The effect of inbreeding in small populations of Harbour seals (Phoca vitulina)

Abstract

The Harbour seal (Phoca vitulina) is a common pinniped species on the west coast of Sweden. However, only one population exists in the Baltic proper. The Baltic population in Kalmarsund has been isolated for several thousand years and a recent study shows that they have lower levels of heterozygosity and genetic diversity compared to any other European population. Historically the population has undergone severe population bottlenecks which in turn have led to higher degrees of inbreeding. Inbreeding is known to influence fitness negatively however, it has been ignored in current ecological risk assessments for harbour seals. In order to fill this knowledge gap, the current study present a literature overview and maps different fitness traits reported to be affected by inbreeding depression in pinnipeds. We run multiple population viability analyses (PVAs) to estimate the effects from inbreeding depression on small, isolated harbour seal populations. The inbreeding depression was applied in the units of lethal equivalents commonly found in mammal populations and the amount of inbreeding (F) was based on recently available values from the Kalmarsund population. Several scenarios were investigated by simulations of an age structured, stochastic population model including the combined effects from epizootic events and reduced fecundity due to endocrine-disrupting pollutants. The simulations also elaborated effects from various initial population sizes and carrying capacities to gain better understanding of how population sizes as such mitigate the effect of inbreeding. The results showed that inbreeding depression at the currently detected level can be expected to have an overall negative effect on the population viability of the Kalmarsund harbour seal population and other isolated seal populations. High levels of inbreeding depression also increased the probability of extinction (34% risk of extinction in 300 years) compared to low levels (5% risk of extinction in 300 years). Populations were severely affected by inbreeding depression in scenarios which simultaneously accounted for reduced fecundity and epizootics, highlighting the need for animal conservation to account for multiple stressors. Understanding how inbreeding affects isolated marine mammal populations can be important for the design of effective conservation programs and to ensure stable and healthy populations.