Consequences on population dynamics following regained connectivity in pike (Esox lucius) spawning location

Abstract: Distributional movements of subpopulations may act as a buffer to prevent the loss of a species in a certain area. However, within subpopulations adaptations may evolve that makes the inhabitants of a certain habitat to better cope with prevailing environmental conditions. If such traits are related to reproduction, they may reduce the opportunity of gene exchange between other subpopulations. Also, a lack of adaptations to a specific habitat may be what prevents a group of individuals, arriving from an adjacent habitat, to successfully colonize an area where a previous subpopulation has been lost.This is the report from a field study conducted in the wetland Lake Långsjön, that in 2018 was restored in order to promote the recruitment of anadromous pike (Esox lucius) to the Baltic Sea. Commonly, wetlands that are restored to promote anadromous pike recruitment, are constructed so that they enable spawning migration from the sea towards the wetland and juvenile emigration towards the sea only. In that sense Lake Långsjön is different, from other wetlands restored for the same purpose, due to that it is connected to both the Baltic Sea and an upstream located freshwater lake. By quantifying the migration of pike (spawners and juveniles) in both directions I explore the consequences that the regained connectivity between the Lake Långsjön and the coast may have on the population dynamics within this wetland; (i) whether it is potentially influenced by allowing mixture between pike with different migratory strategies for spawning (anadromous and potamodromous), (ii) what drivers there are of pike fry emigration and how they may influence the pike fry emigration route and (iii) whether or not the pike of potamodromous origin, resident in the upstream located lake, may work as a source, providing the Baltic Sea with pike juveniles. Pike spawners arriving in the wetland were caught in traps between March - April. Pike fry were caught withing the wetland with fyke nets and by netting. Emigrating pike fry were caught in fyke nets. Findings suggest that spawning migration patterns do not differ between anadromous and potamodromous pike. However, the spawners arriving from the Baltic Sea I suggested are to be composed by offspring of potamodromous origin, possibly hatched during the previous season, and that they as juveniles swam downstream. This, in turn, indicates that the potamodromous stock can help establish an anadromous stock in the Baltic Sea. Still, due to the observation of pike fry displaying an emigration behaviour upstream, origin is identified as a factor that may influence the pike fry emigration route. Also, this emigration pattern seems to indicate a heritable trait that has not been described before among pike, that of downstream spawning. The restoration of the wetland and the regained connectivity is key, both for the ability to restock the Baltic Sea with pike juveniles but also to ensure the conservation of a fascinating stock of pike exhibiting a unique spawning strategy