Nine years after the epidemic mass die-off in 1988, the Wadden Sea common seal population is still increasing rapidly. A maximum total of 12,927 seals has been recorded in the four subregions (NL, Nds, SH, DK), exceeding the count of 1996 by as much as 17.3%. This is, however, partly attributable to the extraordinarily high number of pups in 1997 (2,783), making up 21.5% of the total (1996: 19.4%). When birth rates differ significantly, net growth may be better indicated by the increase of non-pup numbers only, which is 14.2% in this case. The present population size is almost 1.5 times that of 1987 and more than 3.5 times that of the mid seventies, when seal stocks in the Wadden Sea were at their minimum in this century. So far, there is no evidence of resource depletion or any other density-related factor that could retard population growth.

Rather than the fact of ongoing increase itself, the lack of consistency in growth rates in the post-epizootic period is remarkable. Annual increase fell from about 18 % in 1991-92 to 10% in 1994-95, but accelerated in 1996 (13 %) and 97. Simultaneously, the proportion of pups in the maximum count dropped from 20.6 % (1990-91) to 18.6 % (1995), but recently, increased again.

As the population size at present is considered well below its maximum possible value, internal factors must account for these parameter fluctuations. It is supposed that structural features of the population attained in the epidemic, particularly disproportional reduction of the adult male and some juvenile age groups, gave way to subsequent dynamics of the sex and age distribution and related changes in the per capita birth and mortality rates. However, stabilization of population parameters is to be expected after some time. The high pup production of 1997 is likely, again, to result in a considerable increase, which is to be verified in 5 coordinated surveys in 1998.

The data presented in this note have been provided by the following institutions: IBN-DLO Texel, NL; Bezirksregierung Weser-Ems, Nds; Landesamt für den Nationalpark Schleswig-Holsteinisches Wattenmeer, SH; Fiskeri- og Søfartsmuseet, Esbjerg, DK.

Authors Address:

Kai. F. Abt
FRZ Westküste, Hafentörn, D - 25761 Büsum
E-mail: kabt@ftz-west.uni-kiel.de