sea by a 9 km storm surge barrier since 1986. This is an open barrier with sluices that can be closed during heavy storms and high water levels. Only the
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most southern estuary, Westerschelde, is still directly connected to the North Sea. Seals haul out on sandbanks in these estuaries and on sandbanks located in the North Sea west of the Delta coastline (the Voordelta).
The SRRC stranding network has grown since its foundation in 1971. Since 1993, data on marine mammal strandings in the southwest Delta have been collected systematically by a single team (coordinated by the second and third author). We therefore analysed stranding records of the period 1993-2008 for harbour porpoises (n=599), common seals (n=348), and grey seals (n=131). As the southwest Delta is a densely inhabited coastline with a well organised stranding network, it can be assumed that the majority of the stranded animals was reported. Stranded animals on the beach are reported to the SRRC stranding network; they are not actively searched for. All strandings in this study concerned strandings of single individuals, no mass strandings were recorded. Fresh carcasses were transported for necropsies, while those in states of late decomposition were transported for disposal after recording the basic parameters (species, length and sex).
When severe decomposition made species identification impossible, the animals were listed either as ‘unspecified seal’ or ‘unspecified cetacean’.
The following parameters were studied for harbour porpoises and seals: (1) the frequency of stranding over the period 1993-2008; (2) the seasonal patterns; and (3) the geographical distribution of strandings. The phocine distemper virus (PDV) epidemic in
Figure 1. The estuaries and Delta Works of the Dutch Delta. The Nieuwe Waterweg (1) has an open
connection to the North Sea. The Haringvliet (2) and the Grevelingenmeer (3) are enclosed water bodies. The Oosterschelde (4) is separated from the North Sea by a storm surge barrier (5). The Westerschelde (6) has a direct connection to the North Sea. The Voordelta (7) is an area with several sandbanks.
The SRRC stranding network has grown since its foundation in 1971. Since 1993, data on marine mammal strandings in the southwest Delta have been collected systematically by a single team (coordinated by the second and third author). We therefore analysed stranding records of the period 1993-2008 for harbour porpoises (n=599), common seals (n=348), and grey seals (n=131). As the southwest Delta is a densely inhabited coastline with a well organised stranding network, it can be assumed that the majority of the stranded animals was reported. Stranded animals on the beach are reported to the SRRC stranding network; they are not actively searched for. All strandings in this study concerned strandings of single individuals, no mass strandings were recorded. Fresh carcasses were transported for necropsies, while those in states of late decomposition were transported for disposal after recording the basic parameters (species, length and sex). When severe decomposition made species identification impossible, the animals were listed either as ‘unspecified seal’ or
‘unspecified cetacean’.
The following parameters were studied for harbour porpoises and seals: (1) the frequency of stranding over the period 1993-2008; (2) the seasonal patterns; and (3) the geographical distribution of strandings. The phocine distemper virus (PDV) epidemic in 2002 caused mass mortality of common seals (Jensen et al. 2002; Rijks et al. 2005). Data of the year 2002 were included in the study of trends over the years. These data were, however, excluded for the study of seasonality in common seals, because the epidemic caused a high peak in strandings in the late-summer of 2002. Stranding data were presented separately for three age categories: juveniles, subadults and adults (Table 1). Length could not always be determined for animals that stranded in severe state of decomposition; the age category of these was classified as ‘unknown’. The frequency of live strandings was determined for each species. Also, the ratio of male and female strandings were studied for each species. For the analyses of stranding locations, we used the geographical information system software Quantum GIS (Development Team 2009).
Figure 1. The estuaries and Delta Works of the Dutch Delta. The Nieuwe Waterweg (1) has an open connection to the North Sea. The Haringvliet (2) and the Grevelingenmeer (3) are enclosed water bodies. The Oosterschelde (4) is separated from the North Sea by a storm surge barrier (5). The West- erschelde (6) has a direct connection to the North Sea. The Voordelta (7) is an area with several sandbanks.
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2002 caused mass mortality of common seals (Jensen et al. 2002; Rijks et al. 2005). Data of the year 2002 were included in the study of trends over the years. These data were, however, excluded for the study of seasonality in common seals, because the epidemic caused a high peak in strandings in the late-summer of 2002. Stranding data were presented separately for three age categories: juveniles, subadults and adults (Table 1). Length could not always be determined for animals that stranded in severe state of decomposition; the age category of these was classified as ‘unknown’. The frequency of live strandings was determined for each species. Also, the ratio of male and female strandings were studied for each species.
For the analyses of stranding locations, we used the geographical information system software Quantum GIS (Development Team 2009).
Results
A total of 1,078 strandings were recorded in the period 1993-2008. These were: harbour porpoises (n=599), common seals (n=348), and grey seals (n=131). Furthermore, there were 71 records of dead-stranded seals of which the species could not be identified, as well as three records of unidentifiable dead cetaceans. Incidentally strandings were recorded for marine mammals of other species that had washed up on the Dutch Delta coast. These pertained to three arctic seal species (harp seal (Pagophilus groenlandicus; n=2), ringed seal (Pusa hispida; n=1), and hooded seal (Cystophora cristata; n=3)) and to nine cetacean species (white-beaked dolphin (Lagenorhynchus albirostris; n=15), striped dolphin (Stenella coeruleoalba; n=2), bottlenose dolphin (Tursiops truncates; n=1), short-beaked common dolphin (Delphinus delphis; n=2), minke whale (Balaenoptera acutorostrata;
Table 1. Age categories.
Species Sex Age category Age range (yr) Length range (cm)
Common seals male juvenile age ≤ 1 length ≤ 107
subadult 1 < age ≤ 4.7 107 < length ≤ 142
adult age > 4.7 length > 142
female juvenile age ≤ 1 length ≤ 103
subadult 1 < age ≤ 3.7 103 < length ≤ 129
adult age > 3.7 length > 129
Grey seals male juvenile age ≤ 1 length ≤ 134
subadult 1 < age ≤ 4.9 134 < length ≤ 174
adult age > 4.9 length > 174
female juvenile age ≤ 1 length ≤ 126
subadult 1 < age ≤ 4.0 126 < length ≤ 158 adult age > 4.0 length > 158 Seals were divided into three age categories, based on standard length as described by (McLaren 1993). For age at sexual maturity we used Härkönen & Heide-Jørgensen (1990) for common seals, and Hauksson (2007) for grey seals.
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n=2), fin whale (Balaenoptera physalus; n=2), humpback whale (Megaptera novaeangliae;
n=1), North Atlantic bottlenose whale (Hyperoodon ampullatus; n=1) and sperm whale (Physeter macrocephalus; n=1).
Numbers over the years
The harbour porpoise was the species that stranded most frequently; averaging 100 individuals per year in the last three years (2006-2008; Figure 2A). The number of strandings increased rapidly over the period covered in this study (1993-2008). Harbour porpoise strandings peaked in 2006, and were slightly lower again in 2007 and 2008. Strandings of common seals showed an increase in numbers over the period 1993-2003 (Figure 2B). Since 2005, strandings have remained stable at around 35 seals annually. There was a peak in dead-stranded common seals during the PDV epidemic in the year 2002. Annual stranding numbers were higher in the years after the 2002 epidemic compared to the years before the epidemic. The number of grey seal strandings has increased since 2002 and has been at a consistently high level since 2005 (Figure 2C). In recent years, around 20 grey seals have stranded annually. For all three species, the increases over this period are not associated with an increase in particular age classes (Figure 2), live or dead strandings, or gender (data not shown).
Only a minority (28/599, 5%) of harbour porpoise stranded alive, while a third of all common seals strandings (103/348, 30%) and half of the grey seals strandings (63/131, 48%) were live strandings. More than half of the porpoise strandings were juveniles (Figure 2A), while for both common seal and grey seal strandings approximately half were juveniles (Figure 3A, Figure 4A). For harbour porpoises, significantly more males stranded (246/403, 61%) than females (χ2=19.7, P<0.001). This was the same for grey seals, with more males (74/121, 61%) stranded than females (χ2=6.0, P<0.05). For common seals, the sex ratio appeared to be even.
Seasonality
Strandings of harbour porpoises were more frequent in the period March to September, with a peak in August (Figure 3A). Common seal strandings in non-epidemic years (n=281) peaked from July to August (Figure 3B). Grey seals strandings peaked from December to January (Figure 3C). For all three species the seasonal peaks in strandings were caused by an increased stranding rate of juveniles (Figure 3). No apparent differences were found in the seasonal patterns of male and female strandings for all three species (data not shown).
Geographical distribution
Strandings of harbour porpoises occurred along the whole western coastline of the Dutch Delta, but appeared to be most numerous in the southern part of the Dutch Delta (Figure 4). Strandings were regularly recorded in the Oosterschelde. Common seal and grey seal strandings occurred along the whole western coastline (Figure 4). Strandings of both seal species were also recorded in the Oosterschelde as well as in the Haringvliet and the Grevelingenmeer.
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A B C re 2. Strandings per age class for harbour porpoises (2A), common seals (2B) and grey seals (2C) over the period 1993-2008. Figures 2B and 2C present data of seal counts in Southwest Delta by Rijkswaterstaat (RWS, the executive arm of the Dutch Ministry of Infrastructure and the Environment) and the Province of Zeeland (CBS et al., 2010). A B C re 3. Strandings per month for harbour porpoises (3A), common seals (3B) and grey seals (3C) over the period 1993-2008. In Figure 3B data of the epidemic year 2002 was excluded.
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Figure 2. Strandings per age class for harbour porpoises (2A), common seals (2B) and grey seals (2C) over the period 1993-2008. Figures 2B and 2C present data of seal counts in the Southwest Delta by Rijkswaterstaat (RWS, the executive arm of the Dutch Ministry of Infrastructure and the Environment) and the Province of Zeeland (CBS et al., 2010).
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A B C Figure 2. Strandings per age class for harbour porpoises (2A), common seals (2B) and grey seals (2C) over the period 1993-2008. Figures 2B and 2C present data of seal counts in the Southwest Delta by Rijkswaterstaat (RWS, the executive arm of the Dutch Ministry of Infrastructure and the Environment) and the Province of Zeeland (CBS et al., 2010). A B C Figure 3. Strandings per month for harbour porpoises (3A), common seals (3B) and grey seals (3C) over the period 1993-2008. In Figure 3B data of the epidemic year 2002 was excluded.
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Figure 4. Stranding locations of harbour porpoises (4A), common seals (4B) and grey seals (4C) for the period 1993-2008. Incidentally, stranded seals were recovered from the sandbanks west of the coast.
A B C Figure 4. Stranding locations of harbour porpoises (4A), common seals (4B) and grey seals (4C) for the period 1993-2008. Incidentally, stranded seals were recovered from the sandbanks west of the coast.ABC
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Discussion
Strandings of harbour porpoises, common seals and grey seals in the Southwest Delta showed a distinct increase in numbers over time up to 2006, and have since remained stable. The harbour porpoise has shown the highest increase in strandings over the period 1993-2008 and currently has the highest annual stranding rate.
The 20th century
The number of harbour porpoise in Dutch waters severely declined in the 20th century (Smeenk 1987). The decline was thought to be caused by a cumulative effect of the rise in PCB levels, the decline in herring stocks, and an increase in by-catch in fisheries.
Similarly, common seals were much more numerous in Dutch waters in the past than they are nowadays. Hundreds of bounties were paid annually for seals caught in the province of Zeeland from 1591 until 1942 (with brief interruptions) (‘t Hart 2007). The highest number of bounties paid was 618 in the year 1620 (‘t Hart 2007). Reijnders (1994) estimated that in 1900, the size of the common seal population in the Delta area was around 11,500 animals.
Until the closure of the hunt in 1963, years of intensive hunting had caused the common seal population in the Netherlands to dwindle to 250 common seals for the Delta area (Van Haaften 1963). By the late 1960s and early 1970s, the population of common seals, which had started to increase, had declined again. This time it was linked to pollution (Koeman et al. 1973; Van Haaften 1974; Van Haaften 1978; Reijnders 1980). In the case of grey seals, remains were found at archaeological sites studying the dwellings of the coast near the Wadden Sea (Clason 1988). It can be thus assumed that grey seals were once abundant in Dutch waters but disappeared, probably due to intensive hunting (Van Bree et al. 1992). It is most likely that the grey seals became extinct in Dutch waters during the 19th century (‘t Hart et al. 1995).
The period 1993-2008
Today, harbour porpoises are seen more often again in Dutch waters (SCANS-II, 2008).
The same trend is observed in the stranding records of the Southwest Delta. Two large- scale cetacean surveys were conducted in the North Sea (the SCANS projects), the first in 1994 and the second in 2005 (Hammond 2002; SCANS-II 2008). Overall abundance in the North Sea did not change substantially between the two SCANS surveys. However, a shift in distribution from the northern North Sea to the southern North Sea (including Dutch waters) was observed. The observed increase in strandings therefore corresponds with a local increase in numbers of harbour porpoises rather than intrinsic population growth.
The observed increase in the numbers of common seal strandings over the previous 16 years coincides with the observed increase in the number of common seals seen during aerial surveys in the Dutch Delta (Strucker et al. 2007; CBS et al. 2010). Interestingly, the number of common seals that stranded in the years after the 2002 epidemic was not lower than in the years before the epidemic, even though 54% of the common seals in Dutch
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waters died (Rijks et al. 2005). Rijks et al. (2005) noted that during the PDV epidemic, seals in the Delta were affected later than those in the Wadden Sea, probably because virus spread was slower in the smaller and more dispersed population. Probably, this resulted in lower mortality of common seals in the Delta area. Since 2006, the number of common seal strandings in the Dutch Delta has not shown an increase, possibly reflecting a stabilisation of the number of common seals in the Delta area. The annual stranding rate of approximately 35 common seals constitutes a substantial part of the number of common seals observed in this area during aerial surveys (154 for 2007; data RWS/
Provincie Zeeland, CBS et al. 2010).
The increase in grey seal strandings is related to their recolonisation of Dutch waters.
Grey seals began to settle again in the Dutch Wadden Sea from the 1980s onwards (Reijnders et al. 1995), with the first grey seal pup being observed in 1985 (‘t Hart et al.
1988). In the Southwest Delta, grey seals were observed during aerial surveys since 2003 (CBS et al. 2010) and grey seal pups have been observed since 2004 (Berrevoets et al. 2005).
The grey seals that stranded before 2003 may have come from adjacent waters. Tagging studies have shown the migration of grey seals from the Wadden Sea as well as from the United Kingdom to the Dutch Delta (Hickling 1962; Hewer 1974; SRRC unpublished data).
Phocine distemper virus infection is more pathogenic for common seals than for grey seals (Rijks et al. 2005). Only five grey seals were among the 2,284 seals stranded dead on the Dutch coast during the 2002 epidemic, and phocine distemper was not identified as a cause of death in the three grey seals for which post-mortem examination was carried out (Rijks et al. 2005). Numbers of grey seals are now growing in the Wadden Sea (Härkönen et al. 2007) and in the Dutch Delta (Strucker et al. 2007; CBS et al. 2010). Although the counts of grey seals (206 animals in 2007; data RWS/Provincie Zeeland, CBS et al. 2010) are higher than those for common seals, the annual number of grey seal strandings is lower (around 20 strandings annually).
Seasonality
For harbour porpoises and common seals, the peak in strandings is during summer. For grey seals the peak in strandings is during winter. For all three species this peak consists of an increase in strandings of juveniles during and after the birth season. Harbour porpoises give birth in summer (Addink et al. 1995), common seals in May-July (Van Haaften 1981), and grey seals in December-January (Bonner 1981; Vedder et al. 1992).
There is a difference in the seasonal pattern of abundance of harbour porpoises at sea and the seasonal pattern of strandings. The peak in strandings is during summer, while line-transect surveys in the southern North Sea show a peak in abundance during spring (Zanderink & Osinga, 2008)
Geographical distribution
The harbour porpoises, as well as the seals in the Dutch Delta are part of larger populations.
It is thought that the harbour porpoises in Dutch waters are part of a population in the
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southern North Sea (Evans et al. 2009). Harbour porpoise strandings are most frequent in the southern part of the Delta area. Since the main direction of the North Sea water flow along the Dutch coast is from southwest to northeast, it is likely that stranded porpoises in this area, especially those in an advanced state of decomposition, may have died in Belgian or even French waters. Common seal and grey seal strandings are spread along the whole coastline of the Delta area. Common seals and grey seals in Dutch waters migrate to adjacent seal colonies and cannot be regarded as isolated populations (Hewer 1974;
Van Haaften 1974; Van Haaften 1981). The growth or decline of adjacent seal colonies will therefore affect the number of seals in the Dutch Delta. Over the last decades the population of common seals has shown an increase in the Wadden Sea (TSEG 2008).
However, there is a general decline in most of the common seal colonies around Great- Britain (Lonergan et al. 2007). While this is not the case in the Wadden Sea or the Dutch Delta, careful consideration of the situation in areas of decline is warranted.
The storm surge barrier of the Oosterschelde apparently does not constitute an insurmountable barrier for the three species. However no information is available on the frequency of migration from the Oosterschelde to the North Sea and vice versa.
In September 2009, a survey was carried out in the Oosterschelde to estimate the total abundance of harbour porpoises. In total 37 individuals were counted of which five were calves (Zanderink & Osinga, 2010). Although dams (with sluices) separate the Haringvliet and the Grevelingenmeer from the North Sea, seals still appear in these waters. They probably reach these waters through sluices; the Haringvliet can be reached via the large sluices in the dam and the Grevelingenmeer via a smaller sluice in the Brouwersdam.
Conservation
Fortunately, the numbers of harbour porpoises, common seals and grey seals have shown a recovery during the period studied (1993-2008). Marine mammals are important indicators of the state of the marine environment, because they are vulnerable to changes in this environment. In the study area, marine mammals have had to endure many changes over the last 40 years. In the 1970s and 1980s, the building of the Delta works was a major construction feat and at present, new land is being reclaimed from the sea to build a harbour in the northwest corner of the study area. Similarly, the Dutch Delta is in close proximity to the harbours of Rotterdam and Antwerp with their busy shipping routes and extensive industrial areas. The Dutch Delta also hosts many recreational activities and supports various types of fisheries. This means that marine mammals have had, and still have, to cope with human activities such as habitat loss, pollution, disturbance, changes in stocks of available prey, and interactions with fisheries, all of which affect the populations.
Although the populations of these three species have shown recovery, they are still not as abundant as in previous centuries. It is essential to continue monitoring strandings in the Dutch Delta as it is the basis for the study of disease and mortality of these animals.
Furthermore, it is key to establishing adequate management actions for the conservation of marine mammals.
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Acknowledgements
We are grateful to all volunteers of the SRRC stranding network. Their dedication, time and enthusiasm are of vital importance for marine mammal research. We are grateful to Professor E. Gruys, Dr. P. ‘t Hart and two anonymous reviewers for their helpful comments on the manuscript.
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