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Information on catch and effort of recreational angling in mixed‐use fisheries (co‐exploited by commercial and recreational fishers) is often scarce, preventing the application of data‐rich stock assessments typically performed for industrialised commercial fisheries. This study shows how data‐poor stock assessment methods developed for marine fisheries, particularly a model class labelled as “catch‐only” models (COMs), offer a possible solution. As a case study, COMs are used to assess a northern pike Esox lucius L. stock around the German Baltic island of Rügen. Multiple COMs were fitted to a time series of total pike removals, and their outputs were used as explanatory variables in ensemble models. The stock was found to be fully exploited and currently declining. This study highlights the potential for using COMs to determine status of previously unassessed coastal and freshwater stocks facing recreational fishing pressure and demonstrates how incorporating recreational removals is crucial for achieving reliable insights into the status of mixed‐use stocks. European Maritime Fisheries Fund (EMFF) of the EU and the State of Mecklenburg‐Vorpommern (Germany)

Intraspecific niche differentiation can contribute to population persistence in changing environments. Following declines in large predatory fish, eutrophication, and climate change, there has been a major increase in the abundance of threespine stickleback (Gasterosteus aculeatus) in the Baltic Sea. Two morphotype groups with different levels of body armor—completely plated and incompletely plated—are common in coastal Baltic Sea habitats. The morphotypes are similar in shape, size, and other morphological characteristics and live as one apparently intermixed population. Variation in resource use between the groups could indicate a degree of niche segregation that could aid population persistence in the face of further environmental change. To assess whether morphotypes exhibit niche segregation associated with resource and/or habitat exploitation and predator avoidance, we conducted a field survey of stickleback morphotypes, and biotic and abiotic ecosystem structure, in two habitat types within shallow coastal bays in the Baltic Sea: deeper central waters and shallow near‐shore waters. In the deeper waters, the proportion of completely plated stickleback was greater in habitats with greater biomass of two piscivorous fish: perch (Perca fluviatilis) and pike (Esox lucius). In the shallow waters, the proportion of completely plated stickleback was greater in habitats with greater coverage of habitat‐forming vegetation. Our results suggest niche segregation between morphotypes, which may contribute to the continued success of stickleback in coastal Baltic Sea habitats., Following declines in large predatory fish, eutrophication, and climate change, there has been a major increase in the abundance of two morphotype groups of threespine stickleback (Gasterosteus aculeatus) in the Baltic Sea. In deeper coastal waters, the proportion of completely plated stickleback was greater in habitats with greater biomass of two piscivorous fish: perch (Perca fluviatilis) and pike (Esox lucius) and in shallow coastal waters. The proportion of completely plated stickleback was greater in habitats with greater percent coverage of habitat‐forming vegetation. Our results suggest niche segregation between morphotypes, which may contribute to the continued success of stickleback in coastal Baltic Sea habitats.

Inroduction. Changes in the species composition and population size of freshwater fihes are determined by deformations of the habitat in Ukraine and are characterized by a pronounced negative dynamics. Deformation of ichthyofauna is caused by extinction of a number of aboriginal fihes and emergence of alien species.Aim. To establish the current species composition and relative population size of fih species in the Oster River and the dynamics of these characteristics over a 50-yearlong period in the context of negative transformations of the ichthyofauna of the rivers of Ukraine.Results. Comparative study of the species composition and relative population size of fih species in the fih catches by small-river fihing gear in the Oster river was conducted during the period since 1971-72 till 2019. During this time considerable changes in species composition took place. The following species disappeared from the catches: Leuciscus leuciscus, Leuciscus idus, Chondrostoma nasus, Carassius carassius, Blicca bojerkna and Neogobius flviatilis, they were replaced by three invasion species (Percottus glenni, Pseudorasbora parva, Proterorhinus marmoratus). The changes in the dominant species took place, thus, in the Oster ichthyocene the species with the short life cycle started to prevail, the share of invasion species increased from 1.4 to 37%, and the share of initial marine fih among them is very low. The comparison with the river systems of the Stugna and the Lower Dnipro represents the fact that the mentioned negative tendencies are common for the whole Dnipro system, however the situation in the Lower Dnipro must be considered less critical than in the Oster and the Stugna rivers. The fact that the catches of industrial fih decreased more than 10 times in the Desna river during this period of time gives the reason to consider that the dramatic reduction of the absolute number of their populations occurred in the Oster river as well. That is why even relatively safe species (Abramis brama, Rutilus ritilus, Scardinius erythrophthalmus, Esox lucius, Perca flviatilis, Alburnus alburnus, Squalius cephalus) are in critical situation in this transformed river system.Сonclusions. In the ichthyocene of the Oster River over a 50-year period there have been signifiant changes in species composition, relative and absolute population numbers associated with the extinction and reduction of population sizes of rheophilic species and fih that are adapted to life in the flodplain. The modern Oster River is dominated by invasive and short-cycle species. The obtained tendencies in the dynamics of the ichthyofauna of the Oster River are similar to the nature of changes in other river systems of Ukraine, although they occur at a faster pace, which is characteristic of the smallest plain rivers.

Concentrations of total mercury (THg) in the axial musculature of northern pike (Esox lucius), walleye (Sander vitreus), lake whitefish (Coregonus clupeaformis), and sauger (Sander canadensis) were analyzed from several regions of Lake Winnipeg and three main inflows, focusing on Mossy Bay in the North Basin of the lake. Length-standardized means (standard means) indicated THg concentrations in pike of 550 mm fork length (0.44 ppm) and walleye measuring 400 mm (0.38 ppm) from Mossy Bay were highest in 1971 and declined to 0.13 and 0.22 ppm, respectively, by 1974. Standard means of both these species have been similar since 2010 (walleye; approximately 0.11 ppm) and 2013 (pike; approximately 0.17 ppm), except for a significant increase to 0.15 ppm in walleye in 2019, potentially related to concurrent changes in trophic interactions with invasive fish and invertebrate species. Standard means of 300 mm long sauger (0.23–0.29 ppm) did not decline significantly between 1971 and 1974, and those of 350 mm long whitefish ranged from 0.006 to 0.028 ppm from 1983 to 2016 with no clear temporal trend. In concert with data from other areas of Lake Winnipeg and the three inflows for 2000–2019, these results indicate higher contemporary concentrations in the South Basin of the lake. This latitudinal gradient in fish THg is assumed to be a result of continuous mercury inputs from the Red and the Winnipeg River, whereas a former point source of industrial mercury in the Saskatchewan River is no longer relevant. According to human consumption limits based on tolerable daily intake calculations and current THg concentrations of fish from Mossy Bay, substantial quantities of whitefish, pike, and walleye fish can be safely eaten.

The aim of this research is to compare the average individual weight of the four most important fish species in fisheries (Cyprinus carpio, Silurus glanis, Esox lucius and Sander lucioperca), caught by angling in different parts of the Sava River catchment in Croatia. Significant differences were expressed between the counties along the Sava River catchment, with the smallest specimens (p˂0.05) caught in Vukovar-Sirmium county. Artisanal fishery is mostly developed in that part of the Sava River, along with angling in both countries – Croatia and Bosnia and Herzegovina. Furthermore, the inflow of the River Bosna passes through urban and industrial cities of Bosnia and Herzegovina. It can be concluded that implementing recreational fishing surveys with the specific aim of obtaining data on average individual weight is useful for fishery management.

Northern pike (Esox lucius) was widely distributed in the high latitudes of the northern hemisphere. In China, northern pike was originally distributed only in the upper reaches of the Irtysh River in Xinjiang and has appeared in many water bodies outside the Irtysh River Basin in Northern Xinjiang. A total of four populations were collected from north to south in Xinjiang, including Irtysh River (RIR), Ulungu Lake (LUL), a small lake nearby Ulungu River (LJD), and Bosten Lake (LBO). We estimated population genomic parameters, performed gene flow analysis, and estimated the effective population size of each population. The proportion of individuals with high inbreeding coefficient (F ≥ 0.0625) accounted for 36.4% (44/121) of all sequenced individuals, approximately 4.5% (1/22) in LUL, 25.9% (7/27) in LBO, 42.9% (18/42) in RIR, and 60% (18/30) in LJD. RIR had the highest mean of genomic relatedness (coancestry coefficient = 0.025 ± 0.040, IBD = 0.036 ± 0.078). Gene flow results showed that the population spreading was from RIR into two branches, one was LBO, and the other continued to split into LUL and LJD, and migration signal from LBO to LUL was detected. Our results suggested that the extinction risk of northern pike was very low in Xinjiang of China, and the controlled capture fishery of northern pike could be developed reasonably.

The premise of catch-and-release angling is that most fish survive fisheries interactions. Therefore, it is common for anglers, management agencies, and other organizations to share information on handling practices and other strategies that are believed to improve fish welfare and survival. Recent media coverage has sensationalized the use of carbonated beverages to treat bleeding fish, an intervention that is purported to stop bleeding but has yet to be validated scientifically. We captured Northern Pike (Esox lucius) via hook and line, experimentally injured their gills in a standardized manner, and treated them with either Mountain Dew, Coca Cola, or carbonated lake water and observed the duration and intensity of bleeding, as well as overall blood loss (using gill colour as a proxy) while the fish was held in a lake water bath. As a control, we had a group of experimentally injured fish that did not have liquid poured over their gills before the observation period. All treatments and the control were conducted at two different water temperatures (11-18 °C and 24-27 °C) to determine if the effects of pouring carbonated beverages over injured gills is temperature dependent. When compared to the control, we found that the duration and intensity of bleeding increased regardless of the type of carbonated beverages used in this study, and there was no effect of water temperature. Use of chilled versus ambient temperature beverages similarly had no influence on outcomes. As such, there is no scientific evidence to support the use of carbonated beverages for reducing or stopping blood loss for fish that have had their gills injured during recreational angling based on the context studied here. This study reinforced the need to scientifically test angler anecdotes and theories when it comes to best practices for catch-and-release fishing.

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.

The oriental river shrimp (Macrobrachium nipponense), an exotic species, has been well adapted and dispersed in Iranian freshwater ecosystems. The feeding ecology and trophic level of this species in the Anzali Wetland, southwestern Caspian Sea, was investigated by both a traditional method and the carbon and nitrogen stable isotope technique. The highest feeding index (FI) was seen in mollusca at 35.9%, followed by detritus at 31.3%. While no significant differences were observed in the preference of food items among different seasons. A Bayesian mixing model was used to estimate the contributions of the stomach contents to the isotopic signature of M. nipponense. According to mixing model results, M. nipponense obtain food from gastropod, fish, shrimp (as cannibalism), insecta, other crustacea, aquatic plants, worms, zooplankton, and unrecognizable content (as detritus) with 30.6%, 16.8%, 12.7%, 5.2%, 4.3%, 3.5%, 0.6%, 0.3%, and 25.9%, respectively. M. nipponense showed a high δ15N and its trophic level (TL = 3.38) was at the top of the Anzali Wetland food web, close to commercially important fish species such as pike, Esox Lucius. Being at this trophic level, this shrimp is expected to possess high nutritional quality for human consumption.

Catch‐per‐unit‐effort is often used as an approximation of population size. However, for the management and conservation of populations, information about the number of individuals is fundamental. Pike, Esox lucius L., is a popular fish species for recreational anglers. In this study, data in the form of journal keeping by anglers were used: date; place; and photographs of the captured fish; pike were identified based on their natural markings in combination with length measurements. The data were analysed by spatial capture–recapture (SCR) models. Results showed that a small and densely vegetated lake (6.7 ha) in south‐eastern Sweden had a population size of 91 (±22 SE) pike ≥60 cm. On one occasion, 10 individuals were caught, that is 11.1% of the population, highlighting that angling may potentially have a substantial impact on the population size. Hopefully, this study can inspire angler–manager collaborations for conservation of fish stocks.

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.

A comprehensive understanding of how interactions between catchments and downstream lakes affect fish growth rate is lacking for many species and systems, yet is necessary for predicting impacts of environmental change on productivity of freshwater fish populations. We investigated among-lake variability in growth rate of Northern Pike (Esox lucius), a fish species of widespread subsistence and commercial importance. Northern Pike were captured from 11 subarctic lakes that span 60,000 km2 and four ecoregions in the Dehcho Region of the Northwest Territories, Canada. Growth rates were related to stable isotope ratios and to lake and catchment physicochemistry. Growth, modelled using increment widths (n = 2953) measured on cleithra (n = 432), was significantly slower (p

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.

The effect of the temperature on the spawining phenology of the fish assemblages and the role of the exogenious and endogenious factors in temporal niche overlaping of fish assemblages were investigated in different water body types of the Dnipro River basin within the “Dnipro-Orilskiy" Nature Reserve (Ukraine in the years 1997-2018. The period of spawning (beginning and end) of the Blicca bjoerkna, Carassius gibelio, Scardinius erythrophthalmus, Abramis brama, Perca fluviatilis, Rutilus rutilus, and Esox lucius was estimated. Community niche overlap was estimated by calculating the pairwise Pianka index. The temperature was discused as an exogenous factor effecting on the fishes spawn timing. From 1998 to 2018 the average temperature in the study area continued to rise. The annual course of the temperature takes the sinusoidal waveform, which can be described by a fourth-degree polynomial. The residuals of the corresponding polynomial trend are specific for each year. The regular components of the residuals variation were revealed by dbMEM-variables. Fishes spawning occurred within the narrow water temperature ranges, which are specific for each fish species. There are statistically significant differences between ponds in water temperatures at which the spawning start occurred. The water temperature at which the spawning start occurred is linearly dependent on the time of the spawning start. There were temporal overlaps between spawning of the different species. The deviation of the Pianka mean niche overlap indexes from random alternatives does not depend on the habitat type. The decreasing trend of the Pianka mean niche overlap indexes was observed during the study period. There is no statistically significant correlation between the average annual air temperature and the level of the niche overlap. The degree of niche overlapping depends on broad-, medium-, and fine-scale temperature patterns.

A decline in northern pike (Esox lucius) abundance in the upper St. Lawrence River is thought to be related to hybrid cattail invasion and disruption of natural water-level periodicity leading to spawning and nursery habitat degradation. Channel-connectivity and spawning-pool excavations were implemented in a tributary to the upper St. Lawrence River to enhance spawning and nursery site complexity. We assessed the effects of excavations on pike by comparing percent survival (from advanced larvae to fall emigration), water quality factors, and larval diets at excavated and non-excavated sites. Lab-reared pike, 16 days post-hatch and 14 mm in length, were released at three site types: non-excavated, spawning pool, and channel. Percent survival in channels and spawning pools after a minimum of 26 days was nearly identical (2.4%), and greater than non-excavated sites (0.13%). Spawning pools displayed higher water temperatures and dissolved-oxygen concentrations than either channel or non-excavated sites. Larvae primarily consumed zooplankton and diets of larvae from spawning pools and non-excavated sites were similar, whereas larvae in channels consumed different taxa. Differences in diet were not reflected in survival, suggesting larvae were not prey-limited. Observed differences in percent survival among sites demonstrates created spawning pool and channel-connectivity sites enhanced conditions for pike.

We examined if restored connectivity within a freshwater coastal wetland, dominated by invasive Typha x glauca, can increase the ecological condition, as assessed by a Floristic Quality Index (FQI), and improve habitat to benefit northern pike (Esox lucius) reproduction. Using an aquatic excavator to simulate natural rivulets, channels were excavated within monotypic Typha stands to create a link to upslope wet meadows. Following excavation, we found channels were rapidly colonized by native vegetation, resulting in higher FQI scores and percent cover of short-emergent vegetation compared to natural reference channels. Excavated channels also retained a community of vegetation with more native species five years following excavation. Fish emigration surveys revealed that excavated channels functioned as early-life habitat and had higher abundances of young-of-the-year northern pike than reference channels. A seven-year dataset (2011–2017) of northern pike outmigration at channel connectivity and pothole complexes was strongly related to water level conditions during the spawning and emigration period. We conclude that the enhancement methods increased the ecological condition of the vegetation community (i.e., FQI) suppressed by invasive Typha while the associated northern pike reproductive success highlights the importance of habitat connectivity and periodic flooding within coastal wetlands.

The brackish Baltic Sea is inhabited by several primary freshwater fish species. The pike (Esox lucius) is the largest of these species and acts as the top predator in the marine food web of the Baltic Sea. Prior studies have shown the importance of brackish bays as spawning grounds for the Baltic Sea pike. To identify the spawning habitat of the pike population around the island of Rugen (southern Baltic Sea)—either freshwater streams flowing into the Baltic or brackish lagoons of the Baltic—we used the ICPMS method to quantify the strontium to calcium (Sr:Ca) ratios in sagittal otoliths of 79 adult pike. In total, only six per cent (n = 5) of the analysed Baltic Sea individuals were classified as being of freshwater origin. Ninety‐four per cent (n = 74) of analysed pike were classified as having hatched in brackish water. Compared to previous studies on Baltic Sea pike, our sample shows a high proportion of pike that is of brackish origin: this highlights the adaptive capacity of this species with regard to its ability to reproduce in different salinities. Our data highlight the importance of brackish bays for pike reproduction in the southern Baltic Sea, even if pike live on the upper limit of their salinity tolerance in this area. This is an important finding considering decreases in commercial catches in the last 30 years indicating a population decline supposedly related to recruitment failures.

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Pike (Esox lucius) occupy coastal streams and rivers of the Baltic Sea, where they attain large sizes (>5 kg). These large sizes are perhaps due to the fact that they can tolerate relatively high salinities and can thus forage in the nearby more productive brackish environments. In an attempt to quantify the extent to which pike utilise brackish environments, and to provide some insight into the underlying causes for brackish water migrations, we tagged 30 pike from a western Baltic river with acoustic transmitters and were able to track 21 individuals for 1 year. Based on experienced from local anglers, this population was assumed to be brackish in nature, where individuals underwent freshwater migrations to spawn. Our findings however suggest that the smallest and most active individuals make short exits into brackish waters and do so on rare occasions. Our results further indicate that neither sex nor size is related to activity level. We suggest that these patterns reflect two distinct behaviours—active and passive—and that large pike can be supported by the food availability in the river, without the need to venture into coastal zones, thus defying the conventional view that Baltic pike are all brackish in nature.

Structural complexity is known to influence prey behaviour, mortality and population structure, but the effects on predators have received less attention. We tested whether contrasting structural complexity in two newly colonised lakes (low structural complexity lake—LSC; high structural complexity—HSC) was associated with contrasting behaviour in an aquatic apex predator, Northern pike (Esox lucius; hereafter pike) present in the lakes. Behaviour of pike was studied with whole-lake acoustic telemetry tracking, supplemented by stable isotope analysis of pike prey utilization and survey fishing data on the prey fish community. Pike displayed increased activity, space use, individual growth as well as behavioural differentiation and spent more time in open waters in the LSC lake. Despite observed differences between lakes, stable isotopes analyses indicated a high dependency on littoral food sources in both lakes. We concluded that pike in the HSC lake displayed a behaviour consistent with a prevalent ambush predation behaviour, whereas the higher activity and larger space use in the LSC lake indicated a transition to more active search behaviour. It could lead to increased prey encounter and cause better growth in the LSC lake. Our study demonstrated how differences in structural complexity mediated prominent changes in the foraging behaviour of an apex predator, which in turn may have effects on the prey community.

Fish stocking is commonly used to enhance recreational fisheries; however, the side effects of stocking at the ecosystem scale have rarely been tested. Avian predators, because of their high position in food webs, can serve as indicators of environmental change, including prey-species abundance. The scope of this study was to understand whether recent changes in northern pike (Esox lucius) stocking on a national scale could cascade through food webs and significantly shape the diet of a predator––the white-tailed eagle (Haliaeetus albicilla). The stocking of pike in state-owned waters in Lithuania has increased two- to threefold since 2005. A significant increase in the proportion of pike in the breeding-season diet of the white-tailed eagle has also been observed. We explain the increase in pike in the predator’s diet as being a causal consequence of pike stocking. We encourage consideration of the entire food web when launching/terminating recreational fish stocking programmes.

The relentless role of invasive species in the extinction of native biota requires predictions of ecosystem vulnerability to inform proactive management strategies. The worldwide invasion and range expansion of predatory northern pike (Esox lucius) has been linked to the decline of native fishes and tools are needed to predict the vulnerability of habitats to invasion over broad geographic scales. To address this need, we coupled an intrinsic potential habitat modelling approach with a Bayesian network to evaluate the vulnerability of five culturally and economically vital species of Pacific salmon (Oncorhynchusspp.) to invasion by northern pike. This study was conducted along 22,875 stream km in the Southcentral region of Alaska, USA. Pink salmon (O.gorbuscha) were the most vulnerable species, with 15.2% (2,458 km) of their calculated extent identified as “highly” vulnerable, followed closely by chum salmon (O.keta, 14.8%; 2,557 km) and coho salmon (O.kisutch, 14.7%; 2,536 km). Moreover, all five Pacific salmon species were highly vulnerable in 1,001 stream km of shared habitat. This simple to implement, adaptable, and cost-effective framework will allow prioritizing habitats for early detection and monitoring of invading northern pike.

Esox lucius L. (northern pike) inhabits river, lake, and brackish water ecosystems of the temperate region in North America and Eurasia. In many areas of its native range, the species is confronted with human-induced disturbances that can affect local and regional genetic diversity. Conservation of genetic diversity constitutes an important policy objective. Therefore, we investigated how the genetic population structure and patterns suggestive of hybridisation among subpopulations from different catchments vary with the type of ecosystem (river vs. lake) and the integrity of the ecosystem assessed using ecological status assessments of the European Water Framework Directive and indices of the wetland quality (in rivers) and trophic state (in lakes). In total, 1,384 pike samples from the North, Baltic, and Black Sea drainages in Germany were analysed. We differentiated between metapopulations from each hydrogeographic region using genotypes from 15 microsatellites and mitochondrial cyt b sequences. Individual populations showed signs of genetic admixture ranging from almost zero to complete replacement by foreign genotypes originating from different catchments. Hierarchical general linear modelling revealed a significantly positive association of the degree of genetic hybridisation with decreasing ecological status. An eroded population substructure indicative of hybridisation was also significantly elevated in rivers relative to lakes and in heavily modified water bodies. Results were consistent when using ecological status assessments by the Water Framework Directive and an alternative environmental quality index based on the health of wetlands in rivers and trophic state in lakes. We speculate that macrophyte loss through loss of wetlands or in response to eutrophication fostered genetic mixing among different subpopulations, which were brought into secondary contact either due to stocking or due to natural dispersal from foreign catchments. Our findings suggest that anthropogenic alteration of freshwater ecosystems may influence the genetic structures present in a freshwater top piscivore at both local and regional scales. ogenic alteration of freshwater ecosystems may influence the genetic structures present in a freshwater top piscivore at both local and regional scales., Funding of the current work was granted by the German Federal Ministry of Education and Research within the project Besatzfisch in the Program for Social-Ecological Research (Grant No. 01UU0907 to R.A.). R.A. received further funding through the European Union (European Maritime and Fisheries Fund) and the State of Mecklenburg-Vorpommern (Germany; Grant MV-I.18-LM-004, B 730117000069) for the project Boddenhecht. A.N. was supported through an ERC starting grant EVOLMAPPING. J.A. was supported by a Ramon y Cajal Grant (grant no. RYC2018-024488-I) and the intramural research project JSATS (grant no. PIE 202030E002) funded by the Spanish Ministry of Science and Innovation, and the Spanish National Research Council. Open access funding enabled and organized by ProjektDEAL.

The practice within recreational fisheries to release captured fish back to the wild, known as catch-and-release (C&R), is an increasingly important strategy to protect fish stocks from overexploitation. However, C&R is a stressor and since animal reproduction is particularly sensitive to stress there is reason to suspect that such a practice induces sublethal fitness consequences. Here, we investigated whether and how C&R fishing influenced the reproductive potential in an anadromous population of Northern pike (Esox lucius). First, female pike were exposed to authentic C&R using rod-and-reel fishing in a coastal foraging habitat prior to the spawning period. Next, we observed the migration to the freshwater spawning habitat and compared both the timing of arrival and maturity stage between C&R-treated and control individuals. Finally, to evaluate effects on the quality and viability of eggs we stripped captured control and recaptured C&R-treated females, measured egg dry mass to assess nutrient content, conducted artificial fertilisations and incubated eggs in a controlled laboratory experiment. We found no evidence of C&R causing alterations in either arrival time, maturity stage, or the quality and viability of fertilised eggs. In combination, our results suggest that long-term effects of C&R-induced stress on key reproductive traits of pike, if any, are minor.

Predation is one of the main drivers of animal grouping. However, predation risk is not shared equally within groups. Despite long-standing theoretical work and strong empirical insights, very little work has examined the real-time dynamics of predators attacking groups of prey nor quantified the features that predict prey predation risk and survival. Here we used high resolution tracking of Northern pike (Esox lucius) attacking large schools of prey (golden shiner fish, Notemigonus crysoleucas) to get a mechanistic quantification of predation at every stage of the attack. We found that pike tended to attack the groups head-on, but did so stealthily, often striking only when partly within the school. From the predator’s perspective, relative position to the prey was the most important feature, with shiners very close and directly in front of the pike being most at risk. From the prey’s perspective, we found that central individuals, with relatively low local neighbour density and alignment, face increased risk. While the majority of attacks were successful, escape was associated with individuals achieving high maximum movement speeds, relative to others. Our study highlights both predator attack strategy and prey behaviour are key factors underlying the predation risk of grouping prey, and our results contradict the long-held assertion arising from Hamilton’s “selfish herd” concept that central positions are safer. To ultimately understand the cost-benefits that underly the evolution of animal grouping it is thus key to consider the multifaceted, and dynamical, nature of predator-prey relationships.

International audience; ContextHydrological and land use changes for human purposes, have resulted in the increased fragmentation of river landscapes and the loss of aquatic habitats, leading to profound changes in fish diversity and productivity.ObjectivesIn the fluvial Lake Saint-Pierre (St. Lawrence River, Canada), we studied how agricultural practices and water-flow regulation have impacted the area and connectivity of spawning habitats of northern pike (Esox lucius).MethodsNorthern pike spawning and nursery habitats were modelled over a 49-year period (1965–2013) to estimate effective spawning area under four contrasting hydrological conditions.ResultsSimulations coupled with land cover analyses revealed that natural flow conditions historically favourable for fish reproduction (high and stable water flows) have been lost due to human activities. The highest potential for reproduction and habitat connectivity have been lost due to (1) intensive agriculture and ploughing of natural vegetation in the upper floodplain that overlaps suitable spawning areas for northern pike, and (2) flow regulation that has lowered and shortened spring floods and dried spawning grounds more frequently.ConclusionsTo restore the St. Lawrence River functions, we propose to reconvert the portion of the floodplains that is vital to fish, but is currently used by intensive agriculture, into natural wetlands or perennial crops and to restore a more natural flow regime by extending the duration of floods between spawning and nursery periods. The highest priority for habitat restoration should target the most effective and recurrent spawning habitats, ditch and stream networks, and connected managed wetlands.

Lake-dwelling fish that form species pairs/flocks characterized by body size divergence are important model systems for speciation research. While several sources of divergent selection have been identified in these systems, their importance for driving the speciation process remains elusive. A major problem is that in retrospect, we cannot distinguish selection pressures that initiated divergence from those acting later in the process. To address this issue, we reconstructed the initial stages of speciation in European whitefish (Coregonus lavaretus) using data from 357 populations of varying age (26-10 000 years). We find that whitefish speciation is driven by a large-growing predator, the northern pike (Esox lucius). Pike initiates divergence by causing a largely plastic differentiation into benthic giants and pelagic dwarfs; ecotypes that will subsequently develop partial reproductive isolation and heritable differences in gill raker number. Using an eco-evolutionary model, we demonstrate how pike’s habitat specificity and large gape size are critical for imposing a between-habitat trade-off, causing prey to mature in a safer place or at a safer size. Thereby, we propose a novel mechanism for how predators may cause dwarf/giant speciation in lake-dwelling fish species.

Maximising growth during the months after hatching is critical for fishes to surpass predator gape limits, make ontogenetic shifts in diet and increase overwinter survival. Constraints on growth during early life stages can have population and community‐level implications, yet are rarely addressed by habitat restoration or fishery management efforts. To assess environmental controls on larval growth rates, we sampled adult and young‐of‐year (YOY) northern pike (Esox lucius) emigrating from spawning grounds in six tributaries of Lake Michigan. We found that YOY growth rates differed significantly among natal streams, in parallel with variation in hatch date and water temperatures. Within each stream, individual growth rates were positively associated with trophic position, as measured by nitrogen isotope enrichment and gut contents. Earlier hatching individuals also grew faster, but the strength of this relationship varied with warming water temperatures (i.e., 21–27°C). Ageing of adult spawners indicated no differences among streams in average growth rates later in life. Our findings suggest that restoration efforts should replicate ecological conditions found in habitats producing large numbers of fast‐growing YOY. Key habitat attributes include adequate water level throughout the spring spawning season, and stable, warm water temperature conditions. Providing these environments during early life history could enhance the robustness of northern pike populations by boosting fitness from hatching to emigration to adult habitats.

Climate change is expected to affect the flow regime, cause loss of habitat, change community composition and behavioural habits of fish. This study assessed the impact of climate change on ecologically relevant streamflow conditions for fish migration and spawning in the Vistula and the Odra river basins. Streamflow simulations obtained with the Soil and Water Assessment Tool (SWAT) for the historical period and two future horizons were driven by nine bias‐corrected EURO‐CORDEX Regional Climate Models under two greenhouse gas concentration trajectories. This study identified a subset of Indicators of Hydrological Alteration (IHA) that are relevant for pike, Esox lucius L., chub, Squalius cephalus (L.), and Atlantic salmon, Salmo salar L. IHA indicators were calculated and compared for different scenarios. An index‐based framework identified that all considered species will be impacted by climate change, with Atlantic salmon facing the largest impact. The model's uncertainty was addressed through an aggregation method that assessed inconsistencies in the model's response.

This contribution directly relates to the paper published by Wheeler in 1978 entitled ‘Why were there no fish remains at Star Carr?’. Star Carr is arguably the richest, most studied and re-interpreted Mesolithic site in Europe but the lack of fish remains has continued to vex scholars. Judging from other materials, the preservation conditions at the site in the late 1940s/early 1950s should have been good enough to permit the survival of fish remains, and particularly dentaries of the northern pike ( Esox lucius L., 1758) as found on other European sites of this age. The lack of evidence has therefore been attributed to a paucity of fish in the lake. However, new research has provided multiple lines of evidence, which not only demonstrate the presence of fish, but also provide evidence for the species present, data on how and where fish were being processed on site, and interpretations for the fishing methods that might have been used. This study demonstrates that an integrated approach using a range of methods at landscape, site and microscopic scales of analysis can elucidate such questions. In addition, it demonstrates that in future studies, even in cases where physical remains are lacking, forensic techniques hold significant potential.

Data on the distribution, size and age structure, and growth of the pike Esox lucius collected in the Rybinsk Reservoir in 1953–2015 were analyzed. The main factors that determine the population size of the pike and dynamics of its structural indices are the intensity of the fishery and changes in the temperature regime of the reservoir. Since the mid-1990s, under the influence of high harvesting load, the population of the pike decreased and its age structure changed toward the predominance of individuals of younger age groups. As a result of climate warming, the water column warming of the Rybinsk Reservoir in the 2000s affected the growth character of large individuals. The ratio of phenotypes with small and high number of vertebrae in the first year depends on the water temperature during embryonic development. Among the large individuals preferring deep-water areas with a lower temperature, approximately 50% have 61–62 vertebrae in the axial skeleton. It is suggested that, with climate warming and the development of pike spawn at a higher temperature, the portion of individuals with a high number of vertebrae may decrease, which will lead to a decrease in the number of recruits of the deep-water part of the population.

Human-induced mercury (Hg) contamination is of global concern and its effects on wildlife remain of high concern, especially in environmental hotspots such as inland aquatic ecosystems. Mercury biomagnifies through the food web resulting in high exposure in apex predators, such as the white-tailed eagle (Haliaeetus albicilla), making them excellent sentinel species for environmental Hg contamination. An expanding population of white-tailed eagles is inhabiting a sparsely populated inland area in Lapland, northern Finland, mainly around two large reservoirs flooded 50 years ago. As previous preliminary work revealed elevated Hg levels in this population, we measured Hg exposure along with dietary proxies (δ13C and δ15N) in body feathers collected from white-tailed eagle nestlings in this area between 2007 and 2018. Mercury concentrations were investigated in relation to territory characteristics, proximity to the reservoirs and dietary ecology as potential driving factors of Hg contamination. Mercury concentrations in the nestlings (4.97–31.02 μg g−1 dw) were elevated, compared to earlier reported values in nestlings from the Finnish Baltic coast, and exceeded normal background levels (≤5.00 μg g−1) while remaining below the tentative threshold of elevated risk for Hg exposure mediated health effect (>40.00 μg g−1). The main drivers of Hg contamination were trophic position (proxied by δ15N), the dietary proportion of the predatory fish pike (Esox lucius), and the vicinity to the Porttipahta reservoir. We also identified a potential evolutionary trap, as increased intake of the preferred prey, pike, increases exposure. All in all, we present results for poorly understood freshwater lake environments and show that more efforts should be dedicated to further unravel potentially complex pathways of Hg exposure to wildlife.

The Quark Archipelago, in the northernmost part of the Baltic Sea, provides valuable reproduction areas for coastal fish such as pike (Esox lucius L.). Here, pike stocks are still in good condition compared to the southern Baltic Sea, where pike stocks have declined. Hence, this area offers a unique opportunity to study the underlying causes of successful pike reproduction and to identify a productive pike reproduction habitat. This study suggests that the key factor defining suitable reproduction habitats for pike in the Quark Archipelago was the level of shelter. The more sheltered a habitat, in terms of morphology and level of isolation of the site and the site's exposure to wave action and shoreline density, the better the reproduction habitat was for pike. Gloes, which are the most isolated successional stages of coastal lagoons, represented the most sheltered habitat type and showed the highest pike larvae abundance. Additionally, sheltered bays and flads, less isolated successional stages of coastal lagoons compared to gloes, were also used as reproduction habitats by pike but with a lower abundance of larvae compared to the gloes. Gradually increasing salinity and the occurrence of sticklebacks (Gasterosteidae) reflected the morphology of the site. Pike larvae were observed in locations without sticklebacks and with low salinity. Pike larvae were found mostly amongst fallen reed (Phragmites australis) and in areas with moss (Fontinalis sp.) growth, which supports earlier findings. This study provides valuable new knowledge on larval pike habitats and highlights the significance of sheltered bays and gloes in the northern Baltic Sea. The information can be utilised for planning conservation and the restoration of pike reproduction habitats to protect and restore pike populations on both a regional and larger scale.

Standardised angler diaries could produce useful proxy data for assessing fish population density and size distribution, but few rigorous studies about their utility exist. We use 62 years of angling diary data (1949–2010), from a large mesotrophic lake, to investigate population structure (abundance, mean size and record size) of European perch ( Perca fluviatilis L.) in relation to the impact of three commercial fishers with different fishing strategies, pike ( Esox lucius L.) predation and temperature. We found that anglers’ harvest rates of perch varied by a factor of 10 over time, indicating large variation in population abundance over decadal time scales. Our statistical analysis revealed that the anglers’ harvest rates of perch were related to pike CPUE (proxy of pike predation), temperature and commercial fishing directly through the harvest of perch and indirectly through the harvest of pike, the top predator of the lake. The size distribution and growth rates of perch caught by anglers also changed substantially during the study period, most likely controlled by density-dependent mechanisms as well as size-selective commercial harvest. The effect of selective harvest on size-structure was stronger than ecological density dependence. We conclude that commercial harvesting may exert strong impacts on the quality of the angling experiences, at least in the studied case. Moreover, our work showcases the value of detailed angler diaries to study and monitor changes in freshwater fish populations, but it also underlines the need for supplementary data on biotic and abiotic factors to reach the full potential of angler diary data.

Body condition indices can be used to investigate fish response to biological or environmental change. We used quantile regression to analyze a 32-year weight–length data set (1982–2013) for smallmouth bass (Micropterus dolomieu) and northern pike (Esox lucius) from the St. Lawrence River to examine changes in condition (weight-at-length) during three ecologically distinct time periods. Condition of both species varied among time periods, with smallmouth bass experiencing greater proportional changes than northern pike. Smallmouth bass weight-at-length was greatest during 2005–2013 and lowest during 1982–1992. Northern pike weight-at-length was greatest during 2005–2013 for quantiles ≥0.5, similar between the 1982–1992 and 2005–2013 time periods for lower quantiles, and lowest during 1993–2004. The most likely weight–length model for smallmouth bass included terms for length, presence of round goby (Neogobius melanostomus), presence of dreissenid mussels, and an interaction between length and dreissenid mussels (wi = 0.93). The most likely model for northern pike included terms for length, presence of round goby, presence of dreissenid mussels, water temperature, conspecific abundance, and sex (wi = 0.56). Invasive species and environmental change related to invasive species have altered the condition of smallmouth bass and northern pike in the St. Lawrence River.

Variation in recruitment and growth of age-0 yellow perch, Perca flavescens (Mitchill), was modelled across a range of nine eastern South Dakota glacial lakes to: (i) estimate factors influencing recruitment and growth dynamics during early ontogeny; and (ii) determine the relative importance of biotic versus abiotic processes in regulating recruitment and growth dynamics. Results provide a framework for future investigations and suggest that abiotic factors were more important in regulating recruitment of age-0 yellow perch, whereas biotic factors were more important in regulating growth. Recruitment was positively related to springtime water levels and temperature and negatively related to spawning stock biomass and springtime wind conditions. By contrast, growth was negatively related to abundance of conspecifics and potential competitors (i.e. bluegill, Lepomis macrochirus Rafinesque) and positively related to abundance of potential predators [i.e. walleye, Sander vitreus (Mitchill) and northern pike, Esox lucius Linnaeus].

Between 2012 and 2013, we estimated predation of non-native Ponto-Caspian gobies by native piscivores in a European mid-sized lowland river by analysing 268 digestive tracts from six native species: perch Perca fluviatilis, burbot Lota iota, European catfish Silurus glanis, pike Esox lucius, Volga zander Sander volgensis and European zander S lucioperca. Species closely associated with the rip-rap bank had a higher proportion of gobies in their diet (burbot 66 % weight proportion [%m], >= 1+ perch 53 %m, = 1+ perch showed positive selection for tubenose goby Proterorhinus semilunaris, while burbot and pike showed a preference for round goby Neogobius melanostonius, though the values may have been affected by the relatively low sample sizes. We estimate that predators consumed approximately 52 % of goby biomass over our study stretch each year (burbot 41%

Morphological differentiation may allow individuals to cope with prevailing environmental conditions. Morphological differentiation in fish characterized by sagittiform shape and ambush predator behaviour, such as northern pike (Esox lucius L., 1758), has rarely been addressed. Morphological differentiation was assessed in two rivers exhibiting contrasting flow regimes: a hydropeaking river characterized by large and frequent fluctuations in flow rates and an unregulated river. An increase in northern pike movement rate was observed in the hydropeaking river. Therefore, morphological features enhancing sustained and burst swimming, as well as manoeuvrability, were expected. Our objectives are to (i) compare morphology between the two rivers and (ii) assess morphological differentiation between sexes. Using geometric morphometrics, shape significantly diverged between rivers irrespective of sex and between sexes in the hydropeaking river. Individuals from the hydropeaking river had more elongated heads, deeper bodies and caudal peduncles, and longer dorsal fin insertions than individuals from the unregulated river. Caudal fin differences between rivers were not consistent between sexes. Morphological differentiation suggested a trade-off among adaptations for sustained and burst swimming, as well as manoeuvrability, to cope with variable flows in a hydropeaking river. Morphological differentiation may allow the exploitation of spatially and temporally variable environmental conditions, including those stemming from river flow regulation.

This article presents the results of analysis of parameters of the immune, antioxidant, and monooxygenase systems in the immunocompetent organs (pro- and mesonephros, spleen, and liver) of species belonging to different systematic and ecological groups, namely, northern pike Esox lucius, zander Sander lucioperca, and bream Abramis brama. Although having the same features in the functioning of homeostatic mechanisms in the species under study, significant differences were revealed between bream and another two species with respect to the parameters of the nonspecific immunity of tissues and enzyme systems that perform protective functions. In all the three species, the basic role in processes of immunopoesis and nonspecific protection is played by the kidneys and spleen, while the greatest values of the parameters of antioxidant defenses and ethoxyresorufin-O-deethylase activity were recorded in the liver.

Studies on catches of anglers usually rely on observational data and are thus uncontrolled with respect to angler skill, bait/lure choice and site choice. We performed a controlled fishing experiment targeting northern pike ( Esox lucius ) in a small (25 ha), weakly eutrophic natural lake situated about 80 km northeast of Berlin (Germany) to understand abiotic, biotic and gear-related factors that relate to catch rates and size of pike captured by angling. The experiment was conducted over two one-week long fishing campaigns where boat-based anglers randomly sampled across 30 pre-determined sites. Sites were systematically exposed to two standardized lure types (soft plastic shad or spoon). We found the catch rates of pike per 15 min to be significantly higher in shallow water and when soft plastic lures were used compared to deeper water and when spoons were used. Catch rates significantly dropped over the course of seven days, suggesting either learning or other reasons moving pike from vulnerable to invulnerable pools (e.g., due to stress caused by capture, sampling and release). Catch rates also varied by season and across anglers and sites as random effects. The variation in size of pike captured exhibited greater stochasticity than variation in catch rate. There was no lure effect on the size of the pike captured, but we found a seasonal effect and a day effect, suggesting larger fish were captured first. Pike captured in sublittoral areas were significantly smaller than those captured in other habitats. Overall, our study documented a novel effect of lure type on the catch rates of pike, but the explanatory power of the predictors was only moderate. Therefore, our results support the idea that the best fishing ingredients are investing time and maximizing encounter probabilities through habitat choice, with only moderate additional effects to be expected from attention to abiotic conditions, day time and choice of type of artificial lure.

Nanayakkara L, Wissel B. 2017. Preliminary investigation of lake-use patterns in prairie lakes, stakeholder perceptions, and resulting management implications. Lake Reserv Manage. 49:49–61.To better understand use and perception of lakes in the prairie region of North America, we distributed 200 lake-use and management surveys at 9 lakes in southern Saskatchewan, Canada, during 2013 and received 65 responses. Survey results indicate recreational uses such as angling, swimming, and boating were most common. Anglers targeted walleye ∼3 times more than other taxa such as northern pike (Esox lucius) and yellow perch (Perca flavescens), and consumption or catch-and-release was based on species and size. Overall, pollution of waterbodies (76% of responses) and overfishing (60% of responses) were stated as issues of highest concern in response to an open-ended question, but results varied among user groups. When asked to rank a set of issues potentially related to lake health, urbanization ranked above b...

Lake Glubokoe (Moscow oblast, Russia) is one of the most investigated water bodies of Europe. We detected new parasite species for the ecosystem of the lake (alien for territory of Europe): coccidian Goussia obstinata (in invasive alien fish Perccottus glenii) and copepod Neoergasilus japonicus (on aboriginal ruff Gymnocephalus cernuus). The given coccidian is host-specific for P. glenii and was introduced in the lake together with its host. Participation of P. glenii in circulation of recorded heteroxenous helminthes (Isthmiophora melis, Azygia lucii, and Raphidascaris acus) was revealed. This is confirmed by the data on trophic links of their obligatory definitive hosts (American mink Neovison vison and northern pike Esox lucius) with P. glenii in Lake Glubokoe.

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 the stomach of a pike from the Danube Delta we have found seven Perccottus glenii individuals. Their message indicated that in the Danube Delta P. glenii has already large population. Also, it proves that in its habitats, the pike can consume numerous individuals of this invasive species. Their gut contents prove that P. glenii hunted near the shores, ingesting numerous terrestrial preys.

During the last century a large portion of small and medium-sized rivers in Latvia were channelized, hydroelectric power stations were also built, which led to changes in the hydrodynamic conditions, geomorphological structure, as well as a change in the fish fauna. Fish are an integral part of any community in natural or man-made bodies of water. They actively participate in maintaining the system, balancing/equilibrium, energy, substance transformation and biomass production. They are able to influence other organisms in the ecosystem in which they live. The aim of the paper “Pike distribution and feeding comparisons in natural and historically channelized river sections” is to find out what pike feed on in different environments in Latvian rivers, such as natural and straightened river sections, as well as what main factors determine the composition of their food. Several points were assessed during the course of the study: the impact of environmental conditions on the feeding habits and the distribution of pike; the general feeding habits of predators in Latvian rivers; the feeding differences of predators in natural and straightened river sections; and lastly, rhithral and pothamal habitats were compared. The study was based on data from 2014 and 2015 on fish fauna monitoring. During the study, 347 pike were collected from 136 plots using electrofishing method.