In Wadden Sea Seals Protected
After Viral Outbreak Harbor Seals Face Pollution & Human Disturbance
Wadden Sea seals not endangered, but get special protection following 1988 die-off. Seals used as monitor of environmental health where contaminant levels are high.
The Wadden Sea Seal Agreement came after the Phocine Distemper Virus (PDV) outbreak in 1988 caused a significant decline in harbor seal (Phoca vitulina) stocks within the Wadden Sea, Kattegat/Skagerrak and North Sea. Grey seals (Halichoerus grypus) were not as seriously affected by the virus but they are included in the Seal Management Plan as the problems of pollution and human disturbance have significant impact on them as well.
The Convention on Migratory Species (CMS) oversees the Wadden Sea Seal Agreement, which is signed by the Netherlands, Germany and Denmark. The three countries have also developed a Seal Management Plan (2002-2006) as a follow up to implement some of the objectives of the Agreement.
Phocine Distemper Virus Outbreaks
In 1988, an virus swept through the seal colonies off of Northern Europe, resulting in the deaths of approximately 18,000 harbor seals, also known as common seals and 500 grey seals. PDV is closely related to the canine distemper virus but is a distinct virus. The disease, while not killing as many grey seals was responsible for higher than normal abortion rates in that species.
In 2002 a second PDV outbreak occurred, but some of the seals that survived the 1988 die-off still had immunity to the virus, so the result was not as devastating. Still 22,000 common seals were lost, 60% of the populations most severely affected. And the heaviest losses were in the areas with the highest levels of pollution.
Pollution and Stress Affect Seal Immunity
In 2007, there was another outbreak of disease in the area. This time PDV was not found and the researchers are attempting to figure out what this new virus is. No matter the name of the virus, the chronic disease outbreaks in the area are testimony to the role of pollution in wildlife health.
It is now accepted that high contaminant levels increase susceptibility to disease. In fact, a study by de Swart et al showed that feeding on contaminated fish lowered T-cell immune response. T-cells are a critical component of response to viral infection.
Stress can be a factor in susceptibility to disease as well. For the seals of the Wadden Sea, human encroachment and industrial development contribute to both stress and contaminant levels. Tourism, dredging and dike building are all issues in coastal habitat destruction in all three countries.
Studying Seals to Monitor the Environment
It is important to note that neither harbor nor grey seals are endangered. Grey seals occur in large numbers on both sides of the Atlantic Ocean. The harbor seal is ubiquitous throughout the world. But awareness of the vulnerability of local populations of common species and response to threats to those species may be important in protecting ecosystems. By overseeing the health of one species, multiple other species and their habitat are protected.
So the seals of the Wadden Sea are the canaries in the Wadden Sea coal mine. Monitoring them provides the three countries with a window on the health of their local marine environment. Perhaps it will become a model for other ecosystem management plans.
Source:
De Swart, R., Ross, PS., Timmerman, HH., Vos, HW., Reijnders, PJH., Vos, JG., and Osterhaus, ADME. 1995. Impaired cellular immune response in harbor seals (Phoca vitulina) feeding on environmentally contaminated herring. Clinical and Experimental Immunology 101:480-486
