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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.

Behavioural responses of animals are often placed conceptually along some axis characterized by the extremes, e.g. bold versus shy or timid. Ecological and evolutionary pressures can be associated with increased frequencies of different behavioural patterns that are also subject to intrinsic factors such as differing metabolic requirements or cognitive abilities. We exposed individual wild-caught juvenile bluegill, Lepomis macrochirus, to kairomones from predatory northern pike, Esox lucius, or to lake water controls during a Z-maze arena assay commonly used to assess relative levels of boldness and exploratory behaviours. Fish were subsequently lethally sampled for liver and brain mass to obtain estimates of relative body condition (hepato-somatic index, HSI) and cognitive capacity (brain-somatic index, BSI), as well as the state of the gallbladder, to test for relationships between these indices and observed boldness-related behaviours. Using principal components analysis (PCA) we identified a boldness–exploration syndrome in bluegill independent of chemical treatment and body metrics where bolder fish emerged from a start chamber in less time, travelled farther through the maze and crossed more lines than less bold fish. Bluegill exposed to kairomones were bolder on average than their counterparts in the control treatment independent of body metrics, suggesting that elevated risk levels indicated by predator kairomones can elicit behavioural responses consistent with boldness, exploration and risk taking. The role of relative cognitive ability in the expression of boldness and exploration is equivocal and may be resolved through repeated assays on individuals at different risk levels.

This work represents the first ultrastructural study of the tegument of a freshwater teleost-infecting aporocotylid blood fluke and the first detailed observations of the surface sensory receptors of any aporocotylid. The work was carried out on Sanguinicola inermis from the branchial arteries of the pike, Esox lucius. Light and scanning electron microscopy observations failed to show the presence of protruding surface spines in S. inermis. Transmission electron microscopy studies revealed, deep beneath the distal cytoplasm of the tegument, sporadic, membrane-bound sarcoplasmic processes, each containing a single spine with the typical crystalline structure of digenean spines. On rare occasions, the spine may protrude above the body surface, but in such cases the distal tegumental cytoplasm is extremely flattened. This finding supports a unique, deep origin of aporocotylid spines when compared to those of other digenean families. This character may well prove to be an autapomorphy of the Aporocotylidae, and the position of the base of aporocotylid spines within sarcoplasmic processes supports their origin as a muscle derivative. Unlike other digenean groups, the distal region of the cytoplasmic processes, connecting sunken tegumental perikarya with the syncytial tegumental cytoplasm, is strengthened by peripheral microtubules. Three types of ciliated and three types of non-ciliated sensory receptors were distinguished in the tegument of S. inermis. One of the distinctive features of the ciliated receptors is the unusual position of the basal region of the cilium within a tegumental outgrowth. Such a position has previously been described for certain receptors types in larval and adult schistosomatids and larval diplostomids, suggesting that this feature, in adult worms, might prove to be an apomorphic trait for some or all groups of blood flukes.

In our study, a systematic development of a new large-scale sperm cryopreservation protocol was carried out in northern pike (Esox lucius). The effect of 2 sugar based (glucose and trehalose) extenders, 3 dilution ratios (1:3, 1:9 and 1:19) 2 vol straws (0.5 and 5 mL) and a 10 mL cryotube, 2 different cryopreservation methods (Polystyrene box-P. box and Controlled Rate Freezer-CRF), as well as 3 different thawing periods (3, 3.5 and 4 min) were investigated on the motility of thawed sperm. The glucose based extender showed significantly higher pMOT (1:3–18 ± 16%, 1:9–20 ± 13%, 1:19–16 ± 12%) at all dilution ratios than in the trehalose based extender (1:3–0.3 ± 1%, 1:9–1±1%, 1:19–4±2%). A similar tendency was recorded in VCL and STR at a ratio 1:3 and 1:9. No significant difference was measured in sperm movement between the P. box and CRF using the 0.5 mL straw. Similarly no significant difference was observed in all motility parameters with 10 mL cryotube frozen in CRF at a ratio 1:3–1:19. An effective and short thawing period (3 min) was experimentally specified for the 10 mL cryotube cryopreserved in the CRF. In all large-scale cryopreservation methods, high pMOT (straw CRF: 57 ± 10%, straw P. box: 50 ± 9%, cryotube CRF: 41 ± 10%), and STR were measured, and no significant difference was recorded in all motility parameters. Our results demonstrate the effectiveness of our newly developed extender and the applicability of 3 different large-scale cryopreservation methods in pike sperm. Our protocols could be new prospective candidates for future exploitation in hatchery practice.

The effect of different treatment agents, namely, carp pituitary homogenate (CPH), Ovaprim ([D-Arg6, Pro9NEt]-sGnRH + domperidone) and a dopamine-receptor antagonist (metoclopramide), on the stimulation of northern pike (Esox lucius) spermiation was tested under controlled conditions. To carry out the experiment, males (n = 84) were divided into four groups: control (n = 21); CPH (n = 21); Ovaprim (n = 21); metoclopramide (n = 21). The control group was given 0.9% NaCl but no hormonal treatment. After 24 h, sperm was collected from seven males belonging to control (n = 7), CPH (n = 7), Ovaprim (n = 7) and metoclopramide (n = 7). This procedure was repeated after 48 and 72 h post-treatment. At each time, sperm was collected from seven males from each group only once. After collection, the quantity and quality of sperm were determined. It was confirmed that the treatment agent and latency time (the time between treatment and sperm collection) are two factors affecting the quantity and quality of northern pike sperm collected under controlled conditions. The highest total sperm volume and total sperm production (TSP) were noted in the CPH group compared to the Ovaprim, metoclopramide and control groups. In contrast, the time of sperm collection affected the sperm concentration (SC), TSP and sperm pH. With increasing time, SC and TSP decreased, which indicated the occurrence of sperm hydration being part of the final sperm maturation process. Sperm maturation is in turn a consequence of increases in sperm pH and seminal plasma osmotic pressure between 48 and 72 h post-treatment. Sperm motility and sperm kinetic parameters were affected by treatment agent and the time of sperm collection. This indicates that the sperm’s ability to move that is achieved in the optimal environment (in spermatic ducts) is dependent on both factors which determine sperm maturation in northern pike under controlled condition.

Although many fish are captured and released following hook removal by recreational anglers, some fish break the line and are confronted by the potential impediment of a lure lodged in the jaw, buccal cavity, or throat. We simulated break-off events by releasing northern pike (Esox lucius) into Lake Opinicon, Canada with custom-built lures that were manufactured to contain radio transmitters. Treatment groups combined hook placements (lower jaw, upper and lower jaw, throat) and hook types (barbed and barbless) to investigate the effects on pike survival, movement, and lure shedding. Fifty-one pike were released (522 ± 64 mm), three of which died (6%; 95% CI = 2–16%). Data were analysed by dummy variable regression to investigate the main effects of hook placements and hook type in pike. Cumulative distance swam after release was significantly reduced by deep hooking and lower jaw hooking. All fish except for one shed the lures within 14 d of release, and barbed hooks and lures lodged in the lower jaw significantly increased the time required for pike to shed lures. In light with previous work, we documented significant short-term behavioural consequences of lure break-off for pike (i.e. hyperactivity) but the experimental fish rapidly and naturally shed the hooks within days in nearly every instance. Given our findings, for pike (and likely related species such as muskellunge), anglers can be reasonably confident that long-term damage to individuals is limited even when a lure is retained by an animal following a break off event. Nonetheless, use of barbless hooks facilitates lure shedding, and all efforts should be taken to avoid break off events in the first instance using appropriate gear (e.g., wire leader, heavy line) especially when angling for fish with sharp dentition.

Halogenated polycyclic aromatic hydrocarbons (HPAHs) were identified in biological samples from the Alberta Oil-Sands Region (AOSR) using gas chromatography coupled with high-resolution time-of-flight mass spectrometry (GC-HRTOF-MS) at a resolving power of 25,000. Knowledge of the electron ionization (EI) fragmentation behavior of individual HPAH isomers, achieved by injecting authentic standards in full-scan MS mode, was paramount in identifying a suite of HPAHs in samples from the AOSR. Confirmation of compounds in biological samples was based on the measured mass accuracy (±3 ppm) of 2 characteristic ions prominent in the EI mass spectra of each compound. Numerous compounds were detected in the high resolution total ion chromatogram in liver extracts of 4 biological species from the AOSR: river otter (Lontra Canadensis), northern pike (Esox lucius), lake whitefish (Coregonus clupeaformis) and snails (Gastropod sp.) many of which remain unidentified. Careful examination of the high-resolution accurate mass data suggests that dichloro-anthracene/phenanthrene, bromo-anthracene/phenanthrene and dibromo-fluorene were present in the biological samples. Lipid corrected concentrations of dichloro-PAHs were estimated to be 16.3 ± 11.4 (n = 4) and 5.5 (n = 1) ng/g in lake whitefish and river otter, respectively. Concentrations of mono-bromo-PAHs were an order of magnitude greater in snails (170.5 ng/g) than in northern pike (12.5 ng/g) while concentrations of dibromo-PAHs were 4 times greater in snails than in northern pike. The detection of these compounds in biota implies that these compounds are bioaccumulative. The liver-based biomagnification factor of the dichloro-PAH congener in the river otter/lake whitefish feeding relationship is much smaller than 1 implying that this compound does not biomagnify.

Biomagnification of Hg and persistent organic pollutants (POPs: polychlorinated biphenyls (PCBs), organochlorine pesticides (OCPs), polybrominated diphenyl ethers (PBDEs)) in aquatic food chains can lead to high pollutant concentrations in top predators, including humans. Despite this threat to human health, research concerning bioaccumulation is still underrepresented in the southern hemisphere and in (sub)arctic and (sub)tropical areas, emphasizing the need for research in these locations. In this study, samples of water, sediment and aquatic biota were analyzed to determine concentrations of POPs and total mercury (THg) in the Ga-Selati river (South Africa) and two rivers Rena and Gudbrandsdalslågen in Norway. Trophic magnification factors (TMFs) were determined to evaluate and compare the biomagnification and the threat to human health due to consumption of the fish was assessed.Concentrations of POPs in sediment and biota samples were generally low except for relatively high concentrations of ∑DDX (dichlorodiphenyltrichloroethane and metabolites) in aquatic biota from the Ga-Selati river (ranging from 1.9 to 133 ng/g ww in invertebrates and 1.9 to 5643 ng/g ww in fish). Dissolved THg concentrations were high in the Ga-Selati river (ranging from 0.009 to 0.036 μg/l) but THg concentrations in sediment and biota were low in studied rivers compared to other studies. Biomagnification occurred for THg, several DDT-metabolites and PCB compounds, TN and CN. Biomagnification of p,p′-DDT and THg differed significantly between the two countries, supporting existing patterns found in literature, although more data is needed to attribute these differences to climatic or other factors. Concentrations in fish from the rivers Ga-Selati and Rena were under the threshold levels reported for THg and POPs, but caution should be taken when consuming Northern pike (Esox Lucius) from the subarctic river Gudbrandsdalslågen, to avoid harmful effects due to both elevated THg and PBDE exposure. Keywords: Persistent organic pollutants, Hg, Bioaccumulation, Trophic magnification factors, South Africa, Norway

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.

Emerging PFAS were recently reported at sites impacted by aqueous film-forming foams (AFFFs) and near major manufacturing centers; however, few studies have evaluated whether these can occur far from release sites. Here, newly identified PFAS were investigated in wild sporting fish from boreal freshwater ecosystems (background sites, 2013–2014 summer seasons), compared to fish impacted by a major AFFF release (summer 2013 and autumn 2014). Different freshwater wild sporting fish species (Esox lucius, Esox masquinongy, Micropterus dolomieu, Sander vitreus, Perca flavescens, and Semotilus corporalis, n = 74) were collected from 13 ecosystems (lakes, reservoirs, and rivers) across Eastern Canada. Of 29 quantitative PFAS, 15 compounds were detected in fish from background sites, including perfluorocarboxylates (C6,8–14), perfluoroalkane sulfonates (C6,8,10), perfluorooctane sulfonamide (FOSA), 6:2 fluorotelomer sulfonate (6:2 FTSA), 7:3 fluorotelomer carboxylic acid (7:3 FTCA), and a zwitterionic PFAS—perfluorooctane sulfonamidoalkyl betaine (PFOSB). To our knowledge, this is the first report of PFOSB in biota. It is also one of the first reports of anionic fluorotelomers (6:2 FTSA, 7:3 FTCA, 9:3 FTCA) in wildlife from background sites. Long-chain fluorotelomer sulfonamidoalkyl betaines (e.g., 8:2 and 10:2 FTAB), fluorotelomer betaines (e.g., 9:3 and 9:1:2 FTB), and fluorotelomer sulfone propanoic acids (e.g., 8:2 FT(SO2)-PA, 10:2 FT(SO2)-PA)) were solely prevalent (up to 97% of summed suspect PFAS) in Smallmouth Bass (M. dolomieu) from the AFFF-impacted site. Perfluorobutane sulfonamide (FBSA), perfluorohexane sulfonamide (FHxSA), 6:2 FTSA and 7:3 FTCA were detected in at least one Smallmouth Bass sample both at the AFFF-impacted and background sites. According to the estimated chronic daily intake and current tolerable daily intake suggested by national agencies, the observed PFOS levels would not pose a health risk to anglers who might consume these wild-caught fish.

The sustainable management of fisheries is increasingly as much about people as it is about stock management and an important prerequisite for managing stakeholder disagreement is an understanding of each party's position and perspective. In Ireland pike stocks are actively managed within specific fisheries for the purpose of protecting brown trout stocks. Associated public discussion is increasingly polarized with a clear message that trout anglers favour and pike anglers oppose such management intervention. A first step towards successfully managing the tension between all stakeholders is a better understanding of the broad spectrum of anglers’ views. Latent-class site-choice models are estimated with choice experiment data to reveal information on anglers’ preferences with the data collected via an online survey from 341 members of a recreational angling research panel. Several policy relevant lessons arise from the analysis. First, anglers’ preferences are not homogeneous, either among pike or trout anglers. Angler stakeholder groups cannot reasonably attest to represent all pike or trout anglers. There are multiple distinct classes of anglers with quite different preferences on pike stock control as well as other fishery attributes, such as bag limits and catch rates. Second, while anglers have strong preferences across a number of fishery attributes (e.g. catch rates, pollution, invasive weeds, etc.), pike stock control is the predominant attribute affecting anglers’ behaviour. Third, among trout anglers there is a sizeable majority opposed to the maintenance of existing pike stock control measures, which is at odds with a vociferous public campaign led by trout angling representative organizations. The preferences of this ‘silent’ majority reflect the fact that approximately 2-in-5 trout anglers also fish for pike. In general, the research helps inform fishery management decisions, dispelling the idea of a clear dichotomy between pike and trout anglers and instead illuminates the complexity of anglers’ preferences.

Human activity has caused longitudinal fragmentation of many rivers. Fishways have been installed worldwide, but their successive use by potamodromous species remains poorly documented, particularly in large river ecosystems. Four vertical slot fishways were installed within a 32-km stretch of the lower Belgian Meuse River basin. From 2012 to 2016, n = 532 individuals belonging to 11 potamodromous fish species (rheophilic, limnophilic and large carnivorous) were continuously captured in the most downstream fishway (M0), tagged with an RFID transponder and released upstream. These could be further detected in upstream river part within three fishways (M1 and M2 in the Meuse, and O1 in an important tributary, the Ourthe) that were equipped with RFID detection stations. In the first downstream stretch (13 km from M0 to M1) we quantified an ascending rate until M1 (number of individuals detected in M1/ number of individuals tagged in M0) of 32.9% including all individuals tagged, with a maximum value of 67.2% for chub (Squalius cephalus), 40% for pike (Esox Lucius) and 35.5% for nase (Chondrostoma nasus), and a progression time M0 to M1 of 1.1 days per kilometre (d/km), with trout (Salmo trutta) as the fastest species (0.3 d/km). Upstream of the M1 fishway, many individuals of rheophilic species (trout and barbel, Barbus barbus) preferred to enter in the Ourthe tributary (detection at the Ourthe fishway) unlike the upstream Meuse (detection at the most upstream fishway in the Meuse), demonstrating a new accessibility to more adapted spawning sites. Most fish were present within fishways mainly in spring during the circum-spawning migration, and during summer and autumn for dispersal and/or seeking-refuge. The diel activity cycle varied depending on the species, with detection during the entire diel cycle (e.g. chub and barbel), during the day (e.g. trout) and during the night (e.g. catfish, Silurus glanis). The installation of fishways in the degraded river Meuse can be considered adequate for the restoration of the free movement because potamodromous species demonstrated their ability to migrate over long distances (>20 km) and to reach potential spawning habitats through the reopened access to a tributary.

The effect of carp pituitary homogenate (CPH, n = 7) at a dose of 2.0 mg kg−1 and (D-Arg6, Pro9NET)-sGnRH + domperidone (Ovaprim, n = 7) at a dose of 0.5 ml kg−1 in northern pike (Esox lucius) sperm maturation under controlled conditions was examined. On the control group, 0.9% NaCl at a dose of 1.0 ml kg−1 (n = 7) was used. Sperm was collected 48 h following injection. Sperm quantity (total sperm volume, total sperm production and sperm concentration), sperm motility using the Computer-assisted sperm analysis (CASA) system and sperm and seminal plasma quality (sperm pH, seminal plasma osmotic pressure and seminal plasma pH) were determined for each male separately in each group. The results of the present study demonstrate that hormonal treatment had a positive effect on sperm maturation in northern pike, regardless of the hormonal preparation used. However, even though no differences were found in total sperm volume and total sperm production between fish injected with either CPH or Ovaprim, it should be highlighted that the highest progressive motile sperm (PRG), straight-linear velocity (VSL) and movement linearity (LIN) was noted in fish treated with Ovaprim. It was also found that it is possible to collect sperm from non-hormonally manipulated fish. However, in such a case, only a small sperm volume (around 0.1 ml) characterised by lowered PRG (below 40%) was noted. Considering the fact that only after Ovaprim application sperm motility with progressive movement of sperm above 50% was observed, the hatchery practice of collection of sperm 48 h after its application (at a dose of 0.5 ml kg−1) may be recommended.

The Athabasca River in Alberta flows through natural sources of eroding oil sands bitumen and oil sands mining operations that may result in low level contamination of surface waters. Northern pike (Esox lucius) are apex predators and important food and game fish species native to the Athabasca River system. This species has the potential to be exposed to both natural and anthropogenic sources of contamination from oil sands related materials throughout its life cycle. Pike are difficult to rear in the laboratory and little information exists on the toxicity of oil sands related materials to this key indigenous fish species. In this study, the potential effects of two sediment samples collected from different areas of one tailings pond in the Athabasca oil sands area are assessed in a daily renewal bioassay on early life stages of northern pike. Gametes were collected from spawning wild pike captured from a reference site outside of the oil sands area. Fertilized eggs were exposed to control water or increasing concentrations of tailings pond sediments for 21days, coinciding with initiation of exogenous feeding and completion of yolk absorption. Developing fish were examined for survival and changes in body weight, length, and development. Embryos exhibited increased developmental abnormalities and decreased growth and survival with increasing sediment concentration. Both sediment samples had similar levels of naphthenic acids and similar types of PAHs, with alkylated PAHs dominating. However, concentrations of total and alkylated PAHs differed between sediment samples and were related to increasing developmental abnormalities and decreased growth and survival. This is consistent with developmental changes observed with exposure to PAHs in other fish species. These results provide information on the effects of tailings pond sediments comprising mixtures of PAHs and alkylated PAHs on the development and survival of a key species in the northern aquatic ecosystem.

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.

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.

Catch-and-release is practiced in recreational fisheries under the premise that released fish will survive with negligible injury and stress. However, hooking injuries may prevent that outcome from being realized. One way to potentially minimize injuries and maximize survival in angled fish is to replace treble hooks on hard plastic lures with single hooks, but the effectiveness of this tactic has yet to be tested. Our study investigated if replacing treble hooks with single hooks on hard plastic lures reduced injuries and handling for angled Northern Pike (Esox lucius), Smallmouth Bass (Micropterus dolomieu) and Largemouth Bass (Micropterus salmoides). Furthermore, we compared fish handling (i.e., need for use of hook removal gear) and injuries between fish that were captured with barbed and barbless hooks. Fish were angled using three types of conventional hard plastic lures (i.e., crankbaits, jerkbaits, and lipless crankbaits). Upon landing, total length of the fish, an array of hooking characteristics (i.e., number of hook points in the fish, anatomical hooking location(s)), and reflex impairment were recorded. Analyses revealed that using barbless J hooks on all lures yielded the shortest unhooking times for all species. For Smallmouth Bass caught on both crank and jerk baits, J hooks tended to result in more shallow hooking than treble hooks. Barbless treble hooks were more likely to be embedded in a sensitive location (e.g., foul hooked, gullet, gills, and/or eyes) compared to barbless J hooks in Smallmouth Bass. No other significant differences in hook types and anatomical locations were found for other species. Hook type and lure type did not influence reflex impairment or survival for any of the species. Using J hooks, especially barbless ones, on lures that traditionally have treble hooks should be considered when encouraging best angling practices for the freshwater gamefish studied here to expedite release, although the extent to which this influences mortality remains unclear.

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.

Recreational angling is a major introduction pathway for large-bodied alien fishes into freshwaters, where the fish are released to enhance angling success and increase angler satisfaction, despite this often resulting in invasive populations. There is a thus a need to understand the role of these alien species in angler catches to enable more informed risk-based decisions to be made on future releases. In England, the invasive, piscivorous pikeperch Sander lucioperca has been present in rivers since the 1960s. Anglers target invasive pikeperch in fisheries where the native piscivorous pike Esox lucius is also present; this includes two rivers in the lower Severn basin, western England (main River Severn and Warwickshire Avon). To assess the contributions of invasive pikeperch to angler catches, the aim here was to compare their catches with those of native pike in the lower Severn basin in relation to angling effort and methods, abiotic conditions, and fish size. In 307 angling sessions across 16 anglers where at least one fish was captured, 428 pike and 266 pikeperch were captured. In a sub-set of data from six anglers who submitted catch returns that included non-capture sessions, 78 % of sessions resulted in the capture of at least one pike or pikeperch. Catch rates of pike were significantly higher than pikeperch in the main River Severn but not the Avon. Captured pike were significantly larger than pikeperch, but pikeperch were larger relative to the maximum size each species reaches in England. Lures generally captured more pike than any other method, with these fish tending to be smaller than those caught on other methods; these patterns were not evident in pikeperch. Both species were captured across a broad range of river flow conditions (Q6 to Q99). Only 19 % of successful angling sessions resulted in the capture of both species, suggesting some species selectivity by anglers. These results emphasise that alien fish species can provide important angling resources in recreational fisheries, although management decisions on future introductions should still consider their ecological risks.

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.

Municipal wastewater effluents (MWWEs) consist of dynamic and complex mixtures of chemical and biological compounds that can alter the health of exposed aquatic organisms. Disturbance of energy metabolism has been reported in fish exposed to MWWEs. However, there is a scarcity of knowledge on the physiological events leading to perturbation of energy balance and thyroid regulation, and associated lipid metabolism. The objective of the present study was to use a set of biomarkers, from gene transcription to body condition, to investigate the effects of a chronic environmental exposure to a major primary MWWE on fatty acid metabolism and thyroid hormone levels in northern pike (Esox lucius) collected from the St. Lawrence River near Montreal (QC, Canada). The exposure of pike to MWWE was examined through determination of a suite of persistent and bioaccumulative halogenated flame retardants in liver as this effluent is a known regional source for these chemicals. Greater hepatic concentrations of polybrominated diphenyl ethers (PBDEs, range: 29.6-465ng/g w.w. and 88.8-823ng/g w.w. in females and males, respectively) and other halogenated flame retardants (e.g., dechlorane-related compounds) were determined in fish collected downstream of the MWWE's point of discharge relative to the upstream site. This exposure in male pike was associated with decreased acyl-coA oxidase (acox1) and fatty acid synthase (fasn) mRNA levels as well as a decreased acyl-coA oxidase (ACOX) activity in liver. In female pike, MWWE exposure was associated with lower circulating free and total triiodothyronine (T3) levels and a tendency for greater total lipid percentages in liver. Present findings provide evidence that chronic exposure of a top predator fish to MWWE can be related to gender-specific effects on fatty acid metabolism and thyroid hormone homeostasis, and highlight the need for further investigation.

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.

After a fish snaps an angler’s line, the hook(s) still embedded in its mouth, the question arises: what will the encounter cost the fish? The consequences of retained gear on the physiology and behaviour of fish is not well understood. This study aimed to quantify the impact of prolonged exposure to a retained lure (simulated break off in recreational angling) to the physiology and behaviour of northern pike (Esox lucius) was studiedin a laboratory setting. A combination of blood-based physiological metrics and metabolic rate measurements were used to provide a comprehensive overview of the physiological consequences of lure retention in this species using two different treble hook sizes on metal casting spoons in three different hooking locations. Fine-scale video observations of pike following simulated break off were collected to assess pike interaction with a retained lure and to quantify activity patterns. We found that the retention of a lure did not significantly affect metabolic rate, blood physiology or locomotor activity of pike. However, gill ventilation rate was found to be elevated in pike hooked deeply in the throat suggesting that lures in obstructive locations may somewhat challenge recovery from exercise. Elevated cortisol levels in these fish compared to wild controls suggests that confinement produced prolonged stress in all treatments that may have affected the physiological and behaviour responses that we observed. Our findings provide important observations about the interpretation of stress-oriented laboratory studies using northern pike and the extrapolation of these results to the wild. Despite our negative findings in relation to lure impacts on pike physiology and behaviour, avoiding break offs would still be advisable for fish welfare reasons.

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.

The aim of this study is to investigate isotopic variability in archaeological ( n = 85) and modern ( n = 29) freshwater fish specimens from Switzerland. Here, carbon (δ 13 C) and nitrogen (δ 15 N) stable isotope ratio analysis was performed on bone collagen samples of pike ( Esox lucius) , perch (Perca fluviatilis), barbel (Barbus barbus), roach (Rutilus rutilus) and carp (Cyprinus carpio ) from eleven archaeological (11th to 18/19th centuries CE) and modern sites. The archaeological vs. modern fish data revealed significant isotopic differences for pike, perch and barbel (δ 13 C p ≤ 0.03; δ 15 N p ≤ 0.008), and provides possible evidence for a temporal change in Swiss aquatic ecosystems from Medieval to modern times. In comparison to archaeological fish (δ 13 C mean ± SD; − 23.3 ± 1.6‰; δ 15 N mean ± SD; 8.3 ± 1.8‰), the modern fish samples show decreased δ 13 C and increased δ 15 N values (δ 13 C mean ± SD; − 27.4 ± 2.3‰; δ 15 N mean ± SD; 12.5 ± 4.1‰) that can be associated with anthropogenic effects: fossil fuel combustion, deforestation and organic waste in the form of sewage and fertilizers. The isotopic signatures of archaeological fish remains indicate a local fishery practice, but also the exploitation of distant fishing grounds and freshwater fish transportation. Furthermore, a diachronic isotopic trend is observed in young perch from sites in Basel, dating between the 12th and 15/16th centuries CE, and the isotopic data from the Rhine freshwater fish (18/19th century CE) suggests that a significant shift in the river's trophic state was possibly caused by organic pollution from urban and industrial wastewater. This retrospective research illustrates possible natural processes and human activities which can cause differences in fish stable isotope data and highlights the ability to elucidate changes in past bodies of water. Furthermore, this study provides an interpretative framework for additional palaeoenvironmental studies and modern restoration projects focused on freshwater ecosystems.

Verifying the link between somatic and otolith growth is crucial for a number of analyses that provide data on the ecology of early life stages of fish. In the current study, sagittal and lapillar otoliths were extracted from larval northern pike (n = 720) that were reared from eggs in RAS systems at three temperatures (10 °C, 15 °C, and 22 °C) for 28 days. The growth rate (SL at age) of larval northern pike was significantly dependent on rearing temperature with the highest rate observed at 22 °C (0.83 mm·d−1), a lower rate at 15 °C (0.48 mm·d−1), and the lowest at 10 °C (0.25 mm·d−1). The maximum value of growth observed for an individual larvae at a certain age was 1.28 mm·d−1. The higher somatic growth rate at higher temperatures was followed by faster lapillus and sagitta linear growth rates. Positive relationships between otolith growth and somatic growth were noted not only among temperatures but also within given temperatures. The sizes of lapilli and sagittae were strongly correlated with fish size, but both somatic growth and temperature had a statistically significant effect on those relationships. Magnitude of this phenomenon was however fish size dependent. We concluded that larval northern pike otoliths (both sagittae and lapilli) are a reliable source of information on somatic growth, and they can be used for increment trajectory width analysis and marginal otolith increment width analysis. However, if growth back-calculation is to be employed, it is recommended to consider the somatic growth and temperature effect on the fish size-otolith size relationship.

The objectives of this study were to determine the specific characteristics of sperm motility and fertilization rates of whitefish (Coregoninae subfamily, Salmonidae, Salmoniformes) and northern pike (Esocidae, Esociformes) in response to cryopreservation using a glucose–methanol extender, which has recently been successfully used for salmonid species. The mean post-thaw sperm motility was 69% for northern pike and 53% for whitefish. Fertilization trials with cryopreserved sperm yielded very high hatching rates for northern pike (79–89%) and for whitefish (54–56%), and these results were similar to those using fresh semen controls. This is the first report demonstrating that the fertilizing ability of cryopreserved northern pike and whitefish semen is similar to that of fresh semen when a sperm:egg ratio as low as 1.5 × 10 5 :1 and 3 × 10 5 :1 was used for northern pike and whitefish, respectively. However, different responses of sperm to the same cryopreservation procedure were observed; therefore, additional studies are required to determine whether the mechanisms underlying of that phenomenon are species–specific and/or related to environmental conditions. Our cryopreservation procedure is applicable not only for salmonid sperm but also for taxonomically distant fish such as northern pike and, therefore, could be used for ongoing northern pike and whitefish conservation and aquaculture efforts. Further studies should focus on the optimization of cryopreservation procedures in order to achieve large–scale fertilization and should aim at defining the minimum efficient sperm:egg ratio for cryopreserved sperm.

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.

To identify the effects of oocyte ageing on egg quality in northern pike, Esox lucius , two experiments were performed as follows: in experiment I, partial volumes of the eggs from 7 pike females were stripped and fertilised with 2-day intervals for 8 days post-ovulation (DPO). In experiment II, the stripped eggs of 3 females were stored in ovarian fluid at 10 °C and fertilised separately with 12-hour intervals for 96 hours post-stripping (HPS). In both experiments, the eggs from each batch were fertilised with mixed milt obtained from 5 males. Eyeing, hatching and eyed-egg mortality rates as well as the occurrence of larval malformations and ploidy anomalies were considered as indices for egg quality. The results of experiment I indicated that the best and most reliable post-ovulatory stripping times in pike were at 2 DPO and up to 4 DPO, respectively, when females were retained at 10 °C. Over-ripening of the eggs occurred from 8 days after ovulation. The eyed-egg mortality and larval malformation rates did not show any significant increase up to 6 DPO, but then increased significantly and reached 36% and 43% at 8 DPO, respectively. The incidence of triploid larvae increased over time, from 0% at 0 and 2 DPO to 11% and 14% in more aged oocytes at 4 and 6 DPO, respectively. The second experiment clarified that the best post-stripping fertilisation time in pike was at 0 HPS. However, stored eggs in ovarian fluid could retain almost 50% of their initial fertilising ability for 24 h after stripping. Complete loss of egg viability occurred after 96 HPS, and the highest eyed-egg mortality rate (27%) and larval malformation rate (33%) were detected in this group. The obtained results suggest that for commercial artificial reproduction procedures in northern pike, the time interval between successive fish examinations for ovulation should not exceed 4 days. In addition, the eggs 1 stripped from the female should be fertilised immediately following collection to secure the highest possible fertilisation rate. Statement of relevance Knowledge of in vivo and in vitro storage of fish eggs can improve hatchery techniques as well as better broodstock management, which are the most important factors affecting synchronization of the artificial fertilisation and consequently the effectiveness of the entire operation. The duration of egg viability after ovulation and stripping has not yet been determined for the pike. A better understanding and refinement of egg storage methods are strongly desirable to maximize the efficacy of mass production in pike hatcheries.

The pike population in the Løpsjøen reservoir and the accessible 21 km section of the inflowing RiverRena was investigated from 2003 to 2013. Telemetry and mark-recapture with Floy tags demonstratedthat most fish had an annual home range of less than 2 km, while some fish performed quite extensivemigrations (up to 14.4 km). Of fish radio-tagged in the reservoir, 57% were positioned in the river atleast once. The population of pike ≥25 cm was estimated at 1002 fish, or 3.9 fish (3.17 kg) ha−1. Meanlength at age was significantly larger for pike caught in the river than in the reservoir for almost all agegroups. CPUE during boat electro-fishing indicated significantly lower population densities and a lowerproportion of juvenile fish in the river than in the reservoir. Diet analysis revealed that almost all availableprey fish species were taken by pike, and that there was a clear diet differentiation between pike sizeclasses, and between fish from the river and reservoir habitats. Small pike (50 cm) in the reservoir had taken brown trout as well as a number of other prey fish, while river pike ofthis size had taken burbot (Lota lota), lamprey and brown trout. Species like perch (Perca fluviatilis), roach(Rutilus rutilus) and in particular grayling (Thymallus thymallus) were rarely found in pike stomachs. Theconstruction of the reservoir in a fast-flowing river caused the establishment of a healthy pike populationand additional predation pressure on the rheophilic salmonids in the system, both in the reservoir andin the inflowing river.© 2015 Elsevier B.V. All rights reserved. River fragmentation, Predation, Habitat utilization, Habitat modification, Northern pike

Eyeing and hatching rates with regard to sperm: egg ratio, the length of time eggs remain viable post-activation in water, and duration of storage time in testicular and stripped sperm were determined in northern pike Esox lucius. In a trial of seven sperm: egg ratios, we found 4 × 105 to be the minimum tested number of spermatozoa per egg to produce an acceptable hatching rate (66 ± 4%). To determine the period of time that eggs remain viable after activation, eggs were placed in hatchery water and fertilized at 0, 15, 30, 45, 60, 90, 120, 240, and 480 s post-activation. With elapsed time, the eyeing and hatching rates decreased significantly (P

Role reversal can shift fish communities from predator to prey dominance. In the Baltic Sea, the increase in abundance of the small-bodied three-spined stickleback (Gasterosteus aculeatus) has co-occurred with declines of coastal predatory fish, such as perch (Perca fluviatilis) and pike (Esox lucius) in many areas. Recent studies have pointed at predator-prey reversal as a possible mechanism explaining these trends, but conclusive evidence on the phenomenon or its relative importance compared to other drivers is at present open to question. In this study, we applied path analysis to quantify the potential role of stickleback in limiting the recruitment of perch and pike in enclosed bays (i.e. flads) along the Swedish and Finnish coasts. We constructed a causal network model based on vast field data combined with data from GIS and orthophotos, representing fish abundance, hydrographic factors, habitat-forming vegetation, anthropogenic disturbances and geomorphological factors. We found that stickleback abundance had as large effect on larval densities of perch and pike as environmental forcing, represented in our best models by the thermal regime of the flads. Larval densities decreased with stickleback abundance and increased with increasing summer cumulative temperature at a similar rate. Stickleback abundance, in turn, was strongly limited by flad openness, expressed as the number of flad openings to the sea, and increased with vascular plant cover (flad averages). Our study shows that predator-prey reversal could be a strong determinant of perch and pike larval survival, and suggests that the prevalence of more enclosed flads, which are less accessible to stickleback, may be crucial for recruitment of these piscivores. This implies that (1) in areas with, or prone to, increased stickleback abundance, habitat restoration and actions to conserve piscivore populations should primarily focus on more enclosed flads, and that (2) dredging and canalization to enhance boating in these areas could have detrimental effects on piscivore recruitment by decreasing water temperature and enhancing stickleback abundance, hence restricting larval growth and survival.

It is difficult to determine the individual behaviour, growth, and survival of large predatory freshwater fish without stressing or even killing them. In this study, we tested the ability of an image recognition program to identify individual recaptured pike (Esox lucius) based on skin pattern. We and local anglers caught 209 pike individuals, including 45 recaptures, over the course of 1.5 years. The software correctly identified the image of all recaptured fish as already present in the database. Based on recaptures, we predicted a population of 560 pike in the lake and showed a much more comprehensive size structure than seen using standardized gillnet sampling which only yielded four pike. Angling and photographic identification involving reporting by citizen-scientists provided valuable temporal data in a non-invasive manner compared to traditional sampling. This methodology is based on freeware software and removes the need for any tagging equipment. It can potentially be used on other freshwater fish species and larger populations if the skin pattern is distinctive. However, it relies on the willingness of anglers to report their catches.

Ice angling is a popular activity during the winter months at northern latitudes. For ice-anglers, tip-up rigs are often used to capture sportfish, including northern pike Esox lucius. When a fish grabs a bait rigged on a tip-up, a flag pops up and alerts the angler to the fish. However, the amount of time spent hooked on a tip-up before a fish is retrieved may vary depending on angler attentiveness. Studies of open water fishing have found that an increase in the time between the strike and the hookset can cause an increased probability in hook ingestion by fish, which can lead to mortality. However, this has not been documented for ice-angled fish. To fill this gap, we ice-angled northern pike using tip-ups rigged with one of two hook types (treble or gap) over a span of 4 days, waiting predetermined periods of time (0–7 min) before setting the hook. Sixty-five pike were successfully captured, and 142 bites were recorded over the 4 days of angling. We then noted the hooking location (lip, mouth, or throat/gills) as well as the angling outcome (whether or not the fish was successfully landed). It was determined that increasing time between strike and hookset (retrieval time) does lead to deeper hooking locations (ordinal regression, p = 0.04). However, regardless of retrieval time, none of the fish fully ingested the hook, and longer times more often resulted in the bait being dropped. Therefore, although retrieval time does influence the depth of hooking, it did not influence hook ingestion in this case. Despite this, since prior work has shown that deeper hooking increases the risk of mortality for angled fish, we recommend anglers pursue fishing practices that minimize the time taken prior to retrieval for ice-angled fish.

Lipids play important roles in growth, reproduction, locomotion, and migration of fish. Municipal effluents, which are complex mixtures of biological and chemical compounds including flame retardants, have been shown to alter lipid metabletabolism in environmentally and experimentally exposed fish. Down-regulation of several genes coding for fatty acid metabolism enzymes has previously been reported in male northern pike (Esox lucius) collected in the St. Lawrence River (QC, Canada) downstream of a major primary wastewater treatment plant (WWTP) point of discharge. The main objective of this study was to investigate the effects of exposure to the Montreal's WWTP effluent on the lipidomic profile (i.e., fatty acids, acylcarnitines, and phospholipids) as well as the transcription of genes related to lipid metabolism in the liver of northern pike collected upstream and downstream of this WWTP effluent. Halogenated flame retardant concentrations were also determined in pike liver and used as markers of exposure to this effluent. Greater concentrations of saturated and monounsaturated lysophosphatidylcholines (LPCs) and lower concentrations of polyunsaturated LPCs were determined in the liver of pike collected downstream of the WWTP compared to those collected upstream. Lower mRNA levels of peroxisome proliferator-activated receptor alpha (pparα), a major regulator of lipid metabolism, were also measured in pike exposed to Montreal's WWTP effluent. In addition, the relative contributions (%) of LPC 18:2 and LPC14:0, compounds used as markers of obesity and inflammation, were significantly correlated with halogenated flame retardant concentrations and fish girth. Results of the present study suggest that chronic environmental exposure to a primary WWTP effluent can modulate the transcription of genes related to lipid metabolism, and hence affect the hepatic phospholipid composition of pike from the St. Lawrence River.

F2-isoprostanes (F2-isoPs) are a reliable biomarker class for oxidative stress in vivo in animals. These compounds are traditionally measured in matrices like liver and plasma, however social and environmental pressures warrant the development of non-lethal and non-invasive methods to assess animal health. Therefore, this study aimed to develop a high-performance liquid chromatography tandem mass spectrometry (HPLC-ESI-MS/MS) method to separate and detect F2-isoPs in fish mucus. The method was developed and validated for four native F2-isoP isomers using Northern pike mucus (Esox lucius). Linearity was observed between 5 and 1000 pg/μL. The limits of detection of the four F2-IsoP isomers ranged from 0.63 to 2.0 ng/g. Recoveries ranged from 78 to 95%, and matrix effects were small (

Northern pike (Esox lucius) sperm equilibration and cryopreservation were conducted using a simple cryopreservation extender (CE) containing 185 mM trehalose, 40 mM KCl, 20 mM Tris at pH 8.5 and supplemented with 10% of methanol. Sperm was diluted tenfold (1:9; sperm:CE), and a subset of the samples was equilibrated (over 15 min) and stored on crushed ice (+4 °C). The remainder of the samples were drawn into French straws of 250 μl capacity and subsequently frozen. Thawing of sperm was carried out in a hot water bath at 40 °C for five seconds. The control group was fresh sperm that was neither diluted in CE nor frozen. After equilibration and cryopreservation, sperm motility parameters were analyzed using a Computer-Assisted Sperm Analysis system. Fertilization using different sperm:egg ratios (i.e. 400, 200, 100, 50, 25 and 10 thousand sperm per egg) was performed to determine the fertilization capacity of sperm. Sperm motility parameters of northern pike remain at the same level in fresh, equilibrated and cryopreserved samples. On the other hand, average sperm longevity was longest in equilibrated (118.0 s.) and cryopreserved (124.5 s.) sperm in comparison to fresh sperm (84.7 s.). Sperm samples equilibrated in CE achieved higher (n = 4. P

Environmental contaminant monitoring traditionally relies on targeted analysis, and very few tools are currently available to monitor “unexpected” or “unknown” compounds. In the present study, a non-targeted workflow (suspect screening) was developed to investigate plastic-related chemicals and other environmental contaminants in a top predator freshwater fish species, the northern pike, from the St. Lawrence River, Canada. Samples were extracted using sonication-assisted liquid extraction and analyzed by high performance liquid chromatography coupled with quadrupole time of flight mass spectrometry (HPLC-QTOF-MS). Ten bisphenol compounds were used to test the analytical performances of the method, and satisfactory results were obtained in terms of instrumental linearity (r2 > 0.97), recoveries, (86.53–119.32%), inter-day precision and method detection limits. The non-targeted workflow data processing parameters were studied, and the peak height filters (peak filtering step) were found to influence significantly the capacity to detect and identify trace chemicals in pike muscle extracts. None of the ten bisphenol analogues were detected in pike extracts suggesting the absence of accumulation for these chemicals in pike muscle. However, the non-targeted workflow enabled the identification of diethyl phthalate (DEP) and perfluorooctanesulfonic acid (PFOS) in pike extracts. This approach thus can be also applied to various contaminants in other biological matrices and environmental samples.

Landscape alterations can obstruct movement corridors and degrade spawning habitats for migratory fishes, requiring expensive restoration efforts. To assess use of natural and artificial waterways for spawning migrations, we monitored adult migrations and young-of-year production of northern pike (Esox lucius) for two years in six adjacent tributaries of southern Green Bay on Lake Michigan, USA. Field observations were compared with natal origins of young-of-year and adults inferred from otolith microchemistry. Individual tributaries varied widely in their production of young-of-year pike. Microchemical differences were apparent only among tributaries whose watersheds differ in land use, and adult pike showed no evidence of homing even to the same tributary land use class where they were born. Though otolith microchemistry suggests a lack of fidelity to natal streams, carbon stable isotope ratios of adult pike showed a latitudinal gradient across tributaries, suggesting that adult pike do not mix freely outside of the breeding season. Both field observations and microchemical tracing suggest that pike can potentially recolonize historical or newly-created breeding habitats after restoration efforts make them accessible.

In recreational fishing, catch and release (CR low temperatures tended to result in deeper hooking. Despite this, water temperature did not affect level of bleeding in pike, and low water temperatures may be beneficial as they likely reduce the overall stress response in released pike. (C) 2014 Elsevier B.V. All rights reserved. (Less)

There is concern regarding exposure of aquatic organisms to chemicals that interfere with the endocrine system. One critical mechanism of endocrine disruption is impairment of steroidogenesis that can lead to altered hormone levels, altered or delayed sexual development, and ultimately reproductive failure. With the current large gap in knowledge and a high degree of uncertainty regarding the sensitivity of fishes native to northern ecosystems to endocrine disrupting chemicals (EDCs), the aim of this study was to develop an in vitro gonadal explant assay enabling the assessment of EDCs on sex-steroid production in wild fish species native to North America. Northern pike (Esox lucius), walleye (Sander vitreus), and white sucker (Catostomus commeroni) were sampled from a reference location in Lake Diefenbaker, Saskatchewan, Canada, at spawn and multiple post-spawn time points. Gonads were excised and immediately exposed for 24h to a model inducer (forskolin) or inhibitor (prochloraz) of steroidogenesis in L-15 supplemented media. Furthermore, seasonal profiles of plasma 11-ketotestosterone (11-KT) and 17-β estradiol (E2) concentrations were characterized. Enzyme-linked immunosorbent assays were used to quantify hormone concentrations in plasma and media. The seasonal profile of plasma hormones was significantly correlated with basal in vitro hormone production. Gonad tissue exposed to forskolin showed a concentration-dependent increase in E2 and a general increase in 11-KT. Gonad tissue exposed to prochloraz resulted in a decrease of concentrations of 11-KT and E2. These results illustrated that gonadal tissue is undergoing steroidogenesis in an in vitro setting that is comparable to in vivo hormone profiles, and which is responsive to chemical exposure in a concentration-dependent manner. The seasonal time point during which gonad explants were excised and exposed had an impact on the potency and magnitude of responses, resulting in a seasonal effect on sensitivity. Male and female white sucker showed greatest sensitivity to forskolin, while male and female walleye showed greatest sensitivity to prochloraz. Also, gonad explants from these species were found to have greater sensitivity than responses previously reported for in vitro explants of other fish species such as the fathead minnow (Pimephales promelas), and stable cell lines currently used as screening applications to detect chemicals that might disrupt the endocrine system. Therefore, current approaches that use stable cell lines or tissue explants from standardized small bodied laboratory species might not be protective of some wild fish species. Future research is required that investigates whether this in vitro gonadal explant assay is predictive of in vivo effects in wild species of fishes.

The population vulnerability of a species to pesticides is determined by the risk of its exposure to the pollutant, its sensitivity to the toxicant and the potential to sustain its populations if adversely affected (population resilience). This study uses population modelling to link the life cycle traits of a set of fish species which are highly susceptible to pesticide exposure, to their population resilience as a contribution to the needed identification of representative vulnerable species for higher-tier ecological risk assessment (ERA) of pesticides in Europe. Leslie matrix models for 21 species were parametrized using data from literature on field populations whose resilience was assessed. Phoxinus phoxinus was identified as the species with the least resilient populations for effects on fertility, Lampetra planeri for effects on juvenile survival and Esox lucius for effects on adult survival. Combining the measured effects from standard fish bio-tests with population models of these species would result in an ecologically relevant and protective risk assessment for field conditions in Europe.

Otoliths of perch ( Perca fluviatilis ), pike ( Esox lucius ) and European whitefish ( Coregonus lavaretus ) caught at different locations in the northern Baltic Sea along the Finnish west coast and at some rivers and lakes were subjected to elemental analyses with particle induced X-ray emission and laser ablation inductively coupled plasma mass spectrometry. The strontium concentration in otoliths from whitefish (∼3300 μg/g) was 2–3 times higher than that of perch and pike (∼1400 μg/g), while within species the strontium concentration of otoliths from fish caught at different locations was in the same range. The strontium concentrations were lowest in fish from the lakes (∼450 μg/g). Whitefish otoliths contained more zinc (∼60 μg/g) than those of pike (∼30 μg/g), while the zinc concentration in perch otoliths were below the detection limit. No spatial intraspecies variations in zinc concentrations were observed. X-ray diffraction showed that the otoliths consisted of aragonite solely.

Cellular automata (CAs) allow for transparent modelling of complex systems based on simple transition rules and are flexible in incorporating individual differences and local interactions. They may therefore be particularly suited to answer river management questions that could not be addressed by existing habitat suitability models, such as the optimal distance between spawning grounds. This study explores the usability of CAs for spatio-temporal modelling of a pike population to support river management. Specifically, we evaluated the usability of the CA model by analyzing its sensitivity to three model parameters: the number of pike in the grid, the initial pike distribution and the grid resolution. The model includes habitat characteristics and basic expert knowledge on the ecology of pike and was tested on a 10 km stretch of the river Yser in Flanders (Belgium). Simulation results showed that the model converged to a realistic pike distribution over the study area only at high pike density and low grid resolution, irrespective of the initial pike distribution. Pike density and grid resolution affected the sensitivity to the initial pike distribution in the grid. Specifically, the sensitivity was high at low pike density and high grid resolution, and absent when pike density was high. This analysis indicated that initial conditions and cell size may have a severe impact on the model output, illustrating the importance of firstly analyzing this impact before conducting further analyses. Depending on the outcome of such analyses, CAs can be a promising modelling technique to evaluate and predict the effect of river restoration on pike populations.

a b s t r a c t To date, most studies of commercial fisheries bycatch have focused on mortality at time of capture as an endpoint. However, sub-lethal indicators of organismal condition have the potential to reveal mechanisms associated with mortality (both at time of capture and post-release) and opportunities for improving fish welfare. In this study, we simulated commercial fishing efforts in inland lakes with fyke nets during a typical fishing season (early April to late June) in southeastern Ontario, Canada, where bycatch of non-target fish species had previously been documented. Using non-target gamefish (i.e., large- mouth bass (Micropterus salmoides, Lacepede), northern pike (Esox Lucius, L.)), as well as a target species (i.e., bluegill (Lepomis macrochirus, Rafinesque)), we examined the sub-lethal consequences of capture (e.g., blood physiology, reflex impairment, and injury) and compared the effects of being retained in the net for two different durations (i.e., two or six days) over a range of water temperatures (i.e., 3-28 " C). Sub- lethal physiological disturbances (i.e., blood glucose and lactate) in largemouth bass and bluegill tended to be greater at higher water temperatures. However, fish retained for six days generally did not exhibit greater stress than those retained for two days, with the exception of plasma glucose in largemouth bass. Reflex impairment was similar among temperature and retention periods. Fish retained in nets experi- enced a range of injuries (including fin frays, scale loss, and mouth damage) that had the potential to facilitate the development of opportunistic pathogenic infections. Greater incidences of injury on fish bycatch tended to be associated with higher temperatures and longer retention. To reduce physiologi- cal disturbances and injury that could lead to delayed mortality, we suggest that regulations for inland commercial fishers require them to check their nets more frequently as water temperatures increase. We suggest that future studies of bycatch incorporate sub-lethal endpoints given that they serve as an objective measure of fish welfare and can provide quantitative mechanistic information to support management actions.

For many decades, pike populations in Belgium have been suffering from a decline of the environmental quality due to habitat deterioration, water pollution and many other degrading phenomena. Since past attempts to rehabilitate the pike populations had only limited success, it is of importance to gain insight into the spatio-temporal dynamics of pike such that more effective restoration programs can be effectuated in the future. Ideally, this can be accomplished by relying on telemetry data, but since the collection of such data is both labour-intensive and costly, researchers often resort to a simulation-based approach, which on its turn requires a sound spatio-temporal model. Therefore, and as a first step towards an integrated individual-based model (IBM) for describing pike dynamics in rivers, an IBM mimicking the movement of pike in the river Yser, Belgium, is proposed in this paper. This model considers the specificities of pike, such as its seasonally dependent migration, swimming speed and habitat preference, and is based upon environmental data from the river Yser. It is shown that the in silico spatio-temporal dynamics coincides with the one that is typically inferred from in situ observations. Amongst other things, the proposed model may be relied upon to identify the most appropriate management and restoration measures through a scenario analysis.

In lakes and rivers of eastern Ontario (Canada) commercial fishers use hoop nets to target a variety of fishes, but incidentally capture non-target (i.e., bycatch) gamefish species such as northern pike (Esox lucius). Little is known about the consequences of bycatch in inland commercial fisheries, making it difficult to identify regulatory options. Regulations that limit fishing during warmer periods and that require frequent net tending have been proposed as possible strategies to reduce bycatch mortality. Using northern pike as a model, we conducted experiments during two thermal periods (mid-April: 14.45 ± 0.32 °C, and late May: 17.17 ± 0.08 °C) where fish were retained in nets for 2 d and 6 d. A ‘0 d’ control group consisted of northern pike that were angled, immediately sampled and released. We evaluated injury, physiological status and mortality after the prescribed net retention period and for the surviving fish used radio telemetry with manual tracking to monitor delayed post-release mortality. Our experiments revealed that injury levels, in-net mortality, and post-release mortality tended to increase with net set duration and at higher temperatures. Pike exhibited signs of chronic stress and starvation following retention, particularly at higher temperatures. Total mortality rates were negligible for the 2 d holding period at 14 °C, 14% for 6 d holding at 14 °C, 21% for 2 d holding at 17 °C, and 58% for 6 d holding at 17 °C. No mortality was observed in control fish. Collectively, these data reveal that frequent net tending, particularly at warmer temperatures, may be useful for conserving gamefish populations captured as bycatch in inland hoop net fisheries.

A commonly practiced method intended to reduce mortality from recreational fisheries is mandatory (e.g., small fish protected by minimum-size limits) or voluntary catch-and-release (C&R), where fish are caught with hook and line and released alive on the assumption that fish survive unharmed. C&R can, however, have both lethal and sub-lethal consequences for fish, with altered behaviour serving as a useful indicator of sub-lethal effects. We here present a mesocosm study on the short-term effects on foraging-behaviour in response to C&R in pike (Esox lucius), when being released alone or into conspecific groups. Due to the potential of cannibalistic attacks or agonistic interactions post-release, we expected that foraging behaviour would be affected by social environment at release. We found that the time to first interest in and attack on prey was significantly delayed in caught and released pike individuals, but these delays were less pronounced in pike released into groups of conspecifics. We also found that the caught and released pike expressed agonistic behaviours in comparable frequencies to unfished group conspecifics. We conclude that the short-term effects of C&R involve altered foraging behaviours, partly depending on the social context at release. Altered feeding, even if only in the short-term, may reduce body growth post-release, which may affect individual fitness and also have effects at the fish community level as a result of changes in pike predation pressure.

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

We developed decision trees (J48 algorithm) to predict the distribution of the pike (Esox lucius). Based on a historical data, 75 sampling sites were considered for the pike in 7 stream basins in Flanders (Belgium). In total, 108 instances were available in the given sites. The measured variables consisted of a combination of the structural-habitat, physical–chemical and biological variables (biomass, abundance and presence/absence of the pike). The predictive power of decision trees was assessed on the basis of the number of Correctly Classified Instances (CCI %) and Kappa statistic (k). In order to reduce the noise in the data and improve the predictive results with regard to complexity and accuracy of the predictions, different Pruning Confidence Factors (PCFs) were tested. The obtained results showed that the prediction of the pike (based on presence/absence data) was acceptable in terms of two model evaluations (CCI > 70% and k > 0.40). The habitat variables had more contribution to the prediction of distribution of pike relative to the water quality ones. The developed model presented a logical relationship between distribution of the pike and distance from the source, slope and followed by depth. These models can as such become essential tools to encourage river managers to make the necessary investments and/or activity changes as desired by society.