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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
Appendix
Recent observations in the Dollard 9
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Recent observations in the Dollard
Common seals in the Dollard were studied in each summer during the pupping and lactation period (2007-2013). Data on seal disturbances were analysed and published for the years 2007-2010 (Chapter 9). Here, we elaborate on two additional facets of this research program; the mother-pup bond and the effect of the placement of an observation screen.
Mother-pup bond
The behaviour of common seal mother and pup during the lactation period seems to have a within-species variability. On the one hand, there are populations showing a very strong and continuous bond between mother and pup (Venables & Venables 1955; Knudston 1975; Renouf et al. 1983; Oftedal et al. 1987). On the other hand, there are also populations for which this bond is temporarily broken while the mother goes on a feeding trip (Wilson 1978; Boness et al. 1992; Boness et al. 1994). A change in behaviour was observed on Sable island, Canada (Bowen 1999). Females used to leave their pups on the beach while they went on foraging trips. But as the population declined, the pups often accompanied their mothers to sea. The authors suggested that single females and those in small groups may be more likely to take their pups with them to reduce the likelihood of their pups wandering off while they are gone.
For the Wadden Sea, a continuous bond between mother and pup was observed in previous studies (Doornbos 1980; Van Wieren 1981). This may be because of a lack of availability of permanently exposed haul-out sites; during high tide in the Wadden Sea there is simply no place to temporarily leave a pup on land. For the colony of seals in the Dollard, observations were made of the mother-pup bond. In the Dollard a continuous bond was observed in all years. However, since 2009 in a few occasions lone pups were observed to be reunited with their suspected mother. This behaviour was only observed for mother and pup pairs that hauled out on the artificial sand ridges and was not observed for any of the other sandbanks in the Dollard estuary. Most likely this is because the parts of the sand ridges adjacent to the dyke are permanently exposed and pups can continue to haul out here with high tide. This behaviour was only observed during the late lactation season (after June 15th) when pups were close to weaning. Therefore with regard to rescue of lone pups that are found after June 15th on the sand ridges of the Dollard, it is advised to first observe the pups to check if there is a mother present.
A large proportion of orphaned common seal pups strand on the Wadden island beaches or along the coast of the mainland. These coastlines are far from the sandbanks that are used by mothers and pups during the lactation season. Lone pups found in these areas therefore have drifted away from the breeding grounds and their mothers. An observation period is not needed here; this would only prolong the suffering of the animal and increase the risk that the pup is harassed in the water by tourists.
Grey seal mothers are known to temporarily leave their pups to make foraging trips (Hickling 1962). Grey seal pups should thus be observed before admission to rehabilitation.
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Appendix 9
Observation screen
Until 2010, the Dollard dyke area had open access to people which caused a high level of disturbance (Chapter 9). Therefore, preventive measures were taken in 2011. Prior to the 2011 pupping season, an observation screen was placed consisting of 3 overlapping panels (Figure 1A). People could walk in between as well as in front of the panels, however their silhouettes were masked by the screen. Prior to the season of 2012 the screen was replaced by a single panel. Another adaptation was made prior to the season of 2013; in that year access to the dyke area next to the screen was blocked by electric fences (Figure 1B).
Figure 1. The observation screen. A. Seaside view of the screen in 2011 when the screen consisted of three separate panels which allowed people to access the sea side of the screen. B. Landside view of the screen in 2013 when the screen consisted of a single panel and the access to both sides of the screen was blocked.
B A
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In contrast to the expectation, the placement of the observation screen in 2011 led to an increase of the level of disturbance (Figure 2). Apparently, people standing in front of the screen were perceived as a threat by the seals, despite the masking of their silhouettes.
Changing the screen to a single panel did reduce the disturbances, however on many occasions people were observed to stand beside the screen. In the year thereafter (2013), the access to the remainder of the dyke area was closed. As a consequence, the number of actual disturbances was strongly reduced, below the level of the situation before the placement of the screen. This low level was achieved despite the fact that in that year many people visited the area (see high level of potential disturbances, Figure 2).
To conclude, the observation screen in the Dollard turned out to be a successful measure in preventing disturbance of seals by pedestrians on the dyke area. The way the screen is constructed however appeared to be crucial; the screen only reduced disturbance when access to the front and sides of the screen was completely blocked.
0 100 200 300 400 500
2007 2008 2009 2010 2011 2012 2013
Potential disturbances Actual disturbances
Figure 2. Actual and potential disturbances from people on the dyke. Actual disturbances means that people were present and disturbed the seals. Potential disturbances mean that people were present but did not disturb the seals.
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References
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(1992). Influence of storms and maternal size on mother-pup separations and fostering in the harbor seal, Phoca vitulina. Canadian journal of zoology 70, 1640-4.
Boness D.J., Bowen W.D. & Oftedal O.T.
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Doornbos G. (1980) Gedrag van zeehonden (Phoca vitulina L.) in het stroomgebied van de oude Lauwers (oostelijke Waddenzee) in 1978. pp. 24. Rijksinstituut voor Natuurbeheer, Texel.
Hickling G. (1962) Grey seals & the Farne islands, pp. 180. Routledge and Kegan Paul, London.
Knudston P.M. (1975) Observations on the breeding behavior of the harbor seal, in Humboldt Bay California. California fish and game 63(1), 66-70.
Oftedal O.T., Boness D.J. & Tedman R.A.
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Renouf D., Lawson J. & Gaborko L. (1983) Attachment between Harbour seal (Phoca vitulina) mothers and pups. Journal of Zoology 199, 179-87.
Venables U.M. & Venables L.S.V. (1955) Observations on a breeding colony of the seal Phoca vitulina in Shetland. Proceedings of the Zoological Society of London 125, 521-32.
Wilson S.C. (1978) Social organization and behaviour of harbour seals, Phoca vitulina concoler, pp. 103. Office of zoological research, national zoological park, Smithsonian Institution, Washington D.C.
Wieren S.E. van (1981) Broedbiologie van de gewone zeehond, Phoca vitulina, in het Nederlandse Waddengebied, pp. 63.
Rijksinstituut voor Natuurbeheer, Texel.
