The common seal and the grey seal in the Waddenzee and Delta area

The seal population has increased greatly in recent years, but the epidemic of virus disease in 2002 resulted in a downturn.

The common seal in decline

Prior to 1959 the common seal declined appreciably because it was hunted. Between 1950 and 1960 the hunters concentrated on young animals, because their pelts are more valuable. The ban on the killing of seals that was implemented in the Delta area in 1961 and in the Wadden area in 1962 was followed by a recovery in the Waddenzee (Wadden Sea), but thereafter in both areas a decline set in and a new deep point was reached. The causes - the low reproduction rate and the high mortality of juveniles - were ascribed to pollution from PCBs, aggravated by increased disturbance from shipping and water tourism. In addition, the construction of the Delta Works led to disturbance and to the dwindling of the habitat.

The common seal recovers

In the Waddenzee the common seal recovered to some extent in the 1970s, partly as a result of immigration from the German and Danish Wadden Sea and an improvement in water quality. But the recovery was nullified by an outbreak of virus disease in 1988 (60% mortality). This was followed by rapid growth of the population in response to high reproduction, low mortality and immigration from the German and Danish Wadden Sea. But another epidemic of the virus disease phocine distemper, which broke out in June 2002, halved the population. The surviving animals are thought to be resistant. In the Delta area it is only recently that there has been some growth. Here, reproduction rates are low. It is unclear to what extent this is the result of water pollution.

Trends in the grey seal

Historical records indicate that the grey seal was once numerous in the Waddenzee but that it disappeared from there in the Middle Ages. It did not return until 1980.

The common seal and grey seal are on the Red List of mammals, in the Habitats Directive (Annexes II and V) and in the Bonn and Berne conventions.

Technical note

Since 1959 there have been annual seal censuses in the Waddenzee and Delta area from the air, carried out at low tide. Given that it is estimated that on average some 70% of the animals are present on the sandbanks at low tide, the actual population will be higher than the figure shown in the graph. In 2001 the total population of the common seal in the Dutch Waddenzee was estimated to be 5300 animals. Calculations for the period prior to 1959, based on the number of seals killed, give an indication of the changes in the population.

The censuses (done from a boat) of the grey seal in 2000 and 2001 show a lower maximum than in 1999, but in fact this is an artefact of the census method. In 2000 and 2001, bad weather forced the census to be postponed until after the period in which the maximum number of animals is usually counted. Also of importance are the changes in the sandbanks on which the animals bask: the Richel is shrinking and the access route to the Jacobsruggen is silting up. In response to these changes the animals are switching to sandbanks outside the census area.

References

Reijnders, P.J.H. (1985). On the extinction of the Southern Dutch harbour seal population. Biological Conservation, 31: 75-84.

Reijnders, P.J.H. (1986). Reproductive failure in common seals feeding on fish from polluted waters. Nature, 324: 456-457.

Reijnders, P.J.H. (1988). Gevolgen virusuitbraak voor zeehonden in het internationale Waddengebied. Waddenbulletin, 23/4: 201-203.

Reijnders, P.J.H., E.H. Ries, S. Tougaard, N. Norgaard, G. Heidemann, J. Schwarz, E. Vareschi en I.M. Traut (1995). Population development of harbour seals Phoca vitulina in the Wadden Sea after the 1988 virusepizootic. J. Sea Research, 38: 161-168.