Spawning habitat of pike (Esox lucius) in the Baltic Sea include brackish water bays, brooks and rivers. Elevated salinity concentrations are one of several stressors that might increase the use and importance of freshwater habitats for spawning. In the Baltic Sea, one of the largest brackish seas in the world, freshwater species like pike, perch (Perca fluviatilis), whitefish (Coregonus sp), bream (Abramis brama), ide (Leuciscus idus), roach (Rutilus rutilus) and burbot (Lola iota) all undertake spawning migrations to freshwater. However, over the last decades populations densities of these species have declined, and recruitment failure has been argued to be at least part of the problem. The importance of brooks and rivers as spawning areas for these species have not been quantified and set in relation to spawning success in brackish bays. In this study, we collected 175 adult pike (Esox lucius) on their foraging grounds in the sea. Fish were collected in two regions on the Baltic coast, more than 600 km apart. Subsequently we determined their origin (freshwater or marine) using otolith chemistry. Sagittal otoliths were analysed for strontium using the PIXE-method. The results show that 80 of the 175 pike were recruited in freshwater, and several of the larger specimens showed reoccurring migration behaviour. Data show that freshwater is an important recruitment habitat for Baltic Sea pike, suggesting that habitat improvements in rivers entering the Baltic Sea might significantly contribute to population restoration.

Strontium (Sr) or Sr:Ca ratios in otoliths have been widely used in the last decade to describe migration histories

of fish between fresh and marine waters. However, reference experimental studies on particular species and

waters are necessary to confirm the underlying assumption and evaluate the usefulness of this tool for field data.

We conducted an experiment with the aim to answer these questions for anadromous pike (Esox lucius L.) in

the Baltic Sea. Juvenile pike were reared in successively increasing salinities (10 steps from 0 to 7 ‰) for 110

days, and then in constant maximum salinity (7 ‰) for an additional 50 days. Pikes in the experiment were

divided into two food treatments, given prey fish from brackish environments (7 ‰) and fish from freshwater

lakes. Sr:Ca in pike otoliths were positively related to Sr in water (i.e salinity) in both treatments, suggesting

that Sr:Ca ratios may be used to describe migration histories between rivers and the Baltic Sea.

A significant difference in Sr.Ca ratios was found between the food treatments, showing that differences in diet

may contribute to variation in otolith Sr:Ca, at least for pike in the Baltic Sea. Maximum Sr:Ca values for pike

given marine food corresponded with field collected fish from the Baltic Sea.

The movement of pike (n = 3415) was investigated in four streams entering the Baltic Sea in the Kalmar Sound

region, Sweden. Migration patterns were monitored during spawning in three of the streams and during the

whole year in the forth stream using passive integrated transponders and outer tags. The study was conducted

yearly between 2006 and 2010. Mature pike returned (22 – 45%) to the same streams year after year and some

fish returned to the same regions of the streams. No incidence of fish visiting another stream than the one

where they were marked was recorded and no fish missed the spawning season to return the next year. These

results indicated that the reason for fish not returning was due to mortality. The proportions of returning fish

indicated homing and the spawning ground fidelity (stream) was high. Return rates were influenced by size

of fish, sex and stream identity. We also investigated if sex, size or individual behaviour influenced timing of

arrival, departure or duration of stay in freshwater. Diurnal activity was examined in one year in each stream

suggesting activity peaks in the morning and evening. The results show that pike in the Baltic Sea are anadromous

and that the migration behavior is likely to create barriers for gene flow. The separation in populations

by spawning are similar to salmonid species and the management of pike in the Baltic Sea must consider this

process in strategies and plans of fishing.

Northern pike (Esox lucius) inhabit the coastal areas of the Baltic Sea. A large part of the fishes show anadromous

behaviour and spawn in streams and rivers but spend most of the time foraging in the sea. We examined

spawning migration in four streams in the southwest part of the Baltic, situated within a radius of 50 km. Using

juvenile pike in the streams as references, otolith analysis by microPIXE revealed unique elemental patterns (Sr,

Zn, Br, Co and Mn) for the juveniles in each of the four different streams. The strontium signal in the otolith

of the juveniles was used as an indicator of freshwater origin and the time spent in the stream (size of juveniles).

Adult marked pike in their migrating spawning phase were caught in each of the streams and otoliths were

analysed. Defining earlier freshwater origin by the Sr signal from the otolith core to the increase in Sr when the

fish as juvenile pike migrated to the sea, element composition was determined. A principal component analysis

showed that the elemental fingerprint during the freshwater phase several years back was similar for adult fish

and juveniles inhabiting the stream today. The results indicated native homing of the adults to a specific stream,

a process further corroborated by results from electronic marking (Pit-tags) with the return of adult individuals

over several consecutive years. We interpret the results as evidence that pike in the Baltic Sea consists of several

sub-populations and are developed by homing to specific spawning streams. The results of the study may have

implications for fishery management as pike in the Baltic Sea cannot be seen as homogenous “stock“, but instead

consists of different, unique populations similar to the pattern demonstrated in salmon (Salmo salar).

Wetlands are important ecosystems, harbouring biodiversity, capturing

nutrients, and providing recruitment habitats for several fish species. The Baltic Sea, among the

largest brackish, semi-enclosed seas in the world, is exposed to large amounts of nutrients as well as

the overexploitation of several fish species. One measure to counteract eutrophication is to restore or

build wetlands for nutrient catchment (phosphorus) or removal (nitrogen). To function optimally for

nutrient removal, these wetlands should cover large areas, be shallow, and be covered by vegetation,

creating zones of high microbial denitrification. Wetland environments are also suitable spawning and

nursery areas for several fish species inhabiting the coastal zone of the Baltic Sea. Pike (Esox lucius) a

predatory species that has decreased in abundance in recent decades, use wetlands as recruitment areas

during their anadromous life cycle. After restoring wetlands near the coast and opening the waterways

towards the sea, pike larval emigration increased from a few thousand to over a hundred thousand

individuals. The habitat and food choice, growth and migration of larvae and juveniles were followed

over time, revealing that 80–95% left the wetlands within one month (at a size <6 cm). An optimal

wetland enriches nutrients and creates a high primary production base for zooplankton that are used

as food for pike larvae. Wetlands near the sea can function both as nutrient traps and as recruitment

areas for fish.