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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.

The Esox genus belongs to the family Esocidae. It is a freshwater fish that can be found in different parts of Europe, Asia, and North America. The Northern Pike, is a carnivorous fish, member of the only remaining genus of the Esocidae. This fish prefers to lead a solitary life due to the disadvantages of the presence of individuals of the same species which compete for food resources and territory. Waiting for a suitable situation to catch prey is a favoured hunting method, which means this specific species is a lie-in-wait predator. The elongated body and the broad snout are the most striking features used for the identification of this species. This lie-in-wait carnivorous predator mostly feeds on invertebrates and fishes. The species spawns annually. The special living conditions of this species and their endangered environment have made it necessary to avoid any acts that can affect their life quality.

Otolith microchemistry has emerged as a powerful technique with which to identify the natal origins of fishes, but it relies on differences in underlying geology that may occur over large spatial scales. An examination of how small a spatial scale on which this technique can be implemented, especially in water bodies that share a large proportion of their flow, would be useful for guiding aquatic invasive species control efforts. We examined trace isotopic signatures in northern pike (Esox lucius) otoliths to estimate their provenance between two reservoirs in the Upper Yampa River Basin, Colorado, USA. This is a challenging study area as these reservoirs are only 11-rkm apart on the same river and thus share a high proportion of their inflow. We found that three isotopes (86Sr, 137Ba, and 55Mn) were useful in discriminating between these reservoirs, but their signatures varied annually, and the values overlapped. Strontium isotope ratios (87Sr/86Sr) were different between sites and relatively stable across three years, which made them an ideal marker for determining northern pike provenance. Our study demonstrates the usefulness of otolith microchemistry for natal origin determination within the same river over a relatively small spatial scale when there are geologic differences between sites, especially geologic differences underlying tributaries between sites.

Across the globe, dozens of species of invasive fish are now found in fresh as well as marine waters, where they alter habitats, compete with native fish for food, and prey on native fishes, exerting both indirect and direct effects on ecosystems and economies. While efforts to understand and control these species are growing, most are still in their infancy; however, a few examples stand out. This special issue is comprised of 11 notable articles on freshwater invasive fish and is the first to address this topic. This introductory article serves as an introduction to these articles which focus on 5 topics on invasive freshwater fish: (1) the damage they cause (one article); (2) techniques to ascertain their presence (one article); (3) techniques to restrict their movement (one article); (4) strategies to control them (three articles); and (5) lessons learned from ongoing management efforts (five articles). This introduction notes that successful management efforts share a few approaches: (1) they develop and use a deep understanding of local species and their abundance as well as distribution; (2) they focus on reducing reproductive success; (3) they use multiple complimentary control strategies; and (4) they use a long-term approach.

Individual variability in dispersal strategies, where some individuals disperse and others remain resident, is a common phenomenon across many species. Despite its important ecological consequences, the mechanisms and individual consequences of dispersal remain poorly understood. Here, riverine Northern pike (Esox lucius) juveniles (age 0+ (young-of-the-year) and 1+ years) were used to investigate the influence of body size and trophic position (at capture) on the dispersal from off-channel natal habitats and the subsequent consequences for body sizes, specific growth rate and trophic position (at recapture). Individuals that dispersed into the river ("dispersers") were not significantly different in body size than those remaining on nursery grounds ("stayers"). For trophic position, 0+ dispersers were of significantly lower trophic position than stayers, but with this not apparent in the 1+ fish. Following dispersal into the river, the dispersers grew significantly faster than stayers and, on recapture, were significantly larger, but with no significant differences in their final trophic positions. Early dispersal into the river was, therefore, not associated with early dietary shifts to piscivory and the attainment of larger body sizes of individuals whilst in their natal habitats, contrary to prediction. These results suggest that despite an increasing risk of mortality for individuals dispersing early from natal areas, there are long-term benefits via elevated growth rates and, potentially, higher fitness. Such early dispersal behaviour could be driven by early competitive displacement.

One of the predators of the ichthyofauna of the Lower Tobol is Esox lucius (Linnaeus, 1758) (Esociformes, Esocidae). The purpose of this work is to study the current state of the E. lucius parasite fauna in the lower reaches of the Tobol River. In the present study 47 individuals of the northern pike of different sex aged from 2+ to 8+ years were examined by the method of complete parasitological dissection. As a result of the research, 23 types of parasites were found in the pike. The largest number of species of parasitic organisms – 20 – were found in May, 12 were found in December and 11 in September. Of these, 10 species are specific to northern pike: Haemogregarina esoci (Nawrotzky, 1914), Chloromyxum esocinum (Dogiel, 1934), Myxidium lieberkuhni (Biitschli, 1882), Myxosoma anurum (Cohn, 1895), Henneguya psorospermica (Thelohan, 1995), Tetraonchus monenteron (Wagener, 1857), Gyrodactylus lucii (Kulakowskaja, 1951), Triaenophorus nodulosus (Pallas, 1781), Azygia lucii (Miiller, 1776), Raphidascaris acus (Bloch, 1779). For the first time an intraerythrocyte parasite – H. esoci – was found in pike in the Lower Tobol, the extensiveness of invasion was 18.7% in December, and 6.2% in May. Epistylis sp. and Trichodinella epizzotica parasitized on the surface of the body of fish only in May, whereas members of the genus Trichodina were found in winter and spring. In all seasons, parasitization by M. anurum and H. psorospermica was established on the gills. The greatest occurrence of spores of M. lieberkuhni and C. esocinum was observed in the spring. Myxosporidia parasitized in the kidneys and T. monenteron parasitized on the gills during all periods of the study (the latter with an extensivity of invasion of 100.0%). All examined fish are infected with T. nodulosus 100.0% of cases. In May, other types of cestodes were found: P. esocis and Dibothriocephalus latus. Three pike were infected with proteocephalis. The larval stage of diphyllobothriid was found in one fish in the liver and gonads in an amount of 139 individuals. In the pikes’ stomach, during all periods of the study, A. lucii was found in all the fish examined. In the autumn period of the study immature R. campanula were found in in the intestine of 4 fish specimens. The nematode R. acus was found in pike only in spring. This nematode was found in fish aged 4+–8+. In May, glochidia with a high intensity of invasion were found on fins, gill covers and gills of fish. E. sieboldi copepods were found on the gills; in May, one specimen was found in one pike and in September in 6 with AI of 0.7. Tetraonchus monenteron was a dominant species, in May its degree of dominance decreased with the greatest uniformity of species in this study period. Having considered the age dynamics of pike infection by various types of parasites, it was found that in the age group 4+–5+ the number of species of parasites was greatest (20), while in groups 2+–3+ and 6+–8+ it was 14. In the pike, the core parasitic fauna were M. anurum, H. psorospermica, T. monenteron, T. nodulosus, A. lucii. Seasonality has virtually no effect on the degree of infection with specific parasites. The age of fish largely determines the qualitative and quantitative composition of the parasite fauna.

Background Salmonids are of major importance both as farmed and wild animals. With the changing environment comes changes in pathogenic pressures so understanding the immune system of all salmonid species is of essence. Major histocompatibility complex (MHC) genes are key players in the adaptive immune system signalling infection to responding T-cells populations. Classical MHC class I (MHCI) genes, defined by high polymorphism, broad expression patterns and peptide binding ability, have a key role in inducing immunity. In salmonids, the fourth whole genome duplication that occurred 94 million years ago has provided salmonids with duplicate MHCI regions, while Northern Pike, a basal sister clade to salmonids, represent a species which has not experienced this whole genome duplication. Results Comparing the gene organization and evolution of MHC class I gene sequences in Northern pike versus salmonids displays a complex picture of how many of these genes evolved. Regional salmonid Ia and Ib Z lineage gene duplicates are not orthologs to the Northern pike Z lineage sequences. Instead, salmonids have experienced unique gene duplications in both duplicate regions as well as in the Salmo and Oncorhynchus branch. Species-specific gene duplications are even more pronounced for some L lineage genes. Conclusions Although both Northern pike as well as salmonids have expanded their U and Z lineage genes, these gene duplications occurred separately in pike and in salmonids. However, the similarity between these duplications suggest the transposable machinery was present in a common ancestor. The salmonid MHCIa and MHCIb regions were formed during the 94 MYA since the split from pike and before the Oncorhynchus and Salmo branch separated. As seen in tetrapods, the non-classical U lineage genes are diversified duplicates of their classical counterpart. One MHCI lineage, the L lineage, experienced massive species-specific gene duplications after Oncorhynchus and Salmo split approximately 25 MYA. Based on what we currently know about L lineage genes, this large variation in number of L lineage genes also signals a large functional diversity in salmonids.

To efficiently manage northern pike (Esox lucius), information is needed on its habitat use and preference.However, knowledge gaps still exist, especially on pike habitat use and preference in rivers characterisedby artificial environments. We investigated the use of the main river, tributaries and side arms at themacro-scale, and the use and preference of riparian habitats by adult pike at the meso-scale in an anthropogenicallyimpacted river basin. Adult pike were followed in winter and spring by radio telemetry.At the macro-scale pike intensively used the main river in winter and spring, but also frequented specificside arms in winter and specific tributaries in spring, which may indicate the importance of thesehabitats to adult pike. At the meso-scale, reedy semi-natural banks were used the most, irrespective ofany assumption on habitat availability or use. The findings underline the value of protecting the leastimpacted, (semi)natural habitats for adult pike in an anthropogenically impacted river system. The largebehavioural differences in habitat use between individuals at both habitat scales further underline theimportance of habitat heterogeneity. The results also provide insight into the impact of riparian habitatrestoration on adult pike and may be used to more efficiently manage pike rivers, e.g. by enhancing thelateral connectivity with river side arms or by reconstructing natural riparian habitats.

In systems with multiple piscivores, co-occurrence is dependent on resource partitioning. This is pronounced in oligotrophic northern lakes, which have simple food webs and short open-water seasons. We used acoustic telemetry and stable isotopes to quantify habitat and dietary partitioning during thermal stratification among three piscivores that commonly co-occur in Canadian sub-Arctic lakes—burbot (Lota lota), lake trout (Salvelinus namaycush), and northern pike (Esox lucius). Spatial core areas and core habitat niches (space and depth) did not significantly overlap among species. Although burbot and lake trout occupied similar mean daily depths (16.2 m and 13.4 m, respectively), and water temperatures (5.4 °C and 6.9 °C, respectively), they were spatially segregated. Burbot were closely associated with the lake bottom on steep drop-offs between the offshore and nearshore zone with moderate substrate complexity, whereas lake trout were located over deep offshore basins and suspended above the lake bottom. Northern pike occupied shallow depths (5.3 m) and warmer water (16.5 °C) within the nearshore region and were closely associated with bottom substrate of highest complexity. Some significant overlap among spatial home ranges and broad habitat niches indicated that these species interact. However, dietary niches did not significantly overlap at either the core or broad levels, suggesting that species were utilizing spatially diverse food sources. Our results highlight the importance of including depth and space when quantifying resource partitioning among fishes and provide insight into the mechanisms that promote piscivore co-occurrence in northern lakes.

Northern pike are an invasive species in southcentral Alaska and have caused the decline and extirpation of salmonids and other native fish populations across the region. Over the last decade, adaptive management of invasive pike populations has included population suppression, eradication, outreach, angler engagement, and research to mitigate damages from pike where feasible. Pike suppression efforts have been focused in open drainages of the northern and western Cook Inlet areas, and eradication efforts have been primarily focused on the Kenai Peninsula and the municipality of Anchorage. Between 2010 and 2020, almost 40,000 pike were removed from southcentral Alaska waters as a result of suppression programs, and pike have been successfully eradicated from over 20 lakes and creeks from the Kenai Peninsula and Anchorage, nearly completing total eradication of pike from known distributions in those areas. Northern pike control actions are tailored to the unique conditions of waters prioritized for their management, and all efforts support the goal of preventing further spread of this invasive aquatic apex predator to vulnerable waters.

Bioaccumulation of mercury in sport fish is a complex process that varies in space and time. Both large-scale climatic as well as fine-scale environmental factors are drivers of these space-time variations. In this study, we avail a long-running monitoring program from Ontario, Canada to better understand spatiotemporal variations in fish mercury bioaccumulation at two distinct scales. Focusing on two common large-bodied sport fishes (Walleye and Northern Pike), the data were analyzed at fine- and broad-scales, where fine-scale implies variations in bioaccumulation at waterbody- and year-level and broad-scale captures variations across 3 latitudinal zones (~5&deg, each) and eight time periods (~5-year each). A series of linear mixed-effects models (LMEMs) were employed to capture the spatial, temporal and spatiotemporal variations in mercury bioaccumulation. Fine-scale models were overall better fit than broad-scale models suggesting environmental factors operating at the waterbody-level and annual climatic conditions matter most. Moreover, for both scales, the space time interaction explained most of the variation. The random slopes from the best-fitting broad-scale model were used to define a bioaccumulation index that captures trends within a climate change context. The broad-scale trends suggests of multiple and potentially conflicting climate-driven mechanisms. Interestingly, broad-scale temporal trends showed contrasting bioaccumulation patterns&mdash, increasing in Northern Pike and decreasing in Walleye, thus suggesting species-specific ecological differences also matter. Overall, by taking a scale-specific approach, the study highlights the overwhelming influence of fine-scale variations and their interactions on mercury bioaccumulation, while at broad-scale the mercury bioaccumulation trends are summarized within a climate change context.

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.

Previous studies on the effects of stocking density on the behaviour of African catfish have shown that at low densities, especially directly after restocking of tanks, increased aggression might occur. This aggression may directly affect the welfare of the fish. In addition, the resulting skin damage may also lead to the release of chemical alarm cues from the skin of the fish, possibly acting as a secondary stressor in a farming situation. Moreover, in a recirculation aquaculture system, a build-up of chemical alarm cues might occur. The objective of this study was to examine the effects of a single chemical alarm cue administration on the behaviour and growth performance of group-housed African catfish. Furthermore, the effects of a single passage over a biofilter on the behavioural response of African catfish to chemical alarm cues were tested. Although African catfish responded to chemical alarm cues with a short-term 35% increase in the number of active fish, no long-term effects were observed on both behaviour and growth performance of the fish. Furthermore, the results indicated that a single passage over a biofilter did not strongly alter the response of African catfish to the alarm cue, indicated by a 25% increase in the number of active fish. In conclusion, the results of this study indicate that chemical alarm cues, at the concentration applied in this study, cannot be considered a stressor for African catfish, although the effects of higher cue concentrations need further study. In addition, further study into the effects of chemical alarm cues on other, non-predatory, farmed fish is recommended.

The biogeography of freshwater fish species during and after late-Pleistocene glaciations relate to how these species are genetically organized today, and the management of these often disjunct populations. Debate exists concerning the biogeography and routes of dispersal for Northern pike (Esox lucius) after the last glaciation. A hypothesis to account for the relatively low modern genetic diversity for E. lucius is post-glacial radiation from refugia, including lakes from within the un-glaciated portions of eastern Beringia. We report the remains of a Northern pike (E. cf. lucius) skull, including bones, teeth, bone collagen and ancient DNA. The remains were preserved at a depth of between 440 and 446 cm in a 670 cm long core of sediment from Quartz Lake, which initiated at ∼11,200 cal yr BP in interior Alaska. A calibrated accelerator mass spectrometer (AMS) radiocarbon age of the collagen extracted from the preserved bones indicated that the organism was dated to 8820 cal yr BP and is bracketed by AMS values from analyses of terrestrial plant macrofossils, avoiding any potential aquatic reservoir effect that could have influenced the radiocarbon age of the bones. Scanning electron microscope images of the specimen show the hinged tooth anatomy typically of E. lucius. Molar C:N (3.5, 1σ = 0.1) value of the collagen from the specimen indicated well-preserved collagen and its mean stable nitrogen isotope value is consistent with the known predatory feeding ecology of E. lucius. Ancient DNA in the bones showed that the specimen was identical to modern E. lucius. Our record of E. lucius from interior Alaska is consistent with a biogeographic scenario involving rapid dispersal of this species from glacial refugia in the northern hemisphere after the last glaciation.