The Baltic grey seal: A 9000-year history of presence and absence

Abstract

The grey seal ( Halichoerus grypus) has been part of the Baltic Sea fauna for more than 9000 years and has ever since been subjected to extensive human hunting, particularly during the early phases of its presence in the Baltic Sea, but also in the early 20th century. In order to study their temporal genetic structure and to investigate whether there has been a genetically continuous grey seal population in the Baltic, we generated mitochondrial control region data from skeletal remains from ancient grey seals from the archaeological sites Stora Förvar (Sweden) and Neustadt (Germany) and compared these with modern grey seal data. We found that the majority of the Mesolithic grey seals represent haplotypes that is not found in contemporary grey seals, indicating that the Baltic Sea population went extinct, likely due to human overexploitation and environmental change. We hypothesize that grey seals recolonised the Baltic Sea from the North Sea. during the Bronze Age or Iron Age, and that the contemporary Baltic grey seal population is direct descendants of this recolonisation. Our study highlights the power of biomolecular archaeology to understand the factors that shape contemporary marine diversity.

Loss of species and genetic diversity during colonization: Insights from acanthocephalan parasites in northern European seals

“…Notably, when placed in a broader geographical context, C. strumosum mtDNA haplotypes representing the Lake Ladoga + Lake Saimaa clade were more closely related to Pacific and Arctic haplotypes than to the main group of Baltic haplotypes (Appendix S3 : Figure S3.2 ). This intriguing finding suggests that the two lake‐endemic C. strumosum populations retain ancestral genetic variation that in the Baltic Sea has been erased by younger haplotypes that might have arrived when gray seals recolonized the Baltic Sea during the Bronze or Iron Ages after being hunted to local extinction by humans during the Mesolithic (Ahlgren et al, 2022 ). While similar replacement of the Baltic ringed seal population is not apparent in archeological data (Ukkonen et al, 2014 ), recent genetic studies have increasingly converged toward the conclusion that the genetic composition of the extant Baltic ringed seal population differs from the one that was present during colonization of Lake Saimaa (Heino et al, 2023 ; Löytynoja et al, 2023 ).…”

“…Notably, when placed in a broader geographical context, C. strumosum mtDNA haplotypes representing the Lake Ladoga + Lake Saimaa clade were more closely related to Pacific and Arctic haplotypes than to the main group of Baltic haplotypes (Appendix S3: FigureS3.2). This intriguing finding suggests that the two lake-endemic C. strumosum populations retain ancestral genetic variation that in the Baltic Sea has been erased by younger haplotypes that might have arrived when gray seals recolonized the Baltic Sea during the Bronze or Iron Ages after being hunted to local extinction by humans during the Mesolithic(Ahlgren et al, 2022).…”

120‐years of ecological monitoring data shows that the risk of overhunting is increased by environmental degradation for an isolated marine mammal population: The Baltic grey seal

“…Alongside hunting, shifts in Baltic grey seal population size have largely been driven by pollution and climate (Ahlgren et al, 2022; Bergman, 1999; Harding et al, 2007; Jüssi et al, 2008). Baltic grey seal fertility fell to reported lows of 9% in the 1970s as the result of persistent organic pollutants such as polychlorinated biphenyls (PCBs), which cause sterility in grey seals (Bergman, 1999).…”

“…This has led to the Baltic's proposed use as a “Darwinian laboratory” to understand the drivers of ecological and evolutionary changes (Reusch et al, 2018). Baltic grey seals have been genetically isolated for at least 4200 years or possibly since the formation of the Baltic 9000 years ago (Ahlgren et al, 2022; Härkönen et al, 2005). Baltic grey seal health and abundance are considered indicators of ecosystem health (Bäcklin et al, 2011; Galatius et al, 2014; Rowe et al, 2020).…”