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

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.Résumé : La compréhension des motifs spatiotemporels de la génétique des populations est importante pour la gestion de la conservation d'espèces d'importance écologique et socioéconomique, en particulier dans les milieux pauvres en espèces comme les eaux saumâtres de la mer Baltique. Nous avons examiné plus de 600 grands brochets (Esox lucius), un prédateur côtier prisé des pêcheurs sportifs, prélevés sur de grandes portions des côtes de la mer Baltique. Nous avons relevé une faible divergence génétique parmi les populations qui indique que la structure génétique des individus frayant en eaux côtières saumâtres diffère de ce qui avait déjà été signalé pour des brochets anadromes de la mer Baltique remontant les cours d'eau douce pour frayer. Un motif d'isolement génétique par la distance soit sur la voie d'eau la plus courte ou principalement le long des côtes continentales en utilisant des îles comme étapes donne à penser que le flux génétique s'exerce principalement entre populations voisines, possiblement avec migration en mer libre. Des données temporelles montrent une structure génétique stable à l'échelle d'une décennie. Sur une année d'échantillonnage, cependant, la divergence spatiale était plus grande durant la période du frai que durant la période d'alimentation, ce qui indique un plus grand mélange de populations durant la période d'alimentation. Les efforts de gestion devraient viser à assurer la connectivité des lieux de frai saumâtres et des populations de grandes tailles dans les zones centrales connues. [Traduit par la Rédaction]

Baltic Sea genetic biodiversity: Current knowledge relating to conservation management

“…Genetic monitoring over years within single decades shows a temporally stable structure and amount of genetic variation in pike, plaice, cod, and three‐spined stickleback (DeFaveri & Merilä, ; Pampoulie, Stefánsson, Jörundsdóttir, Danilowicz, & Daníelsdottír, ; Was, Gosling, & Hoarau, ; Wennerström et al, ), but with some temporal differences observed in turbot (Nielsen, Nielsen, Meldrup, & Hansen, ). Within‐season genetic differences at herring spawning sites are interpreted as genetically different populations using the same spawning grounds (spawning waves; Jørgensen, Hansen, Bekkevold, Ruzzante, & Loeschcke, ; Jørgensen, Hansen, & Loeschcke, ).…”

“…Delimitation of population genetic structure in order to identify more or less isolated populations is of importance to define management units. Such information exists for fish species including Atlantic salmon (Salmo salar; Koljonen, Jansson, Paaver, Vasin, & Koskiniemi, 1999;Säisä et al, 2005), northern pike (Esox lucius; Laikre, Miller, et al, 2005;Wennerström et al, 2017), whitefish (Coregonus lavaretus; Olsson, Florin, Mo, Aho, & Ryman, 2012) and for habitat forming species such as bladderwrack (Fucus vesiculosus; Johannesson, Johansson et al, 2011). Also, identification of local adaptations is of particular importance in variable environments such as that of the Baltic Sea.…”

“…Of the more than 200 published scientific studies identified in the literature search, this type of data was available only for four species; northern pike ( Esox lucius ), three‐spined stickleback ( Gasterosteus aculeatus ; DeFaveri, Jonsson, & Merilä, ), bladderwrack ( Fucus vesiculosus ) and turbot ( Scophthalmus maximus ; Vandamme et al, ). The pike data were generated by ourselves (Wennerström, Olsson, Ryman, & Laikre, ) and the bladderwrack data were kindly provided by Professor Kerstin Johannesson and colleagues, Gothenburg University. Data for three‐spined stickleback and turbot were available in the Dryad data repository (http://www.datadryad.org).…”

“…Vandamme et al, 2014). The pike data were generated by ourselves (Wennerström, Olsson, Ryman, & Laikre, 2017) and the bladderwrack data were kindly provided by Professor Kerstin Johannesson and colleagues, Gothenburg University. Data for three-spined stickleback and turbot were available in the Dryad data repository (www.…”

Comparing the Performance of Microsatellites and RADseq in Population Genetic Studies: Analysis of Data for Pike (Esox lucius) and a Synthesis of Previous Studies

“…Unfortunately, declines in pike populations in the Baltic Sea have been observed during the last decades (Lehtonen et al, 2009;Ljunggren et al, 2010;Nilsson et al, 2019;Olsson, 2019), and management actions, e.g., restoration of spawning habitats, have therefore been conducted to support the populations (Larsson et al, 2015). Population assignments that could be used to aid in decisions concerning such management efforts have mainly been based on studies utilizing neutral microsatellite markers Wennerström et al, 2016;Eschbach et al, 2019;Nordahl et al, 2019).…”