Trophic flexibility and opportunism in pike Esox lucius

The first comprehensive investigation of pike Esox lucius trophic ecology in a region (Ireland) where they have long been thought to be a non‐native species is presented. Diet was investigated across habitat types (lake, river and canal) through the combined methods of stable‐isotope and stomach content analyses. Variations in niche size, specialization and the timing of the ontogenetic dietary switch were examined, revealing pronounced opportunism and feeding plasticity in E. lucius, along with a high occurrence of invertivory (up to 60 cm fork length, L_F) and a concomitant delayed switch to piscivory. Furthermore, E. lucius were found to primarily prey upon the highly available non‐native roach Rutilus rutilus, which may alleviate predation pressure on brown trout Salmo trutta, highlighting the complexity of dynamic systems and the essential role of research in informing effective management.     

Experimental evidence of cannibalism and prey specialization in Arctic charr,Salvelinus alpinus

Synopsis Use of a radiographic technique enabled the study of prey selection and individual specialization in Arctic charr, Salvelinus alpinus, fed with small charr and dry pellets under laboratory conditions. Both naive and experienced fish (mean weight 475 g, mean length 34.9 cm), selected the smallest individuals when offered juvenile charr (6–16 cm) as prey. The selected prey were, on average, 22% of the predator length. Cannibalism appeared to involve individual specialization, since when groups of large charr were given the opportunity to feed on juveniles one day every two weeks, the same individuals were cannibalistic throughout the sampling period of two months. When large charr were presented with alternate cycles of different food types consisting of dry pellets and fish prey, the charr exhibited a strong and consistent feeding specialization with three distinct groups being recognised: cannibals, pellet eaters and non-feeding fish.     

Four decades of opposing natural and human-induced artificial selection acting on Windermere pike (Esox lucius)

The ability of natural selection to drive local adaptation has been appreciated ever since Darwin. Whether human impacts can impede the adaptive process has received less attention. We tested this hypothesis by quantifying natural selection and harvest selection acting on a freshwater fish (pike) over four decades. Across the time series, directional natural selection tended to favour large individuals whereas the fishery targeted large individuals. Moreover, non-linear natural selection tended to favour intermediate sized fish whereas the fishery targeted intermediate sized fish because the smallest and largest individuals were often not captured. Thus, our results unequivocally demonstrate that natural selection and fishery selection often acted in opposite directions within this natural system. Moreover, the two selective factors combined to produce reduced fitness overall and stronger stabilizing selection relative to natural selection acting alone. The long-term ramifications of such human-induced modifications to adaptive landscapes are currently unknown and certainly warrant further investigation.     

The potential for intracohort cannibalism in age-0 walleye pollock,Theragra chalcogramma,as determined under laboratory conditions

Synopsis In laboratory experiments, we tested the capability of larger age-0 walleye pollock to consume smaller members of their cohort. In separate aquaria, 81 pairs of juveniles covering a wide range of size differences (total lengths differing by 12 to 61 mm) were held and monitored over a 4 day period. Complete consumption, in which a smaller fish was swallowed whole by a larger fish, occurred 11% of the time. In 36% of the pairs, attacks by the larger fish resulted in mortality of the smaller fish. The mouth width:body depth ratio between the larger and smaller fish of a pair differed significantly depending on whether the smaller fish survived, was killed but not consumed, or was ingested whole by the larger fish. Cannibalistic individuals could consume fish close to the maximum size physically possible under gape limitation; at this size the length of the cannibal was approximately 1.7 times the length of the prey. Length-frequency distributions of age-0 pollock in field concentrations suggested that, at least in some geographical areas, potential cannibals and prey commonly co-occur. Unsuccessful predatory attacks by larger individuals may have additional detrimental effects on smaller pollock in natural populations.     

Sibling cannibalism in Koi carp, Cypvinus carpio L., larvae and juveniles reared under controlled conditions

The potential impact of sibling cannibalism on the size and population structure of captive populations of Koi carp, Cyprinus carpio L., which were fed only dry foods, was determined. The rate of cannibalism was found to be positively density-dependent, with the highest rate being recorded at initial stocking densities of 40 larvae 1 (33.3% of the fish had been eaten by cannibals by day 52). Two types of cannibalism could be distinguished: type I, where the head of the prey was rejected once the caudal region of the prey had been progressively ingested; type II, where the prey were caught either head-first or tail-first and were ingested completely. The first type was limited to the period from the ninth to the 15th day of rearing, and the second type started on the 18th day. Measurement of the mouth, head and tail allowed the calculation of predicted cannibal -prey relationships. The consequences and implications of coeval sibling cannibalism in culture systems are discussed.     

Foraging success of juvenile pike Esox lucius depends on visual conditions and prey pigmentation

Young-of-the-year pike Esox lucius foraging on copepods experienced different foraging success depending on prey pigmentation in water visually degraded by brown colouration or algae. Both attack rate and prey consumption rate were higher for E. lucius foraging on transparent prey in brown water, whereas the opposite was true in algal turbid water. Pigments in copepod prey may have a cryptic function in brown water instead of a photo-protective function even if prey-size selectivity was stronger than selection based on pigmentation in juvenile E. lucius.     

Early life characteristics of pike, Esox lucius, in rearing ponds: temporal survival pattern and ontogenetic diet shifts

Survival, biomass and diet of pike, Esox lucius , larvae and juveniles were studied over 3 years by stocking free embryos at a density of six fish m⁻² in 12 drainable outdoor ponds. The ponds were sequentially drained at six larval/juvenile developmental stages, up to a total length (T.L.) of 139 mm. The mean rate of survival at harvest decreased irregularly over time and the highest mortality rates were recorded during the early larval period (13 to 27 mm t.l .) and two intervals of the juvenile period (46 to 99 mm T.L. and 121 to 139 mm t.l .). Mean biomass increased dramatically between 46 mm T.L. (19.8 kg ha⁻¹) and 121 mm T.L. (181.8 kg ha⁻¹) and stabilized between 121 and 139 mm t.l . Sharp increases in the mean weight coefficient of variation were recorded during the early larval period (13 to 27 mm t.l .) and between 74 and 121 mm t.l . (development of cannibalism). Diet breadths were relatively narrow in pike larvae and reached maximum levels in 99 mm t.l . juveniles. Average-sized pike exhibited a sequence of size-dependent shifts from a diet composed primarily (in terms of weight) of micro-crustaceans (at 13 mm t.l .), to chironomid larvae (at 74 to 99 mm t.l .), and finally macrocrustaceans (at 121 to 139 mm t.l .). Cannibalism was detected first among the largest fish at the 74 mm t.l . stage. Between-year diet similarity at various developmental stages was consistently high. In 70 mm t.l . fish harvested from different ponds, we found significant among-pond differences in diet composition; however, similar trends of diet changes in relation to fish size were observed from pond to pond. Our results are discussed in light of existing knowledge of young pike trophic ecology and current aquaculture practices.     

Shift in pike, Esox lucius L., predation pressure following the introduction of ruffe, Gymnocephalus cernuus (L.) to Loch Lomond

Rapid and dramatic change in the fish community of Loch Lomond has resulted from a series of fish introductions in recent years. A comparison of the diet of pike, Esox lucius L., in 1989–1990 with data from 1955–1967, prior to recent introductions, demonstrates a shift in prey choice. In 1955–1967 powan, Coregonus lavaretus (L.), dominated in the diet (57% of prey by number) by 1989–1990 the introduced ruffe, Gymnocephalus cernuus (L.), was the commonest prey species (44% by number). This shift in pike predation to an abundant population of introduced ruffe has consequences for native species. Assuming that the pike population has not increased in response to increased food availability due to the introduction of ruffe, pike predation pressure on native species will be relieved. This is likely to have the greatest effect on powan. Comparison of the predation rate in 1955–1967 with 1989–1990 supports the hypothesis that the rate of predation on powan has declined, although the effect that this may have on the powan population is unclear, as the role of predation in the regulation of population size is unknown for this species.     

Northern pike (Esox lucius L.) and aquatic vegetation,tools in the management of fisheries and water quality in shallow waters

Species and size composition of fish communities in shallow stagnant waters appear to be associated with the type, abundance and pattern of the vegetation. Man-induced impacts as eutrophication, and suppression of vegetation for reasons of water quantity management or angling pleasure may induce irreversible changes in the aquatic ecosystem. Water quality management should aim at restoring former pike habitat. Submerged weeds are important to that effect. These habitats are characterized by relatively low densities of fish stocks. Stocking of bream and carp interferes strongly with these objectives. With respect to fish stocks, interests of water quality and fishery management may be opposed.     

Temporal stability in size distributions and growth rates of three Esox lucius L. populations. A result of cannibalism?

The population total length ( L _T) structures and individual growth trajectories for three stream living pike Esox lucius populations were studied for 7 years. All three populations exhibited small variation in both population L _T structure and individual growth trajectories over time. These dynamics contrasted to the much more variable population L _T structure of perch Perca fluviatilis studied previously. The difference in population dynamics between the two species was related to differences in prey:predator size ratios. The pike populations in the more open and larger streams grew to larger sizes, but this difference in life history did not affect population dynamics of pike. It is concluded that (1) cannibalistic population dynamics may be predicted from individual life-history characteristics such as minimum and maximum victim:cannibal size ratios and (2) the cannibal-driven population dynamics observed in pike seems to be robust to variation in environmental conditions (system openness).     

General morphology and axonal ultrastructure of the olfactory nerve of the pike,Esox lucius

Summary The olfactory nerve of the European pike (Esox lucius) contains 5.1 × 10⁶ axons with an average diameter of 0.20 ± 0,04 m and a length of 5.5 cm in 1 meter long pike. Each axon contains an average of 4 microtubules as well as neurofilaments, smooth endoplasmic reticulum and about 500 mitochondria per centimeter. The number of neurofilaments ranges from zero in 15% of the cross sections to over 10 in 6%. Neurofilaments generally occur in clusters located opposite to microtubule regions. Smooth ER can not be identified in 14% of the cross sections suggesting that this structure may not be continuous. Microtubules often display annular regions (halos) of low electron density ranging in size from 800 to 1300 Å. Halos from adjacent tubules usually merge into regional halos. The ratio of axoplasm to glial cytoplasm is 4.4:1, while the ratio of axonal plasma membrane to glial plasma membrane exceeds 7:1. A 4 cm nerve contains 1280 cm² of axolemma. This nerve represents an extreme in high density axonal packing and is therefore exceptionally well suited for biochemical, biophysical and physiological investigations.     

The annual reproductive cycle of pike, Esox lucius L., in two Scottish lakes

Histological changes in the ovaries of pike from two shallow, adjacent, Scottish lakes are described and related to a macroscopic scale of maturity stages and to changes in gonad weight. There was no evidence of pre-ovulatory degeneration although occasional residual oocytes were present in spent ovaries. The gonadosomatic index (GSI) was high in males from October to March, and in females it increased through the winter months to a maximum immediately prior to spawning. Maximum ovarian weight was 8.3–9.0% of total body weight compared with only 0.9–1.1% for testes. These GSIs were low compared with other reports for the British Isles, suggesting lower reproductive output. Spawning occurred in the first two weeks of April at a water temperature of 6.0–7.5° C. Fecundity was described from a regression of egg number (F) on fork length (f.l.): log₁₀F= -1.51 + 3.33 log₁₀ f.l. (cm). Mean egg diameter after preservation varied from 1.1 to 2.4 mm over the size range examined. Males were mature at age 2 at a length of ≥ 27 cm, while females first spawned at age 3 at a length of ≤ 28 cm, although a minority were mature at age 2. The sex ratio was 1.2 males: 1 female at Kinord and 1.4 : 1 at Davan. Greater egg production at Davan, 50 678 ha^?1, compared with Kinord, 33 557 ha^?1, was due to higher individual fecundity-at-age resulting from better growth. Fecundity (19.29 × 10³ calculated for a pike of 55 cm F.L.), relative fecundity of only 10–24 eggs per gram wet weight of adult, and GSI values were low compared with pike in North America and Eurasia. Food supply may restrict reproductive output of pike in Lochs Kinord and Davan. The abundance ratio of predator: prey is high with the main prey species, perch, present in low numbers, and the only alternative species is eel in these simple fish communities.     

Genomic inbreeding and population structure of northern pike (Esox lucius) in Xinjiang,China

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.     

Population regulation of a tropical damselfly in the larval stage by food limitation,cannibalism, intraguild predation and habitat drying

The relative importance of intraspecific, interspecific, and seasonal causes of larval mortality were investigated for aquatic larvae of the giant damselfly Megaloprepus coerulatus in Panama. These larvae live in water-filled holes in fallen and living trees, where they and three other common odonate species are the top predators. By mid wet season, M. coerulatus larvae were found in nearly half of all tree holes that harbored odonates. Although M. coerulatus were typically, but not always, eliminated from holes inhabited by larger hetero-specifics, M. coerulatus were more likely to encounter conspecifics than other odonate species. Hole with less than 11 of water rarely contained more than a single larva. In large holes where M. coerulatus was the only odonate species present, multiple larvae coexisted at a density of one larva per 1–21 of water. There the absence of 2–4 of the 5 larval size classes, despite a continuous input of eggs, suggested that cannibalism was a common cause of mortality. The size of the final instar, which determined adult body size, was correlated positively with tree hole volume for male, but not female, larvae. Experiments showed that when two larvae were placed together in 0.4–1 holes with abundant tadpole prey, the larger larva killed the smaller one. Often the larva that was killed was not eaten. Small larvae were more tolerant of each other than were pairs of medium or large larvae. Before killing occurred, the presence of larger larvae reduced the growth of smaller individuals, relative to controls. Obligate killing was density-dependent. In 3.0–1 holes with ad libitum prey, conspecific killing occurred until the larval density stabilized at one larva per 1–1.5 I, similar to the density found in large holes under field conditions, For M. coerulatus, cannibalism functions to reduce the number of potential competitors for food in addition to providing nutrition. When interactions between paired larvae in small holes were experimentally prevented, competition for food reduced the growth of one or both larvae relative to controls. Holes that were watered during the dry season supported larval densities similar to those in the wet season. Thus, dry season mortality could not be attributed to a decrease in available prey. Rather, M. coerulatus larvae could not survive more than 1 month of complete drying. Because the dry season typically lasts more than 6 weeks, habitat drying is a secondary source of mortality, affecting second- or third-generation larvae that fail to emerge before tree holes dry out completely.     

Visibility conditions and diel period affect small-scale spatio-temporal behaviour of pike Esox lucius in the absence of prey and conspecifics

Pike Esox lucius in the absence of prey and conspecifics were shown to have the highest habitat-change activity during dusk and to decrease preference for complex habitats in turbid water. As the behaviours indicate routine responses in the absence of behavioural interactions, E. lucius spatio-temporal distributions should be directly affected and thereby more easily assessed and avoided by prey, with potential consequences for encounter rates.     

The population dynamics of pike,Esox lucius,and perch,Perca fluviatilis,in a simple predator-prey system

Synopsis The population dynamics and predator-prey relationship of pike, Esox lucius, and perch, Perca fluviatilis, were examined in simple fish communities in two adjacent shallow lakes, Lochs Kinord and Davan, Deeside, Scotland. Few perch survive to age 3 but Z is low for fish > 3 years and perch live up to 17 years. Population fecundity remained relatively high and constant in perch because of the multi-age spawning stock and the presence of older more fecund perch. Growth rates of perch in both lochs are relatively high as a consequence of low stock abundance. The N, B, and P of adult perch were unusually low. The age range of pike, and N, B, P, and growth were in the range of values reported elsewhere. There was little variation in the strength of pike year classes and the importance of cannibalism and low occurrence of alternative prey in the lochs suggest that the populations were self-regulating. Cannibalism by adults was responsible for most of mortality in perch larvae, and predation by pike and adult perch was responsible for the majority of juvenile losses. This conclusion is supported by the high biomass ratios of pike:juvenile perch of 1.0–30.8. While the number of adult fish was almost equal, the biomass of adult pike was 2–3 × that of perch in Kinord and 6 × in Davan. In L. Kinord, where year class strength was stable, high predation pressure from perch and pike reduced perch abundance rather than eliminated year classes. Perch year classes fluctuated in abundance in L. Davan and were eliminated in the first summer in two sampling years. The pike, and particularly the perch populations, have features characteristic of fish communities in unperturbed ecosystems: namely, a wide range of age classes, stability in biomass with variation dampened by longevity, and low production.     

Species-specific probe,based on 18S rDNA sequence,could be used for identification of the mucilage producer microalga <Emphasis Type="Italic">Gonyaulax fragilis</Emphasis> (Dinophyta)

Brook trout (Salvelinus fontinalis) in Appalachia experience prolonged periods of poor feeding conditions, particularly during summer and fall. To determine which prey organisms are important in sustaining brook trout populations, we monitored the feeding patterns of a population of brook trout over the course of 2 years with an emphasis on seasonal change. We employed a bioenergetics model to estimate whether or not each fish had obtained enough energy to meet daily metabolic demand. As a result, qualitative comparisons between fish feeding above maintenance ration (successfully feeding fish) and fish feeding below maintenance ration (unsuccessfully feeding fish) were possible. With the exception of winter, brook trout derived significantly more energy from terrestrial organisms than aquatic organisms. During each season, successfully feeding brook trout fed on greater proportions of specific prey types. Terrestrial Coleoptera and Lepidoptera consistently proved to be important prey during warmer seasons, while large organisms such as vertebrates and crayfish appeared to be important during winter. Our findings suggest that terrestrial organisms are more important than aquatic organisms in sustaining brook trout populations. Further, certain large and abundant terrestrial taxa are critical in providing energy to brook trout.     

The link between respiratory capacity and changing metabolic demands during growth of northern pike, Esox lucius L.

The relationship between metabolic rate of pike (Y, mgO2) and body weight (X, g) over the range 40–1291 gat 15° C is of the form: Y=aX^b. For resting metabolic rate (V_o2, rest), the scaling coefficient, b , is 0.80 and for maximum metabolic rate measured after exhaustive swimming (V₀₂, max), b is 0.99. Factorial metabolic scope (V₀₂, max/ V₀₂, rest) increases with body weight. Peak postprandial oxygen consumption (V₀₂, ASDA) is a constant multiple of V₀₂ rest for any discrete meal (expressed as % of body weight) up to 10% body weight. V₀₂ASDA after a single meal can utilize the entire metabolic scope (V₀₂, max—V₀₂, rest) of juvenile but not adult pike.     

Response of Ceratitis capitata to citrus chemicals under semi-natural conditions

Responses of Mediterranean fruit fly Ceratitis capitata (Wiedemann) (Diptera: Tephritidae) adults to chemicals from citrus fruits were quantified in choice tests conducted in large field cages housing naturally planted orange trees, on the Greek island of Chios. Both sexes responded strongly to chemicals released from artificial cuts made in the pulp of peeled oranges and also to natural or commercial orange juice applied to the surface of yellow 7.0 cm diameter spheres. Males also responded strongly to chemicals released from cuts made in the oily region of the orange peel, or flavedo and less to sour orange and lemon. In contrast, females did not respond to flavedo chemicals. Male response to peel chemicals occurred throughout the day and increased as the number of cuts per fruit increased from 2 to 8. Contact of male mouthparts with sap from cuts, and mating attempts between males on or near the cuts were frequently observed. The males also responded to commercial essential orange oils applied to yellow spheres, but less to essential mandarin oils. The role of citrus chemicals in host finding behaviour of C. capitata and their potential use for the development of effective traps for monitoring and controlling the fly are discussed. The experimental method followed in this study can be used as a standard technique for evaluation of the quality of flies to be used in sterile insect technique programmes.     

The population biology of pike, Esox lucius L., in two gravel pit lakes, with special reference to early life history

The population biology of pike in two gravel pit lakes of contrasting habitat type were studied. The density of pike (1 + and over) in November 1987 in the Main Lake was 7.7 fish ha⁻¹ (19 kg ha⁻¹) and in St Peter's was 71 fish ha'(53 kg ha⁻¹). The age distribution showed that recruitment was consistent in St Peter's but variable in the Main Lake. The main factor causing this difference in recruitment is abundance of aquatic vegetation. Survival up to November 1987 of 0-group pike in the Main Lake was 0.00175% from potential egg deposition, producing 6.6 fish ha⁻¹ (0.83 kg ha⁻¹). In St Peter's survival was 0.00315%, producing 23 fish ha⁻¹ (3.04 kg ha ¹).