Population structure, movements and site fidelity of grey seals in the Baltic Sea

Abstract: The management and conservation of wildlife must rely on a solid understanding of key ecological and demographic factors. To devise management and conservation strategies for species affected by human activities it is essential to define populations.The Baltic grey seal(Halichoerus grypus ) population is far from the 100,000 animals it was estimated to be one hundred years ago. The population declined during the 20th century, due to hunting and reduced reproductive capacity caused by environmental contaminants mainly PCBs and DDT. A ban on hunting and on the use of PCBs and DDT, stopped the decline and promoted the current growth of the population. The population has increased from less than 4,000 in the late 1970s to approximately 12,000 in the year 2000 and is currently growing at a rate of 6.8% per year. Although the population is increasing, the Baltic grey seals can notbe considered as healthy as the Atlantic grey seals. The growing number of seals has led to a dramatic increase in the interactions between seals and fisheries. The conflict has become a problem of such magnitude that the Swedish Environment Protection Agency recommended a cull of grey seals starting in 2001, disregarding the still valid Helcom recommendation 9:1 that bans hunting of seals in the Baltic. The total hunting quota 2002 was close to 500 seals for the whole Baltic region. The model used when calculating the quota is based on the assumption that Baltic grey seals comprise one homogeneous population, with a high degree of migration and exchange between haul-outs and breeding sites in the different Baltic subbasins. There is very little data to support the assumption of a homogenous population since few studies have been carried out with the purpose of exploring population structure, movements and site fidelity of Baltic grey seals. The current hunting regime, combined with other human induced mortality factors such as bycatch and illegal hunting, might pose a threat to local grey seal stocks.This thesis is the first attempt to compile data on population structure, movements and site fidelity of Baltic grey seals. The results shows that the Baltic grey seal population most likely is differentiated into several stocks, but that the differentiation occurs on an ecological rather than an evolutionary time scale. Baltic grey seals might also be moresedentary and show a higher degree of site fidelity than we previously have had reason to suspect. Grey seals are capable of moving hundreds of kilometres in a short period of time, but most animals studied preferred to stay in the vicinity of one or a few haul-out areas.Movements between the different Baltic sub-basins seem to occur only rarely. These studies suggest that the current practise of treating the whole Baltic as a single management unit is ill founded and mistaken, and may lead to an increased risk of depleting local grey seal stocks.Unfortunately, the data set does not allow us to make any suggestion about the appropriate number of management units for the Baltic Sea. But it highlights the need for using small rather than large management units for the Baltic grey seal population to ensure a viable grey seal population all over its range.