Population structure of Harbour seal (Phoca vitulina) determined by microsatellite markers

Introduction

The population of Harbour seals (Phoca vitulina) inhabiting Danish and neighbouring waters has had a very turbulent history. Thus shifting climate regimes, intensive hunting and at least two outbreaks of Phocine Distemper Virus (PDV) in 1988 and 2002 respectively have evidently resulted in dramatic recurrent population declines.
It is widely recognised that such fluctuations can reduce genetic variation, facilitate inbreeding depression, and increase the population’s susceptibility to anthropogenic and/or natural disturbances. However, apart from the observed declines in population size, very little is known as too the likely effects of these fluctuations on the population/subpopulations of Harbour seal inhabiting Danish waters.
Current management plans are based on population surveys and a relatively limited amount of tagging and telemetry studies, which support a division of the population into seven more or less geographically-defined management regions (Figure 1). The genetic diversity and population structure has however never been investigated and it is yet uncertain if the geographic subdivision also reflects a biological. That is, if the geographically-defined subpopulations also constitutes relatively isolated genetic units with limited breeding in-between.

Objectives

In assuming that the population is not truly panmictic but divided into a number of more or less well-defined subunits the objectives of this study is to:

• Determine the levels of within and between subpopulation genetic diversity.
• Estimate the level of gene flow between subpopulations.
• Estimate present and past population sizes.
• Establish if the observed population declines have resulted in detectable bottlenecks.
• Evaluate the validity of the current division of the population into seven management regions.
• Provide conclusions relevant for the future management and conservation of Harbour seals - in Denmark as well as internationally.

Materials and Methods

Tissue from approximately 270 individuals of Harbour seal originating from 12 more or less broadly defined localities in Denmark and Sweden was sampled following the 2002 PDV epidemic (Figure 1). DNA was isolated and genetic variation at 18 polymorphic microsatellite loci was analysed.

Perspectives

At present the data has not been analysed, but it is expected that the study will:

• Provide information about the genetic differentiation between and within subpopulations.
• Contribute to more precise estimates of distribution ranges and levels of migration between localities or subpopulations in both Denmark and Sweden.
• Shed some light on the extent of past population fluctuations and the genetic implications of these.

And thereby:

• Contribute to better assessments of the vulnerability on population and subpopulation level to diseases (e.g. PDV), construction work at sea (e.g. windmill parks), recreational activities, hunting, competition from small- and large-scale fishing, traffic, local or regional pollution (e.g. oil spills), and other disturbances.
• Provide some information about the degree of bias - due to movements between localities - to be expected in population surveys.
• Be useful as a tool for the future assignment of individuals of unknown origin to a genetically and geographically defined subpopulation.
• Provide a supplement to existing and future studies on population demographics, foraging behaviour and food preferences, and measures of the geographic variation in contaminants – just to mention a few.

Supervisors

Liselotte Wesley Andersen – Department of Wildlife Ecology and Biodiversity – National Environmental Research Institute (NERI)
Hans Siegismund – Department of Evolutionary Biology – University of Copenhagen
Rune Dietz & Jonas Teilmann – Department of Arctic Environment – National Environmental Research Institute (NERI).

Thanks to

The National Environmental Research Institute (NERI) and the Swedish Museum for Natural History for providing me with Harbour seal tissue samples from respectively Danish and Swedish localities.