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Northern pike (Esox lucius L) populations have been declining along the east coast of Sweden, including the Gävleborg coast. This negative trend is believed to be caused by factors related to pike recruitment including loss of suitable spawning habitats. The purpose of this study is to gain a deeper understanding for the recruitment process of pike by investigating their preferences when selecting spawning habitats. Data from a rod fishing inventory targeting adult pike was used to estimate pike densities in 20 assumed spawning bays along the coast of Gävleborg. By using the number of pike caught per hour as a measure of pike density I examined how the morphological characteristics of bays and fish communities influenced pike densities during the spawning period in the studied bays. The results suggest that the density of pike (# caught per hour) tend to increase in bays with deeper average depths, larger surface area, and higher densities of sticklebacks. These results contradict the hypothesis in the study and somewhat previous research. Possible explanations for these unexpected findings are discussed, and one theory suggest that the rod fishing method used may have influenced the results. Probably improvements can be made to make the density estimates from the fishing inventory more accurately, especially when comparing bays of varying sizes. Suggestions for enhancing density estimation include adjusting the fishing duration based on bay size or using a specific number of throws per casting rod and defined surface area and then move to next surface area.

Northern pike (Esox lucius L) populations have been declining along the east coast of Sweden, including the Gävleborg coast. This negative trend is believed to be caused by factors related to pike recruitment including loss of suitable spawning habitats. The purpose of this study is to gain a deeper understanding for the recruitment process of pike by investigating their preferences when selecting spawning habitats. Data from a rod fishing inventory targeting adult pike was used to estimate pike densities in 20 assumed spawning bays along the coast of Gävleborg. By using the number of pike caught per hour as a measure of pike density I examined how the morphological characteristics of bays and fish communities influenced pike densities during the spawning period in the studied bays. The results suggest that the density of pike (# caught per hour) tend to increase in bays with deeper average depths, larger surface area, and higher densities of sticklebacks. These results contradict the hypothesis in the study and somewhat previous research. Possible explanations for these unexpected findings are discussed, and one theory suggest that the rod fishing method used may have influenced the results. Probably improvements can be made to make the density estimates from the fishing inventory more accurately, especially when comparing bays of varying sizes. Suggestions for enhancing density estimation include adjusting the fishing duration based on bay size or using a specific number of throws per casting rod and defined surface area and then move to next surface area.

1. Internal seiches are common in stratified lakes, with significant effects on stratification patterns, hydrodynamics and vertical nutrient transport. In particular, seiches can change the vertical distribution of the thermocline and the cold hypolimnetic and warm epilimnetic water masses by several metres on a timescale of a few hours, leading to rapid and strong changes in temperature profiles and oxygen availability, with profound effects on mobile and sessile organisms. This could affect fish communities directly, through physiological stress and elevated mortality, and indirectly, through prey distribution. 2. The aim of this study was to analyse the effects of internal seiche dynamics on lacustrine fish behaviour, and to characterise fish reaction patterns, with the main focus on vertical movement of fish in the vicinity of a shifting thermocline, and avoidance of cold hypolimnetic water. 3. The analysis was based on acoustic telemetry data from Lake Milada, a post‐mining lake in the Czech Republic, with a total of 61 tracked individuals of four species: northern pike (Esox lucius), wels catfish (Silurus glanis), tench (Tinca tinca) and rudd (Scardinius erythropthalmus). 4. The effects of seiche dynamics on the four species studied were weak but significant during the day, while at night they affected only rudd. Upward seiches elicited stronger responses in fish than downward seiches, and the impacts occurred only during the strongest seiche events. 5. Thermocline shifting during seiche events may induce a transient reduction in habitat for seiche‐reacting species, and thus affect predation and other inter‐ and intra‐specific interactions, as well as fish community dynamics.

The population of northern pike in the Baltic sea has seen a drasticdecrease in numbers in the last couple of decades. The reasons for this are believed to be many, but the majority of them are most likely anthropogenic. Today, many measures are being taken to prevent further decline of pike populations, ranging from nutrient runoff control to habitat restoration. This inevitably gives rise to the problem addressed in this project, namely: how can we best monitor pike populations so that it is possible to accurately assess and verify the effects of these measures over the coming decades? Pike is currently monitored in Sweden by employing expensive and ineffective manual methods of individual marking of pike by a handful of experts. This project provides evidence that such methods could be replaced by a Convolutional Neural Network (CNN), an automatic artificial intelligence system, which can be taught how to identify pike individuals based on their unique patterns. A neural net simulates the functions of neurons in the human brain, which allows it to perform a range of tasks, while a CNN is a neural net specialized for this type of visual recognition task. The results show that the CNN trained in this project can identify pike individuals in the provided data set with upwards of 90% accuracy, with much potential for improvement.

The population of northern pike in the Baltic sea has seen a drasticdecrease in numbers in the last couple of decades. The reasons for this are believed to be many, but the majority of them are most likely anthropogenic. Today, many measures are being taken to prevent further decline of pike populations, ranging from nutrient runoff control to habitat restoration. This inevitably gives rise to the problem addressed in this project, namely: how can we best monitor pike populations so that it is possible to accurately assess and verify the effects of these measures over the coming decades? Pike is currently monitored in Sweden by employing expensive and ineffective manual methods of individual marking of pike by a handful of experts. This project provides evidence that such methods could be replaced by a Convolutional Neural Network (CNN), an automatic artificial intelligence system, which can be taught how to identify pike individuals based on their unique patterns. A neural net simulates the functions of neurons in the human brain, which allows it to perform a range of tasks, while a CNN is a neural net specialized for this type of visual recognition task. The results show that the CNN trained in this project can identify pike individuals in the provided data set with upwards of 90% accuracy, with much potential for improvement.

Translocation into a novel environment through common fisheries management practices, such as fish stocking, provides opportunities to study behavioural and fitness impacts of translocations at realistic ecological scales. The process of stocking, as well as the unfamiliarity with novel ecological conditions and the interactions with resident fish may affect translocated individuals, leading to alterations of behaviours and causing fitness impacts. Our objectives were to investigate how aquatic top predators behaviourally establish themselves and compete with resident individuals following introduction in a novel lake environment and to investigate the resulting fitness consequences. Using high‐resolution acoustic telemetry, we conducted whole‐lake experiments and compared the activity, activity‐space size and fate of translocated and resident individuals in two model top predators, the northern pike Esox lucius (n = 160) and European catfish Silurus glanis (n = 33). Additionally, we compared the reproductive success of translocated and resident northern pike. The experiment was conducted with large (adult) individuals of different origins, resilient to predation, but subject to agonistic interactions and competition with resident fish. Over a period of several months, the translocated catfish exhibited consistently larger activity‐space sizes than resident catfish, but did not differ from residents in activity and survival. The pike from one of the two translocated origins we tested also showed elevated space‐use, and both translocated origins revealed higher mortality rates than their resident conspecifics, indicating maladjustment to their novel environment. When non‐resident pike reproduced, they overwhelmingly produced hybrid offspring with resident fish, indicating that introductions fostered gene flow of non‐native genes. Our study indicates that fish introductions result in behavioural and fitness impacts even in large‐bodied top predators that experience lo, Leibniz Community, Besatzfisch (German Federal Ministry for Education and Research), Peer Reviewed

The establishment of new lakes has gained momentum throughout Europe and North America as a way to remove nutrients, manage extreme rain events, establish recreational areas, and increase biodiversity. We compared freshwater fish colonization and community development of re-established Lake Fil, a 900-ha lake in Denmark, with fish communities in lakes in close proximity as well as those throughout the region. We found that fish migrated quickly into the lake from nearby refugia with fast initial colonization followed by saturation of the species pool. A total of 14 fish species migrated into the lake during the first five years. Over this period, the lake showed increasing similarity not only to the two directly connected lakes, but also to other lakes in the region. This could be seen in terms of species occurrence similarity, catch per unit of effort (CPUE) and biomass per unit of effort (BPUE). The first immigrant was the three-spined stickleback (Gasterosteus aculeatus), followed by roach (Rutilus rutilus) and pike (Esox Lucius), which both came to dominate the fish biomass in the following years. However, the pike population showed no reproduction and perch (Perca fluviatilis) had low immigration and reproduction, pointing to a limited success, overall. One solution might be to stock Lake Fil and other new lakes with predatory fish, which might ensure a more balanced food web structure which positively influence the turbidity of the lake water and provide recreational value for anglers.

Top predators in aquatic ecosystems often have strong top-down effects on the ecosystem. Northern pike (Esox lucius) has been documented to cause whitefish (Coregonus lavaretus) populations to diverge into different ecomorphs. This can facilitate piscivory in other predators as a novel resource becomes available to them in the form of dwarf whitefish. The aim of this study is to examine whether the presence of pike causes Eurasian perch (Perca fluviatilis) to shift their diet from insectivory to piscivory, and whether this is directly driven by whitefish polymorphism. Stomach contents of 147 perch from lakes with and without pikes were analyzed. The results show that the presence of pike has a clear influence on the diet of the perch. In lakes without pike, perch are mostly insectivorous, and in lakes with pike, they are mostly piscivorous. This diet shift appears to be driven by whitefish availability, as a majority of the diet of perch in pike lakes consisted of whitefish, while none of the fish eaten by perch in non-pike lakes was whitefish. In addition, the results showed that perch undergo the diet shift from insectivory to piscivory at a smaller size when coexisting with pike. This study can be added to the growing body of evidence for the ecological significance of pike.

Top predators in aquatic ecosystems often have strong top-down effects on the ecosystem. Northern pike (Esox lucius) has been documented to cause whitefish (Coregonus lavaretus) populations to diverge into different ecomorphs. This can facilitate piscivory in other predators as a novel resource becomes available to them in the form of dwarf whitefish. The aim of this study is to examine whether the presence of pike causes Eurasian perch (Perca fluviatilis) to shift their diet from insectivory to piscivory, and whether this is directly driven by whitefish polymorphism. Stomach contents of 147 perch from lakes with and without pikes were analyzed. The results show that the presence of pike has a clear influence on the diet of the perch. In lakes without pike, perch are mostly insectivorous, and in lakes with pike, they are mostly piscivorous. This diet shift appears to be driven by whitefish availability, as a majority of the diet of perch in pike lakes consisted of whitefish, while none of the fish eaten by perch in non-pike lakes was whitefish. In addition, the results showed that perch undergo the diet shift from insectivory to piscivory at a smaller size when coexisting with pike. This study can be added to the growing body of evidence for the ecological significance of pike.

Paternal, compared to maternal, contributions were believed to have only a limited influence on embryonic development and larval fitness traits in fishes. Therefore, the perspective of male influence on early life history traits has come under scrutiny. This study was conducted to determine parental effects on the rate of eyed embryos of Ide . Leuciscus idus and Northern pike . Esox lucius. Five sires and five dams from each species were crossed using a quantitative genetic breeding design and the resulting 25 sib groups of each species were reared to the embryonic eyed stage. We then partition variation in embryonic phenotypic performance to maternal, paternal, and parental interactions using the Restricted Maximum Likelihood (REML) model. Results showed that paternal, maternal, and the paternal. ×. maternal interaction terms were highly significant for both species; clearly demonstrating that certain family combinations were more compatible than others. Paternal effects explained 20.24% of the total variance, which was 2-fold higher than the maternal effects (10.73%) in Ide, while paternal effects explained 18.9% of the total variance, which was 15-fold higher than the maternal effects (1.3%) in Northern pike. Together, these results indicate that male effects are of major importance during embryonic development for these species. Furthermore, this study demonstrates that genetic compatibility between sires and dams plays an important role and needs to be taken into consideration for reproduction of these and likely other economically important fish species.

Les activités humaines ont profondément transformé les paysages aquatiques des grands fleuves en modifiant l’hétérogénéité et la connectivité de leurs habitats. Ces transformations ont mené à la disparition et au déclin de plusieurs espèces de poissons jadis abondantes. Malgré les actions entreprises afin d’atténuer l’érosion de la diversité ichtyologique, certaines populations de poissons sont encore en situation précaire. Ce constat témoigne de la nécessité de mieux comprendre en quoi les caractéristiques et modifications du paysage aquatique influencent l’organisation spatiale des commmunautés de poissons afin ultimement de définir des unités de gestion écologique. La présente thèse vise à mieux comprendre le rôle des caractéristiques hydromorphologiques et des modifications anthropiques sur l’hétérogénéité et la connectivité du paysage aquatique à différentes échelles spatiales et organisationnelles dans le fleuve Saint-Laurent (Québec, Canada). Partant d’une perspective à l’échelle de la portion fluviale portant sur la diversité des communautés de poissons, cette thèse se consacre ensuite, à une échelle spatio-temporelle plus fine, à l’accès aux habitats vitaux d’une espèce structurante pour la communauté. En effet, la connectivité entre les habitats assurant la survie des jeunes stades de vie du grand brochet (Esox lucius) vise à mieux comprendre le rôle des caractéristiques hydromorphologiques et des modifications anthropiques sur le déclin de cette espèce emblématique. Sur l’ensemble de la portion fluviale du Saint-Laurent (~350 km), un échantillonnage gouvernemental standardisé des communautés de poissons (1995-2013) a été analysé grâce à la combinaison d’analyses univariées sur les indices de diversité et d’analyses multivariées sur la structure d’abondance des espèces. Les analyses révèlent des résultats contrastés selon les indices utilisés et soulignent l’importance de considérer plusieurs facettes de la diversité. En effet, seul l’indice de diversité t

The temperature dependence of predation rates is a key issue for understanding and predicting the responses of ecosystems to climate change. Using a simple mechanistic model, we demonstrate that differences in the relative performances of predator and prey can cause strong threshold effects in the temperature dependence of attack rates. Empirical data on the attack rate of northern pike (Esox lucius) feeding on brown trout (Salmo trutta) confirm this result. Attack rates fell sharply below a threshold temperature of +11 degrees C, which corresponded to a shift in relative performance of pike and brown trout with respect to maximum attack and escape swimming speeds. The average attack speed of pike was an order of magnitude lower than the escape speed of brown trout at 5 degrees C, but approximately equal at temperatures above 11 degrees C. Thresholds in the temperature dependence of ecological rates can create tipping points in the responses of ecosystems to increasing temperatures. Thus, identifying thresholds is crucial when predicting future effects of climate warming.

This project investigated the influence of community structure in reservoirs on trophodynamics, growth and mercury biomagnification in the apex consumer and fisheries target species northern pike (Esox lucius). Where lake whitefish (Coregonus clupeaformis) were abundant in the prey community, pike were found to be piscivorous, consuming predominantly lake whitefish and other large-bodied fish species. Benthivorous feeding behavior was observed in reservoirs without abundant lake whitefish stocks, and amphipods formed the predominant prey consumed by pike. Growth rates of pike were found to be significantly lower under the benthivorous trophic regime. Benthivorous pike had lower mercury concentrations, consistent with lower dietary exposure, than piscivorous pike, but higher biomagnification consistent with low food conversion efficiency and slower growth. A bioenergetics analysis showed elevated feeding rates and activity levels in benthivorous pike on a diet of comparatively low energy density, relative to piscivorous pike, confirming a trophic bottleneck in response to trophic simplification and highlighting the importance of trophic coupling between pelagic resources and apex consumers in reservoirs.

This project investigated the influence of community structure in reservoirs on trophodynamics, growth and mercury biomagnification in the apex consumer and fisheries target species northern pike (Esox lucius). Where lake whitefish (Coregonus clupeaformis) were abundant in the prey community, pike were found to be piscivorous, consuming predominantly lake whitefish and other large-bodied fish species. Benthivorous feeding behavior was observed in reservoirs without abundant lake whitefish stocks, and amphipods formed the predominant prey consumed by pike. Growth rates of pike were found to be significantly lower under the benthivorous trophic regime. Benthivorous pike had lower mercury concentrations, consistent with lower dietary exposure, than piscivorous pike, but higher biomagnification consistent with low food conversion efficiency and slower growth. A bioenergetics analysis showed elevated feeding rates and activity levels in benthivorous pike on a diet of comparatively low energy density, relative to piscivorous pike, confirming a trophic bottleneck in response to trophic simplification and highlighting the importance of trophic coupling between pelagic resources and apex consumers in reservoirs.

This project investigated the influence of community structure in reservoirs on trophodynamics, growth and mercury biomagnification in the apex consumer and fisheries target species northern pike (Esox lucius). Where lake whitefish (Coregonus clupeaformis) were abundant in the prey community, pike were found to be piscivorous, consuming predominantly lake whitefish and other large-bodied fish species. Benthivorous feeding behavior was observed in reservoirs without abundant lake whitefish stocks, and amphipods formed the predominant prey consumed by pike. Growth rates of pike were found to be significantly lower under the benthivorous trophic regime. Benthivorous pike had lower mercury concentrations, consistent with lower dietary exposure, than piscivorous pike, but higher biomagnification consistent with low food conversion efficiency and slower growth. A bioenergetics analysis showed elevated feeding rates and activity levels in benthivorous pike on a diet of comparatively low energy density, relative to piscivorous pike, confirming a trophic bottleneck in response to trophic simplification and highlighting the importance of trophic coupling between pelagic resources and apex consumers in reservoirs.

This project investigated the influence of community structure in reservoirs on trophodynamics, growth and mercury biomagnification in the apex consumer and fisheries target species northern pike (Esox lucius). Where lake whitefish (Coregonus clupeaformis) were abundant in the prey community, pike were found to be piscivorous, consuming predominantly lake whitefish and other large-bodied fish species. Benthivorous feeding behavior was observed in reservoirs without abundant lake whitefish stocks, and amphipods formed the predominant prey consumed by pike. Growth rates of pike were found to be significantly lower under the benthivorous trophic regime. Benthivorous pike had lower mercury concentrations, consistent with lower dietary exposure, than piscivorous pike, but higher biomagnification consistent with low food conversion efficiency and slower growth. A bioenergetics analysis showed elevated feeding rates and activity levels in benthivorous pike on a diet of comparatively low energy density, relative to piscivorous pike, confirming a trophic bottleneck in response to trophic simplification and highlighting the importance of trophic coupling between pelagic resources and apex consumers in reservoirs.

This project investigated the influence of community structure in reservoirs on trophodynamics, growth and mercury biomagnification in the apex consumer and fisheries target species northern pike (Esox lucius). Where lake whitefish (Coregonus clupeaformis) were abundant in the prey community, pike were found to be piscivorous, consuming predominantly lake whitefish and other large-bodied fish species. Benthivorous feeding behavior was observed in reservoirs without abundant lake whitefish stocks, and amphipods formed the predominant prey consumed by pike. Growth rates of pike were found to be significantly lower under the benthivorous trophic regime. Benthivorous pike had lower mercury concentrations, consistent with lower dietary exposure, than piscivorous pike, but higher biomagnification consistent with low food conversion efficiency and slower growth. A bioenergetics analysis showed elevated feeding rates and activity levels in benthivorous pike on a diet of comparatively low energy density, relative to piscivorous pike, confirming a trophic bottleneck in response to trophic simplification and highlighting the importance of trophic coupling between pelagic resources and apex consumers in reservoirs.

Fish is an important part of the human diet, but also a good indicator of environmental contamination by a number of substances, including trace metals in freshwater systems, notably because fish are at the top of the food chain in the water ecosystem (Noël et al., 2013). Much attention has been paid to hazardous elements such as lead (Pb), cadmium (Cd), mercury (Hg), and arsenic (As) and the effects of exposure to these elements have been comprehensively studied (Castro-González and Méndez-Armenta, 2008; Has-Schön et al., 2008). Indeed, due to their toxicity, persistence and bioaccumulation in water and sediment, when occurring in high concentrations, these elements become severe poisonous for all living organisms. In terms of public health food safety, Pb, Cd and Hg, are monitored in fish with maximum levels of heavy metals in foodstuffs fixed by Commission Regulation (EC) No. 1881/ 2006 (EC, 2006) amended by EC 629/2008 (EC, 2008). For As, no maximum level has yet been established at European level, but discussions are ongoing on this topic and it is anticipated that limits will be set for arsenic in the near future, as the methodology for the determination of arsenic and its speciation improves (Noël et al., 2013). Although many studies have examined the relationship between metal exposure, accumulation and toxicity under laboratory conditions, prediction of toxic effects based on environmental or tissue concentrations remains difficult under natural exposure conditions (Visnjic-Jeftic et al., 2010; Skorić et al., 2012). Data from the literature indicate that the contents of these elements in fish varied as a function of the different localities, but depended on species and feeding behaviour and also, by various biotic and abiotic factors (Has-Schön et al., 2006; Noël et al., 2013). This study was undertaken to determine the contamination data of Pb, Cd, Hg and As in the muscle of two fish species (common carp-Cyprinus carpio and Northern pike- Esox lucius) from, Riba predstavlja važan deo ljudske ishrane, a takođe, riba je dobar pokazatelj kontaminacije životne sredine od strane jednog broja supstanci, uključujući i tragove metala u slatkovodnim sistemima, posebno zbog toga što se ribe, kao tercijelni potrošači, nalaze na vrhu lanca ishrane u vodenom ekosistemu (Noël i sar., 2013). Mnogo pažnje posvećeno je elementima kao što su olovo (Pb), kadmijum (Cd), Živa (Hg) i Arsen (As) i efekti izloženosti ovim elementima su sveobuhvatno istraženi (Castro-González and Méndez-Armenta, 2008; Has-Schön et al., 2008). Zbog svoje toksičnosti, otpornosti i bioakumulacije u vodi i sedimentima, kada se nalaze u visokim koncentracijama, ovi elementi predstavljaju opasnost za sve žive organizme. U pogledu bezbednosti javnog zdravlja, kod riba, prate se koncentracije olova, kadmijuma i žive, gde postoje jasno definisani maksimalni nivoi teških metala u namirnicama koji su određeni od strane Evropske Komisije, br. 1881/2006 (EC, 2006), izmenjena i dopunjena EC 629/2008 (EC, 2008). Za arsen, maksimalni nivo još nije uspostavljen na evropskom nivou, ali se očekuje da će granične vrednosti za arsen biti postavljene u bliskoj budućnosti, kao i metode za određivanje arsena (Noël et al., 2013). Za utvrđivanje koncentracije teških metala u tkivu riba prikupljeno je deset uzoraka dve različite vrste riba - šarana (Cyprinus carpio) i evropske ili severne štuke (Esox lucius). Uzorci su ulovljeni od strane profesionalnih ribara tokom rane jeseni 2013. godine iz četiri različita ribnjaka na području Beograda. Dobijeni rezultati su izraženi kao srednja vrednost ± standardna devijacija. Statistička analiza je urađena je korišćenjem Studentovog t-testa i analizom varijanse (ANOVA) sa višestrukim poređenjem Turkey test za utvrđivanje značajnih razlika između srednjih vrednosti. Primenjen je nivo značajnosti od 0.01 i 0.05. U različitim ribnjacima, koncentracija metala u mišićnom tkivu šarana je varirala u zavisnosti od vrste metala. Između sva četiri poređena

This project investigated the influence of community structure in reservoirs on trophodynamics, growth and mercury biomagnification in the apex consumer and fisheries target species northern pike (Esox lucius). Where lake whitefish (Coregonus clupeaformis) were abundant in the prey community, pike were found to be piscivorous, consuming predominantly lake whitefish and other large-bodied fish species. Benthivorous feeding behavior was observed in reservoirs without abundant lake whitefish stocks, and amphipods formed the predominant prey consumed by pike. Growth rates of pike were found to be significantly lower under the benthivorous trophic regime. Benthivorous pike had lower mercury concentrations, consistent with lower dietary exposure, than piscivorous pike, but higher biomagnification consistent with low food conversion efficiency and slower growth. A bioenergetics analysis showed elevated feeding rates and activity levels in benthivorous pike on a diet of comparatively low energy density, relative to piscivorous pike, confirming a trophic bottleneck in response to trophic simplification and highlighting the importance of trophic coupling between pelagic resources and apex consumers in reservoirs.

This project investigated the influence of community structure in reservoirs on trophodynamics, growth and mercury biomagnification in the apex consumer and fisheries target species northern pike (Esox lucius). Where lake whitefish (Coregonus clupeaformis) were abundant in the prey community, pike were found to be piscivorous, consuming predominantly lake whitefish and other large-bodied fish species. Benthivorous feeding behavior was observed in reservoirs without abundant lake whitefish stocks, and amphipods formed the predominant prey consumed by pike. Growth rates of pike were found to be significantly lower under the benthivorous trophic regime. Benthivorous pike had lower mercury concentrations, consistent with lower dietary exposure, than piscivorous pike, but higher biomagnification consistent with low food conversion efficiency and slower growth. A bioenergetics analysis showed elevated feeding rates and activity levels in benthivorous pike on a diet of comparatively low energy density, relative to piscivorous pike, confirming a trophic bottleneck in response to trophic simplification and highlighting the importance of trophic coupling between pelagic resources and apex consumers in reservoirs.

This project investigated the influence of community structure in reservoirs on trophodynamics, growth and mercury biomagnification in the apex consumer and fisheries target species northern pike (Esox lucius). Where lake whitefish (Coregonus clupeaformis) were abundant in the prey community, pike were found to be piscivorous, consuming predominantly lake whitefish and other large-bodied fish species. Benthivorous feeding behavior was observed in reservoirs without abundant lake whitefish stocks, and amphipods formed the predominant prey consumed by pike. Growth rates of pike were found to be significantly lower under the benthivorous trophic regime. Benthivorous pike had lower mercury concentrations, consistent with lower dietary exposure, than piscivorous pike, but higher biomagnification consistent with low food conversion efficiency and slower growth. A bioenergetics analysis showed elevated feeding rates and activity levels in benthivorous pike on a diet of comparatively low energy density, relative to piscivorous pike, confirming a trophic bottleneck in response to trophic simplification and highlighting the importance of trophic coupling between pelagic resources and apex consumers in reservoirs.

This project investigated the influence of community structure in reservoirs on trophodynamics, growth and mercury biomagnification in the apex consumer and fisheries target species northern pike (Esox lucius). Where lake whitefish (Coregonus clupeaformis) were abundant in the prey community, pike were found to be piscivorous, consuming predominantly lake whitefish and other large-bodied fish species. Benthivorous feeding behavior was observed in reservoirs without abundant lake whitefish stocks, and amphipods formed the predominant prey consumed by pike. Growth rates of pike were found to be significantly lower under the benthivorous trophic regime. Benthivorous pike had lower mercury concentrations, consistent with lower dietary exposure, than piscivorous pike, but higher biomagnification consistent with low food conversion efficiency and slower growth. A bioenergetics analysis showed elevated feeding rates and activity levels in benthivorous pike on a diet of comparatively low energy density, relative to piscivorous pike, confirming a trophic bottleneck in response to trophic simplification and highlighting the importance of trophic coupling between pelagic resources and apex consumers in reservoirs.

This project investigated the influence of community structure in reservoirs on trophodynamics, growth and mercury biomagnification in the apex consumer and fisheries target species northern pike (Esox lucius). Where lake whitefish (Coregonus clupeaformis) were abundant in the prey community, pike were found to be piscivorous, consuming predominantly lake whitefish and other large-bodied fish species. Benthivorous feeding behavior was observed in reservoirs without abundant lake whitefish stocks, and amphipods formed the predominant prey consumed by pike. Growth rates of pike were found to be significantly lower under the benthivorous trophic regime. Benthivorous pike had lower mercury concentrations, consistent with lower dietary exposure, than piscivorous pike, but higher biomagnification consistent with low food conversion efficiency and slower growth. A bioenergetics analysis showed elevated feeding rates and activity levels in benthivorous pike on a diet of comparatively low energy density, relative to piscivorous pike, confirming a trophic bottleneck in response to trophic simplification and highlighting the importance of trophic coupling between pelagic resources and apex consumers in reservoirs.

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.

1. Fish and ducks often belong to the same local food web, and several studies indicate that there is a general negative effect of fish on breeding ducks. This pattern has so far been addressed mainly within the framework of competition for common invertebrate prey, while predation by large fish as a force behind settlement and abundance patterns in ducks remains largely unknown. This is the first study to address the effect of fish predation on breeding ducks, isolated from that of competition, and the first experiment to explore the ability of ducks to identify and avoid lakes with high risk of fish predation. 2. We used a before-after control-impact design and 11 naturally fishless lakes. Waterfowl on the lakes were surveyed during the breeding season of 2005. Large adult pike (Esox lucius) were added to two lakes in early spring 2008, and waterfowl surveys were repeated on all 11 lakes. 3. Pike introduction did not affect the number of pairs on lakes during the nesting season in any of three focal duck species (mallard Anas platyrhynchos, teal Anas crecca, and goldeneye Bucephala clangula). During the brood-rearing season, however, there was a decrease in duck days in teal and goldeneye in lakes with pike, with similar trends observed in mallard. The number of goldeneye ducklings was also significantly lower in lakes with pike. We were unable to determine whether the response was attributable to direct pike predation or to broods leaving experimental lakes, but in either case, our study demonstrates high fitness costs for ducks breeding on lakes with pike. 4. The apparent inability of nesting ducks to detect pike and the clear fitness implications may influence the annual recruitment of ducks on a larger scale as pike are both common and widespread. Vegetation complexity and food abundance are likely to be of overriding importance when breeding ducks are choosing a nesting site. As pike have a strong influence on breeding birds, relying on vegetation and cues of

Genetic variation is essential for biological evolution, for maintaining viability of populations, and to ensure ecosystem resilience. Increased human exploitation and environmental change result in rapid loss of biological variation, including genetic diversity. Measures to halt this trend require that biological diversity is assessed and monitored. Assessment of biodiversity includes identifying patterns of distribution of genetic variation within individual species. This thesis focuses on spatial genetic structure and assessment of units for conservation in continuous environments without apparent migration barriers. Empirical data refer to Scandinavia and the model species are northern pike (Esox lucius), brown trout (Salmo trutta), and harbour porpoise (Phocoena phocoena). Questions regarding monitoring genetic diversity and releases of alien populations are also addressed. The spatial genetic structure of the northern pike in the Baltic Sea is characterized by isolation by distance and continuous genetic change. Positive genetic correlation was found among pike within geographical distances of less than 150 km. This distance may be used to suggest management units in this area. For the brown trout, genetic monitoring identified two sympatric populations within a small mountain lake system. The situation is characterized by a clear genetic but no apparent phenotypic dichotomy. Scientific support for a genetically distinct Baltic harbour porpoise population is limited, and the spatial genetic structure of the harbour porpoise in Swedish waters needs to be clarified. Data for launching conservation genetic monitoring programs is available for only a few Swedish species. Millions of forest trees, fish, and birds are released annually in Sweden and the documentation on these releases is poor. To meet responsibilities of safeguarding biodiversity and surveying biological effects of releases, there is an urgent need for studies aimed at evaluating genetic diversity., At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 2: Manuscript.

Genetic variation is essential for biological evolution, for maintaining viability of populations, and to ensure ecosystem resilience. Increased human exploitation and environmental change result in rapid loss of biological variation, including genetic diversity. Measures to halt this trend require that biological diversity is assessed and monitored. Assessment of biodiversity includes identifying patterns of distribution of genetic variation within individual species. This thesis focuses on spatial genetic structure and assessment of units for conservation in continuous environments without apparent migration barriers. Empirical data refer to Scandinavia and the model species are northern pike (Esox lucius), brown trout (Salmo trutta), and harbour porpoise (Phocoena phocoena). Questions regarding monitoring genetic diversity and releases of alien populations are also addressed. The spatial genetic structure of the northern pike in the Baltic Sea is characterized by isolation by distance and continuous genetic change. Positive genetic correlation was found among pike within geographical distances of less than 150 km. This distance may be used to suggest management units in this area. For the brown trout, genetic monitoring identified two sympatric populations within a small mountain lake system. The situation is characterized by a clear genetic but no apparent phenotypic dichotomy. Scientific support for a genetically distinct Baltic harbour porpoise population is limited, and the spatial genetic structure of the harbour porpoise in Swedish waters needs to be clarified. Data for launching conservation genetic monitoring programs is available for only a few Swedish species. Millions of forest trees, fish, and birds are released annually in Sweden and the documentation on these releases is poor. To meet responsibilities of safeguarding biodiversity and surveying biological effects of releases, there is an urgent need for studies aimed at evaluating genetic diversity., At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 2: Manuscript.

Genetic variation is essential for biological evolution, for maintaining viability of populations, and to ensure ecosystem resilience. Increased human exploitation and environmental change result in rapid loss of biological variation, including genetic diversity. Measures to halt this trend require that biological diversity is assessed and monitored. Assessment of biodiversity includes identifying patterns of distribution of genetic variation within individual species. This thesis focuses on spatial genetic structure and assessment of units for conservation in continuous environments without apparent migration barriers. Empirical data refer to Scandinavia and the model species are northern pike (Esox lucius), brown trout (Salmo trutta), and harbour porpoise (Phocoena phocoena). Questions regarding monitoring genetic diversity and releases of alien populations are also addressed. The spatial genetic structure of the northern pike in the Baltic Sea is characterized by isolation by distance and continuous genetic change. Positive genetic correlation was found among pike within geographical distances of less than 150 km. This distance may be used to suggest management units in this area. For the brown trout, genetic monitoring identified two sympatric populations within a small mountain lake system. The situation is characterized by a clear genetic but no apparent phenotypic dichotomy. Scientific support for a genetically distinct Baltic harbour porpoise population is limited, and the spatial genetic structure of the harbour porpoise in Swedish waters needs to be clarified. Data for launching conservation genetic monitoring programs is available for only a few Swedish species. Millions of forest trees, fish, and birds are released annually in Sweden and the documentation on these releases is poor. To meet responsibilities of safeguarding biodiversity and surveying biological effects of releases, there is an urgent need for studies aimed at evaluating genetic diversity., At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 2: Manuscript.

Genetic variation is essential for biological evolution, for maintaining viability of populations, and to ensure ecosystem resilience. Increased human exploitation and environmental change result in rapid loss of biological variation, including genetic diversity. Measures to halt this trend require that biological diversity is assessed and monitored. Assessment of biodiversity includes identifying patterns of distribution of genetic variation within individual species. This thesis focuses on spatial genetic structure and assessment of units for conservation in continuous environments without apparent migration barriers. Empirical data refer to Scandinavia and the model species are northern pike (Esox lucius), brown trout (Salmo trutta), and harbour porpoise (Phocoena phocoena). Questions regarding monitoring genetic diversity and releases of alien populations are also addressed. The spatial genetic structure of the northern pike in the Baltic Sea is characterized by isolation by distance and continuous genetic change. Positive genetic correlation was found among pike within geographical distances of less than 150 km. This distance may be used to suggest management units in this area. For the brown trout, genetic monitoring identified two sympatric populations within a small mountain lake system. The situation is characterized by a clear genetic but no apparent phenotypic dichotomy. Scientific support for a genetically distinct Baltic harbour porpoise population is limited, and the spatial genetic structure of the harbour porpoise in Swedish waters needs to be clarified. Data for launching conservation genetic monitoring programs is available for only a few Swedish species. Millions of forest trees, fish, and birds are released annually in Sweden and the documentation on these releases is poor. To meet responsibilities of safeguarding biodiversity and surveying biological effects of releases, there is an urgent need for studies aimed at evaluating genetic diversity., At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 2: Manuscript.

Genetic variation is essential for biological evolution, for maintaining viability of populations, and to ensure ecosystem resilience. Increased human exploitation and environmental change result in rapid loss of biological variation, including genetic diversity. Measures to halt this trend require that biological diversity is assessed and monitored. Assessment of biodiversity includes identifying patterns of distribution of genetic variation within individual species. This thesis focuses on spatial genetic structure and assessment of units for conservation in continuous environments without apparent migration barriers. Empirical data refer to Scandinavia and the model species are northern pike (Esox lucius), brown trout (Salmo trutta), and harbour porpoise (Phocoena phocoena). Questions regarding monitoring genetic diversity and releases of alien populations are also addressed. The spatial genetic structure of the northern pike in the Baltic Sea is characterized by isolation by distance and continuous genetic change. Positive genetic correlation was found among pike within geographical distances of less than 150 km. This distance may be used to suggest management units in this area. For the brown trout, genetic monitoring identified two sympatric populations within a small mountain lake system. The situation is characterized by a clear genetic but no apparent phenotypic dichotomy. Scientific support for a genetically distinct Baltic harbour porpoise population is limited, and the spatial genetic structure of the harbour porpoise in Swedish waters needs to be clarified. Data for launching conservation genetic monitoring programs is available for only a few Swedish species. Millions of forest trees, fish, and birds are released annually in Sweden and the documentation on these releases is poor. To meet responsibilities of safeguarding biodiversity and surveying biological effects of releases, there is an urgent need for studies aimed at evaluating genetic diversity., At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 2: Manuscript.

Author Posting. © Elsevier B.V., 2007. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Comparative Biochemistry and Physiology Part C: Toxicology & Pharmacology 147 (2008): 331-338, doi:10.1016/j.cbpc.2007.12.003., Many laboratory studies have documented that mercury can be toxic to fish, but it is largely unknown if mercury is toxic to fish in their natural environments. The objective of our study was to investigate the toxic effects of mercury on northern pike (Esox lucius) at Isle Royale, Michigan. In 124 northern pike from eight inland lakes, concentrations of total mercury in skin-on fillets ranged from 0.069 to 0.622 µg/g wet wt. Concentrations of total mercury in livers increased exponentially compared with concentrations in fillets, to a maximum of 3.1 µg/g wet wt. Methylmercury constituted a majority of the mercury in livers with total mercury concentrations <0.5 µg/g wet wt, but declined to 28-51% of the mercury in livers with total mercury concentrations >0.5 µg/g wet wt. Liver color (absorbance at 400 nm) varied among northern pike and was positively related to liver total mercury concentration. The pigment causing variation in liver color was identified as lipofuscin, which results from lipid peroxidation of membranous organelles. An analysis of covariance revealed lipofuscin accumulation was primarily associated with mercury exposure, and this association obscured any normal accumulation from aging. We also documented decreased lipid reserves in livers and poor condition factors of northern pike with high liver total mercury concentrations. Our results suggest (i) northern pike at Isle Royale are experiencing toxicity at concentrations of total mercury common for northern pike and other piscivorous fish elsewhere in North America and (ii) liver color may be useful for indicating mercury exposure and effects in northern pike at Isle Royale and possibly other aquatic ecosystems and other fish species., Financial support was provided by the U.S. Environmental Protection Agency STAR Graduate Fellowship Program to P.E.D.

Author Posting. © Elsevier B.V., 2007. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Comparative Biochemistry and Physiology Part C: Toxicology & Pharmacology 147 (2008): 331-338, doi:10.1016/j.cbpc.2007.12.003., Many laboratory studies have documented that mercury can be toxic to fish, but it is largely unknown if mercury is toxic to fish in their natural environments. The objective of our study was to investigate the toxic effects of mercury on northern pike (Esox lucius) at Isle Royale, Michigan. In 124 northern pike from eight inland lakes, concentrations of total mercury in skin-on fillets ranged from 0.069 to 0.622 µg/g wet wt. Concentrations of total mercury in livers increased exponentially compared with concentrations in fillets, to a maximum of 3.1 µg/g wet wt. Methylmercury constituted a majority of the mercury in livers with total mercury concentrations <0.5 µg/g wet wt, but declined to 28-51% of the mercury in livers with total mercury concentrations >0.5 µg/g wet wt. Liver color (absorbance at 400 nm) varied among northern pike and was positively related to liver total mercury concentration. The pigment causing variation in liver color was identified as lipofuscin, which results from lipid peroxidation of membranous organelles. An analysis of covariance revealed lipofuscin accumulation was primarily associated with mercury exposure, and this association obscured any normal accumulation from aging. We also documented decreased lipid reserves in livers and poor condition factors of northern pike with high liver total mercury concentrations. Our results suggest (i) northern pike at Isle Royale are experiencing toxicity at concentrations of total mercury common for northern pike and other piscivorous fish elsewhere in North America and (ii) liver color may be useful for indicating mercury exposure and effects in northern pike at Isle Royale and possibly other aquatic ecosystems and other fish species., Financial support was provided by the U.S. Environmental Protection Agency STAR Graduate Fellowship Program to P.E.D.

1. Invasions of top predators may have strong cascading effects in ecosystems affecting both prey species abundance and lower trophic levels. A recently discussed factor that may enhance species invasion is climate change and in this context, we studied the effects of an invasion of northern pike into a subarctic lake ecosystem formerly inhabited by the native top predator Arctic char and its prey fish, ninespined stickleback. 2. Our study demonstrated a strong change in fish community composition from a system with Arctic char as top predator and high densities of sticklebacks to a system with northern pike as top predator and very low densities of sticklebacks. A combination of both predation and competition from pike is the likely cause of the extinction of char. 3. The change in top predator species also cascaded down to primary consumers as both zooplankton and predator-sensitive macroinvertebrates increased in abundance. 4. Although the pike invasion coincided with increasing summer temperatures in the study area we have no conclusive evidence that the temperature increase is the causal mechanism behind the pike invasion. But still, our study provides possible effects of future pike invasions in mountain lakes related to climate change. We suggest that future pike invasions will have strong effects in lake ecosystems, both by replacing native top consumers and through cascading effects on lower trophic levels.

Two populations of pike Esox lucius have been identified on the eastern coast of Sweden, in the Baltic Sea. These live together, but separate into two different populations during the spawning season. One is an obligate freshwater spawner and the other not only reproduces in ambient Baltic salinities, but appears not to enter fresh water at all.

An Aquatic Habitat Appraisal Guide (AHAG) was developed to evaluate fish habitat in the Upper Mississippi River System (UMRS). AHAG numerically rates existing and future habitat quality according to Habitat Suitability index (HSI) values, accounts for impacts of an engineering project, and documents fishery benefits of environmental features (e.g., creation of slack-water habitat and construction of weirs) incorporated in the project design. A software program of AHAG was developed to facilitate modifications of the default matrix of HSI values based on site-specific data and calculate final HSI scores for an array of project alternatives. The default matrix consists of HSI values for eight species of fish common in the UMRS: white bass, emerald shiner, river darter, northern pike, smallmouth buffalo, walleye, largemouth bass, and bluegill. This report provides instructions on installing and using AHAG software to evaluate fish habitat according to season and life stage of the species.

As the limited water resources of the western United States are developed for agricultural and industrial uses, there is an ever increasing need to utilize waste water to its fullest potential. This report describes one example where waste water from an energy development site (coal mine operation) was used to enhance the fisheries of an adjacent reservoir. The details of this report provide an example of what considerations may be necessary for similar projects; illustrate potential problem areas in the planning, construction and operation of such a project; and stimulate ideas for developing other innovative uses of waste water for fish and wildlife habitat enhancement.

Literature describing the habitat preferences of the northern pike (Esox lucius Linnaeus) is reviewed, and the relationships between habitat variables and life requisites are synthesized into a Habitat Suitability Index (HSI) model. HSI models are designed for use with the Habitat Evaluation Procedures (HEP) in impact assessment and habitat management activities. p48

Ohland holds a northern pike.