On the causes of the decrease in the harbour seal (Phoca vitulina) population in the Dutch Wadden Sea : population dynamics and tissue levels of heavy metal and organochlorine compounds

P.J.H. Reijnders

    Research output: Thesisexternal PhD, WU


    The harbour seal population in the Dutch Wadden Sea decreased significantly during the last decades. Calculations based on bounty data revealed that the population decreased from about 2700 in 1950 to about 900 in 1959. Annual aerial surveys - which were carried out since that time - showed a slight increase due to the stop of hunting. However, after 1964 a new decline occurred and since 1974 the population stays at a level of about 450 specimens.<br/>During 1974 to 1978 the population dynamics of the population have been studied. Frequent aerial surveys provided data on the size of the population. These data have been used in a simulation model and the birth rate and the initial juvenile mortality were calculated. During boat trips an indication of the age composition was obtained by measuring track widths. By comparing these results with similar results from a stable population in Schleswig Holstein it appeared 1) that juvenile mortality in the first weeks in the Dutch population is higher than that in Schleswig Holstein but the overall mortality of pups in both areas in their first three months of life is of the same order; 2) that pup production in the Dutch population is low compared to the population in Schleswig Holstein; 3) that apparently immigration from elsewhere occurs.<br/>To investigate the contribution of environmental pollution to the decline of the Dutch seal population tissues of dead, stranded animals originating from Schleswig Holstein, Denmark and The Netherlands were collected and analysed for PCB's, o,p'-DDT, p,p'-(DDT, DDE, TDE), dieldrin, aldrin, endrin, endosulfan, α, β, γ, -HCH, HCB, QCB (pentachlorobenzene), HEPO, total mercury, methylmercury, selenium and bromium.<br/>Tests on possible differences in residue levels between Schleswig Holstein plus Denmark on one hand and The Netherlands on the other revealed that especially PCB levels were significantly higher in Dutch adult seals. Considering epidemiological and experimental data an the effects of PCB's on mammalian reproduction, strong support is obtained for the hypothesis that PCB's are responsible for the decreased reproduction<br/>in seals from the Dutch Wadden Sea.<br/>If PCB's are responsible for the decrease of the Dutch seal population, measures of all North Sea countries are urgently required because these PCB's may not only be a hazard to seals but also to other animals and even man. Production and use of PCB's should be limited and used PCB's should be collected and destroyed.<br/>However, even if an immediate ban on PCB's would be effected, the effects of PCB's - because of their persistent character - are likely to last many years. For that reason it may be assumed that the seal population in the western Wadden Sea still faces a difficult period in the years to come. Presently this population shows an insufficient reproductive rate and is partly dependent on immigration from other populations. In order to lead this population through this difficult period all possible measures enhancing its continued existence should be taken. Since the seals in the Wadden Sea form one population maximum result may be expected from measures applying to all seals in the Wadden Sea. Thus international cooperation is necessary.<br/>The following measures are proposed:<br/>- continued interdiction of hunting in order to obtain a maximum number of animals taking part in reproduction.<br/>- establishment of seal reserves and, additionally supporting seal nursery stations in order to guarantee a maximum reproductive output.<br/>However, it has to be stressed that these measures are not effective if at the same time the main cause of the decrease is not found and under control.<p/>
    Original languageEnglish
    QualificationDoctor of Philosophy
    Awarding Institution
    • Moerzer Bruyns, M.F., Promotor, External person
    Award date5 Mar 1980
    Place of PublicationWageningen
    Publication statusPublished - 1980


    • phocidae
    • population dynamics
    • polychlorinated biphenyls
    • wadden sea


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