Temporally stable, weak genetic structuring in brackish water northern pike (Esox lucius) in the Baltic Sea indicates a contrasting divergence pattern relative to freshwater populations

Lovisa Wennerström,Jens Olsson,Linda Laikre,Nils Ryman

doi:10.1139/cjfas-2016-0039

Abstract

Understanding spatiotemporal population genetic patterns is important for conservation management of ecologically and socioeconomically important species. This is particularly so in species-poor environments such as the brackish Baltic Sea. We examined over 600 northern pike (Esox lucius), a coastal predator and treasured sport fish, collected over major parts of the Baltic Sea coastline. We found low genetic divergence among populations, indicating a contrasting genetic structure of brackish water coastal spawners compared with previous reports on anadromous Baltic pike migrating up freshwater streams for spawning. A pattern of genetic isolation by distance either over shortest waterway or primarily along the mainland coast with islands as stepping stones suggested that gene flow is primarily taking place among neighboring populations, possibly with some migration over open water. Temporal data showed a stable genetic structure over a decade. Within a single sampling year, however, spatial divergence was larger during spawning than feeding season, indicating increased mixing of populations during the feeding season. Management should assure connectivity among brackish spawning grounds and large population sizes at identified core areas.

Highlights

Understanding patterns of spatial and temporal population genetic structure is important in evolutionary and conservation biology (Allendorf et al 2008)
All samples were genotyped for 11 microsatellite markers, 10 developed for northern pike and one for the closely related muskellunge (Esox masquinongy): Elu 87, Elu 51, Elu 76, Elu 19, Elu 276, Elu 64 (Miller and Kapuscinski 1996), EluB e INRA, EluB 38 INRA (Launey et al 2003), Elu 10, Elu 12 (Hansen et al 1999), and EMA D4 (Sloss et al 2008)
Geographic distance was measured in three ways: i) shortest waterway distance among sampling areas was used to reflect potential gene flow in any direction, and ii) shortest coastal distance was used to reflect gene flow only along the mainland coastline

Summary

Introduction

Understanding patterns of spatial and temporal population genetic structure is important in evolutionary and conservation biology (Allendorf et al 2008). Research over the last decade has shown that genetic diversity can affect species productivity, viability, resilience, and adaptive potential (Frankham 2005; Reusch et al 2005; McGinnity et al 2009; Hellmair and Kinziger 2014) Insights from these and earlier studies are reflected in international conservation policies such as the Convention on Biological Diversity (CBD), which calls for conservation and sustainable management of genetic diversity, within and between populations (www.cbd.int). 65 The northern pike (hereafter pike) is a Baltic Sea species to which the CBD Aichi Target 13 66 typically applies, and for which assessment of genetic diversity is warranted (Ljunggren et al 2010; Karlsson et al 2014) It is one of few predatory fish species in Baltic Sea coastal areas and is ecologically important. Declines of pike populations have been related to trophic cascades yielding mesopredator fish release and eutrophication symptoms https://mc06.manuscriptcentral.com/cjfas-pubs

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Conclusion