Baltic Sea populations of the northern pike (Esox lucius) have declined since the 1990s, and they face additional challenges due to ongoing climate change. Pike in the Baltic Sea spawn either in coastal bays or in freshwater streams and wetlands. Pike recruited in freshwater have been found to make up about 50 % of coastal pike stocks and to show natal homing, thus limiting gene flow among closely located spawning sites. Due to natal homing, sub-populations appear to be locally adapted to their freshwater recruitment environments. Management actions should therefore not involve mixing of individuals originating from different sub-populations. We offer two suggestions complying with this advice: (i) productivity of extant freshwater spawning populations can be boosted by modifying wetlands such that they promote spawning and recruitment; and (ii) new sub-populations that spawn in brackish water can potentially be created by transferring fry and imprinting them on seemingly suitable spawning environments.

We examined the element pattern in the otoliths of a migratory fish species that inhabit the coastal areas in the brackish of the Baltic Sea. The northern pike (Esox lucius) show migratory behaviour, spawning in streams and rivers and foraging in the sea. We examined spawning migration in four nearby streams in the south-west part of the Baltic. Otolith analysis by microPIXE revealed unique elemental patterns (Sr, Zn, Br, Co and Mn) for the juveniles in each of the 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. Adult pike in their migrating spawning phase were caught in each of the streams. The elemental composition in otoliths in their freshwater phase (using juvenile pike in the streams as references) was determined. A principal component analysis showed that the elemental fingerprint during the freshwater phase several years back in time was similar for the adult fish and for juveniles inhabiting the stream today. The results indicated natal homing of the adults to a specific stream, a conclusion that was strengthened by the fact that marked fish returned to spawn over consecutive years. Anadromous pike in the Baltic Sea may thus be divided in subpopulations. The results of the study may have implications for fishery management, as pike in the Baltic Sea cannot be seen as homogenous population.

In recent decades, wetlands have been constructed or restored around the Baltic sea to counteract the eutrophication of its coastal waters. Some of these wetlands could also be suitable spawning and nursery areas for anadromous northern pike (Esox lucius L.). We studied juvenile pike production in three coastal wetlands along the south-eastern coast of Sweden that were restored in different ways. Where terrestrial vegetation was temporarily flooded, pike larval/juvenile emigration increased from a few thousand individuals before restoration to over a hundred thousand afterwards. We suggest that vegetation was the key to this successful reproduction, as wetlands where vegetation was removed or reduced saw no similar increase in pike production. Flooded vegetation in shallow waters offers optimal spawning conditions, increased food resources, and refuge from predation. The growth and emigration of larvae and juveniles were followed over time, revealing that 80-95% of individuals left the wetlands within 1 month (at a size < 6 cm). This emigration probably represents an adaption to seasonally decreasing water levels but may also be a way to avoid cannibalism.

The strontium (Sr) or Sr:Ca ratio in otoliths has been widely used in the last decade to describe the migration histories of fish between fresh and marine waters. However, reference experimental studies of particular species and waters are necessary to confirm the underlying assumptions and evaluate the applicability of this tool to field data. Laboratory experiments indicated that juvenile, anadromous pike (Esox lucius L) from the Baltic Sea reared in successively increasing salinities (from 0 to 7 parts per thousand) for 110 d accumulated Sr in their otoliths according to a positive relationship with waterborne Sr. When the pike were given prey fish from brackish (7 parts per thousand) environments, the otolith Sr:Ca ratio increased more than in fish given prey from freshwater lakes. Pike held at constant salinity (7 parts per thousand) and given prey fish from the same salinity environment had an Sr:Ca ratio of 6.9 x 10(-3). The ratio decreased successively for fish given prey from freshwater (4.4 x 10(-3)) or kept in freshwater and given food from brackish water (3.1 x 10(-3)). Fish exposed to freshwater and given prey fish from freshwater displayed no increase in Sr:Ca ratio (1.6 x 10(-3)). The experiments demonstrated that the Sr:Ca ratio may be used to describe the migration history of pike between rivers and the Baltic Sea. The maximum Sr:Ca value for pike given marine-origin food corresponded to those of fish collected from the Baltic Sea.

The pike (Esox lucius) is a major predator and top-down regulator in the Baltic Sea where it exists in two sympatric forms. One spawn in streams and rivers and the other one spawn in the sea. During the last decades, the habitats for both of these forms have developed in a negative way. In some freshwater systems, up to 90 % of the water areas have disappeared, mainly through drainage and straightening of watercourses for agricultural purposes. In the sea, reproduction habitats decrease due to construction of harbours and human activities that create disturbances. The perhaps largest single factor negatively affecting recruitment of pike in the sea is the eutrophication. Bottoms are overgrown with filamentous algae and shallow bays are covered with dense Phragmites belts decreasing the habitats suitable for spawning. Further on, a predator on egg and fish larvae, the three-spined stickleback (Gasterosteus aculeatus) has increased in abundance. It is difficult to restore and enhance pike production in the sea and probably the only economically viable alternative is to make restorations in freshwater. However, there is a limited knowledge about the freshwater spawning pike in the Baltic Sea. Thus in this thesis I, together with my coauthors, set out with an aim to increase the knowledge base regarding anadromous pike behaviour.

We found that pike of natal freshwater origin were common in the Baltic Sea. Through Sr:Ca studies in otoliths, about 45 % of the pike were interpreted to be of freshwater origin. The majority of the pike had emigrated out of freshwater at a length below 6 cm. These results indicate that freshwater recruitment is successful, contrasting the vast areas available for spawning in the sea. This creates incitements that restoration measures in these watercourses could have a significant effect on the pike population in the Baltic Sea.

Further, in four streams running out in the Baltic Sea, more than three thousand pike were marked to study spawning migration. About 30-40 % returned to the same river the subsequent year. Most of the pike used the lower parts of the stream for spawning. The homing of pike to a watercourse indicate that freshwater pike in the Baltic Sea consist of specific populations and this is crucial information when taking decisions on fish restoration measures.

Three wetlands adjacent to streams were restored for pike production. The most successful restoration involved minimal digging, with flooded grasslands providing optimal conditions for spawning. The first spawning season after restoration increased the pike production hundredfold.

In conclusion, the anadromous pike are numerous in the Baltic Sea. To compensate for the decline in pike populations in the sea, “pike-factories” created along the coastline are probably the most justifiable option.

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.

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).