bluuebunny/crossref_metadata_2025_split huggingface dataset:
Danger at dawn: experimental support for the twilight hypothesis in shoaling minnows
Authors: Pitcher T. J. , Turner J. R. 
Published: January 2006

This experiment aimed to compare responses to predator threat at dawn and in daylight. Using silicon‐intensified video, shoaling European minnows, Phoxinus phoxinus (L.), were observed in large tanks furnished to simulate natural conditions and equipped with artificial dawn and dusk illumination. In the dark, fish were observed under infra‐red light. The appropriate light level for dawn predator exposure was determined behaviourally. During dawn, the behaviour of individual focal minnows was systematically recorded at five light levels set by the behaviour of fish in pilot work.On six predetermined dawn and daytime occasions over a period of 3 weeks, minnows feeding on an artificial vertical feeding patch were subjected to a standardized stalk by a model pike. Under daytime illumination (8 μEm−2 s−1), the minnows were presented with patches once per day. The feeding patches were presented early in the artificial dawn (0.5–1.5 μm−2 s−1) only on days when fish were to be subjected to the model pike.Fish became active at the onset of dawn. A behaviour which appeared at this time is reported for the first time: minnows rose to the surface, appeared to take in and expel bubbles of air, and descended. Shoaling responses and swimming speeds increased soon after the start of dawn. Foraging increased slightly later than this, when illumination had reached about one‐fifth of the daytime level. This was the level chosen for predator exposure.When approached by the pike model at dawn, minnows abandoned feeding on the patch only when the pike reached one‐third of the distance which it obtained under daylight. Individual fish detecting the approaching threat showed skitter and inspection behaviour similar to that observed in daylight, but these behaviours peaked much later in the pike's stalk than in daylight. We conclude that minnows at dawn fail to detect the approaching pike until it is much closer than in daylight; we believe that the work therefore provides the first experimental support for the ‘twilight hypothesis’ that, under low light levels, stalking predators have an advantage over shoaling prey.
Are ‘preferences’ and ‘tolerances’ in cichlid mate choice important for speciation?
Authors: Crapon de Caprona M.‐D. 
Published: January 2006

Although early experience influences the development of preferences in cichlid mate choice, the same species can be very ‘tolerant’ and spawn with members of other species appearing very different from conspecifics. The conditions under which such interspecific spawnings occur in captivity are described, and are considered to exist in the field.Many cichlid species are genetically similar, and interspecific spawning often produces fertile hybrids. Pre‐mating behavioural barriers to hybridization are important. The ability of cichlids to develop preferences based on subtle cues, but also to produce fertile hybrids, is discussed in the context of speciation: genetic isolation of morphs (and possibly of hybrids) from their parental species, through social learning.
Changes in the species composition of anglers' catches in the River Trent (England) between 1969 and 1984
Authors: Cowx I. G. , Broughton N. M. 
Published: January 2006

Data on anglers' catches on the River Trent were collected for the seasons 1969/70 to 1983/84 using postal questionnaires. During this period 60.4t of fish were caught during a total fishing effort of 526 871 man‐h or 60.15 man‐years, and more than 20 species of fish were recorded. The percentage of anglers making a catch (70–80%) and the catch per unit effort, mean 114.7g man‐h −1, were comparable with those reported for other waters.Roach and gudgeon were the most commonly captured species with chub, bleak, bream, eels and dace also forming major components of anglers' catches. Since 1969 chub, bream, eels and perch have made an increasingly greater contribution to catches, coupled with a decline in numbers of roach and dace. It is suggested that improvement in water quality and its implication on interspecific competition was the most likely cause of the changes.
Studies on lesions caused by plerocercoids of Triaenophorus nodulosmin some fish of an alpine lake, the Königssee
Authors: Hoffmann R. W. , Meder J. , Klein M. , Osterkornj K. , Negele R. D. 
Published: January 2006

Lesions caused by plerocercoids of Triaenophorus nodulosus in the second intermediate hosts were studied in fish of an alpine lake, the Königssee. Four species (Arctic charr, Salvelinus alpinus; minnow, Phoxinus phoxinus; burbot, Lota lota: perch, Percafluviatilis) could be shown to be affected, whereas seven other fish species did not have macroscopical or histological signs of invasion. The capsules of T. nodulosus plerocercoids were usually located in liver tissue. Whereas only living parasites were found in charr and minnow, dead and destroyed plerocercoids could be demonstrated additionally in burbot and perch.The histological picture is characterized by capsules surrounded by connective tissue and by fresh or older migration routes in charr and minnow. Furthermore, larvae migrating through the biliary ductules were found regularly in charr. In burbot and perch, different stages of destruction of the parasitic larvae took place. The ability to reject the parasite by immunological reactions seems to depend on the species of host. Even if heavy changes in the liver tissue could be demonstrated, no negative influences on condition and growth of affected fish could be established.
Scanning cytophotometry of carp, Cyprinm carpio L., erythrocyte populations: the influence of short‐term hypoxic environment and the recovery period following severe bleeding
Authors: Schindler J. F. , de Vries U. 
Published: January 2006

Scanning stage absorbance cytophotometry was used to examine haemoglobin contents in individual cells or carp erythrocyte populations. Changes in frequency distributional profiles in response to respiratory stress caused experimentally by hypoxia and/or bleeding were studied at intervals.Histograms were drawn of haemoglobin contents of individual red blood cells in populations obtained from the same carp on four consecutive days after the fish was temporarily exposed to hypoxic environment. The modes and means of the haemoglobin values increased during the 3 days following to the stimulus, whereas a decline was measured on the 4th day. Erythrocyte counts showed the total number of red blood cells to be approximately constant during the period of investigation. On the basis of these findings, it is concluded that the mature, nucleated red blood cells of carp are capable of resuming haemoglobin synthesis after stimuli such as reducing ambient oxygen concentration.Frequency distributional profiles covering a period of 4 weeks following severe loss of blood showed that it took 10–12 days after bleeding until masses of immature erythrocytes appeared in the peripheral blood and that there were then two distinct populations of red blood cells. In the course of about 2 weeks the precursor cells developed into mature erythrocytes.
An electron microscopic examination of the gastrointestinal epithelium in Dover sole, Solea solea (L.)
Authors: Macdonald N. L. 
Published: January 2006

The gastrointestinal tracts of adult and juvenile Dover sole, Solea solea (L.), were examined using scanning (SEM) and transmission electron microscopy (TEM). SEM showed little differentiation of the internal morphology of the gastrointestinal tract in adult fish, with longitudinally arranged mucosal folds present in all gut regions. Mucosal folds had a similar arrangement to that in the goldfish. Goblet cells were identified in the mucosal epithelium in all regions of the gut while microscopic ducts/pores of possible pancreatic origin were observed in the foregut region. SEM of juvenile gut samples showed a similar arrangement of longitudinal mucosal folds to that found in the adult fish. There was no visible evidence of goblet cells or secretory ducts/pores in any region of the juvenile gastrointestinal tract.In TEM it was observed that apical microvilli were a feature of epithelial cells from all regions of the gastrointestinal tract in adult and juvenile Dover sole. Cells from the distal regions of the adult and juvenile gut showed invaginations and vacuolation of the apical cytoplasm. A high degree of vacuolation in cells from the juvenile hindgut‐rectum indicated the possible occurrence of intracellular digestion of absorbed nutrients in this gut region.
Experimental evidence for juvenile Chinook salmon, Oncorhynchus tschawytscha Walbaum, orientation at night and in sunlight after a 7° change in latitude
Authors: Taylor P. B. 
Published: January 2006

A group of eighteen young chinook salmon, Oncorhynchus tschawytscha Walbaum, trained to orientate in the direction 270°, showed a unimodal orientation of 264° and a mean bimodal axis of orientation of 258°/078° (magnetic) within confidence limits of 218°←→285°: 038°←→105° when tested under controlled conditions in Auckland. Two years later 12 of these fish were transferred to a new test arena in Christchurch, south (7° latitude) of the original location, for individual re‐testing at night in a light‐proof room and also in sunlight; in the absence of ‘local’ Auckland cues/clues. These fish had been kept in artificial light but six fish were exposed to sunlight in Auckland 2 weeks before moving to Christchurch.At night each fish showed consistent non‐random orientation and nine out of 10 fish showed a mean bimodal orientation that fell within the confidence limit established in Auckland; the mean of means axis of orientation for 10 fish was 270°/090°. In sunlight each individual fish showed a consistent, non‐random mean bimodal orientation that fell outside the confidence limits established in Auckland and, at night, in Christchurch. The mean of means axis of orientation was 001°/181° (magnetic); the map direction from Christchurch to Auckland is 15°. The principal findings in this study were:(a) ‘Local’ Auckland cues were not essential for location of the learnt direction. (b) A non‐visual ‘universal’ cue was used as a reference for location of direction at night. (c) Daylight orientation was influenced by anisotropic radiance, even though the sun image was obscured by the test arena cover. (d) The concentration of data at midday and sunset in artificial light and perhaps at midnight, may indicate the presence of a non‐visual zeitgeber. (e) A correlation between time and direction, in light and in darkness, which by implication involves a co‐ordination process. (f) The fish under test apparently imprinted on some aspect of the sun (in Auckland) within a 2‐day period at the age of 2 years. (g) Displacement, by 7° in latitude, may have been detected by both visual and non‐visual means.It is proposed that directional information from the geomagnetic field and the anisotropic radiance of sunlight provides a complementary cross reference which would be sufficient for positional orientation, in relation to some fixed landmark, within coastal waters; but would not suffice for spatial orientation in mid‐ocean.
The immunomodulating effect of the microsporidan Glugea stephani on the humoral response and immunoglobulin levels in winter flounder, Pseudopleuronectes americanus
Authors: Laudan R. , Stolen J. S. , Cali A. 
Published: January 2006

A quantitative study of serum immunoglobulin levels and haemagglutination titres has shown that an injection of the microsporidan Glugea stephani caused a decrease in the humoral response of the winter flounder. These examples demonstrate the effect of temperature on the decrease in total IgM levels and haemagglutination titres in winter flounder simultaneously injected with either G. stephani or horse red blood cells.Transfer of serum from fish injected with spores 3 weeks earlier may also implicate soluble serum factors, such as prostaglandins or leukotrienes, causing the observed decrease in serum IgM levels.
The enzyme cytochemistry and composition of elasmobranch granulocytes
Authors: Hine P. M. , Wain J. M. 
Published: January 2006

Peroxidase, alkaline phosphatase, acid phosphatase, β‐glucuronidase, α‐naphthyl acetate esterase (ANAE), α‐naphthyl butyrate esterase, naphthol AS‐D chloroacetate esterase, acetyl‐L‐tyrosine‐α‐naphthyl esterase (ATNE), tosyl‐L‐lysine‐α‐naphthyl esterase (TLNE) and periodic acid‐Schiff (PAS) were studied in 17 species of elasmobranchs in which granulocytes had previously been identified at the ultrastructural level.Eosinophils, eosinophilic and neutrophilic granulocytes contained variable acid phosphatase, esterases and PAS, but they were strongest in neutrophilic granulocytes; particularly ANAE. Esterases were released into surrounding plasma and therefore probably function as ectoenzymes. In eosinophils and some neutrophilic granulocytes there were indications of weak peroxidase, but this could not be conclusively demonstrated. Alkaline phosphatase was diffuse between granules in some eosinophils of Pavoraja, and (β‐glucuronidase was diffuse in neutrophilic granulocytes of Etmopterus baxteri, otherwise granulocytes lacked these enzymes. Neutrophilic granulocytes stained moderately to strongly for ATNE and weakly and inconsistently for TLNE in Squalus acanthias and Dalatias licha. with a similar reaction in granular lymphocytoid and thrombocytoid cells of Galeorhinus ausiralis and Raja nasuta. The enzyme composition of these granulocytes is discussed.
The levels of liver metallothionein and zinc in plaice, Pleuronectes platessa L., during the breeding season, and the effect of oestradiol injection
Authors: Overnell J. , McIntosh R. , Fletcher T. C. 
Published: January 2006

Wild plaice from two locations were examined for liver metallothionein and liver and serum zinc content, before, during and after the breeding season. During the early stages a number of females had very high liver metallothionein and zinc levels. Egg formation and ripening were accompanied by a reduction in serum zinc. In males, the metallothionein levels did not reach such high values and were not correlated with gonad development.There was a correlation between the zinc concentration of the liver and the metallothionein concentration of the liver. Above the threshold value for metallothionein formation, approximately half the additional zinc was found in the metallothionein and half in non‐metallothionein pool(s).Intramuscular injection of oestradiol 3‐benzoate into immature females caused a significant increase in liver weight but a depression in metallothionein concentration relative to the controls.
Quantitative histology of the seasonal changes in the gonads of summer whiting, Sillago ciliata
Authors: Goodall J. A. , Capra M. F. , Blackshaw A. W. 
Published: January 2006

The histology of the gonads of free and captive summer whiting, Sillago ciliata, were compared to assess the potential of this fish for aquaculture. There were no qualitative differences between the free and captive males: both completed spermatogenesis through to spermiation. The captive females were non‐reproductive, reaching only the vitellogenic stage before the eggs were resorbed by atresia.A reproductive score was devised to assess reproductive potential in the comparison of the two populations. The females in the captive population had lower reproductive potentials, as shown by the score, and started development a month later than the normal free population. The captive males had higher reproductive scores than the free fish but they were also not spawning and, therefore, the higher score indicated that this potential was not realized. The score provides a standard against which experimental results can be compared and subsequently assessed.
A comparison of the behavioural responses of parasitized and non‐parasitized three‐spined sticklebacks, Gasterosteus aculeatus L., to progressive hypoxia
Authors: Giles N. 
Published: January 2006

Eighty sticklebacks (32 parasitized, 48 uninfected) were subjected to a serial reduction in dissolved oxygen (DO) concentration until they showed continuous aquatic surface respiration for longer than 10s (> 10s ASR). An analysis of results showed that (1) parasitized adult male and female sticklebacks reacted to low DO significantly earlier than non‐parasitized controls; (2) severity of parasite infection was significantly correlated with DO at which > 10s ASR occurred; (3) gravid, non‐parasitized females show ASR at a relatively high DO, with a threshold response similar to that of parasitized fish.It is suggested that the metabolic respiratory demand of the parasitic tapeworm larvae causes the stickleback to modify its behaviour at lowered DO. This behavioural modification causes the fish to surface, where they are likely to be at risk from predatory birds. Field data on DO levels at the study site show that environmental conditions in the shallow lake margins approach the range required to affect the behaviour of heavily parasitized sticklebacks.
Migration of farmed adult Atlantic salmon with and without olfactory sense, released on the Norwegian coast
Authors: Hansen L. P. , Døsving K. B. , Jonsson B. 
Published: January 2006

The dispersal and migration of farmed Atlantic salmon, Salmo salar, allowed to escape during the summer was studied. Three groups of 4–year–old fish of the River Imsa stock were released in coastal waters off south‐western Norway: one group, with functional olfactory organs, was released at a fish farm 4 km away from the R. Imsa; two other groups, one with transected olfactory nerves and the other with functional olfactory organs, were released in the sea 90 km from the R. Imsa. To compare them with the migration pattern of reared, large smolts of the Imsa stock, a group of 3 + smolts was released in the R. Imsa.Adults of salmon released as 3–year–old smolts homed with high precision to the R. Imsa. Four– year–olds released in the sea were recaptured in the fjord and in the coastal current, the majority north of the places of release. Immatures migrated to feeding areas in the North Atlantic. Matures seemed to enter rivers at random when ready to spawn. There was no difference in migration pattern between anosmics and controls. The olfactory sense was not mandatory for entering fresh water. The results indicate that the homing behaviour of Atlantic salmon is not a direct consequence of a single imprinting of the smolts, and that there is not a direct genetic link for return to a particular river. The present results support the sequential imprinting hypothesis proposed by Harden Jones (1968).
Effects of anaesthetic MS 222 on the orientation of juvenile salmon, Oncorhynchus tschawytscha Walbaum
Authors: Taylor P. B. 
Published: January 2006

Juvenile salmon, Oncorhynchus tschawytscha Walbaum, were trained to orientate in a direction (270°) and then anaesthetized with ethyl m–aminobenzoate methane sulphonate (MS 222) in a test to determine whether anaesthesia affected the learnt orientation.Before anaesthesia the control group of twelve fish showed a mean unimodal orientation of 264° and a bimodal axis of orientation of 258°/78° with a confidence limit (second order analysis) of 218°←→285°. After administration of MS 222 nine out of ten fish showed marked changes in orientation and random behaviour, persisting in two fish for more than 14 days. The mean of means unimodal orientation, after anaesthesia, was 337° (random) and the bimodal axis of orientation was 337°/157° (random).Eight weeks after completion of the anaesthesia trial the fish were retested. Each fish, except one, showed a mean bimodal axis of orientation that fell within the confidence limit of the control. The mean axis of orientation for ten fish over a 3–day period was 270°/90°, (r) = 0.28.The results support the view that the reference orientation in this study (270°) was a learnt, not an innate directional preference. The effects anaesthesia may have on salmon behaviour during migration are discussed.
Circuli spacing and annulus formation: is there more than meets the eye? The case for sockeye salmon, Oncorhynchus nerka
Authors: Barber W. E. , Walker R. J. 
Published: January 2006

This study describes a pattern of circulus spacing on the scales of sockeye salmon, Oncorhynchus nerka, which has not been reported previously. The average distance between circuli formed during the second year of ocean growth is much less than that of first year or in subsequent years. This pattern was persistent for fishes of two river systems, six year–classes and two age groups (2.2 and 2.3) and was due to summer growth. Statistical analyses were conducted only on age 2.2 fish because of small sample sizes in four of the six years for age 2.3 fish. There were significant (p < 0.05) differences in the distance between circuli formed in the summer and in the two stocks for various years at sea. No significant variation in average spacing between circuli was explained by oceanic temperatures.Annuli were formed between November and January. However, oceanic temperatures were declining at this time and do not reach minima until April. Thus, annuli are formed about the time of the equinox and during a declining temperature regime. We hypothesize that the pattern of circuli spacing and annulus formation are responses to photoperiod and food availability which the fish experience during their existence at sea.
The food of the Nile perch, Lates niloticus (L.), after the disappearance of the haplochromine cichlids in the Nyanza Gulf of Lake Victoria (Kenya)
Authors: Ogari J. , Dadzie S. 
Published: January 2006

After the disappearance of the haplochromine species in the Nyanza Gulf of Lake Victoria as a result of predation by Lates niloticus, the latter has turned its attention to aquatic invertebrates and other fish. Changes in the diet of the Nile perch with increase in its size have been observed: young L. niloticus preyed mostly on invertebrates, including crustaceans and various small aquatic insects; large, immature L. niloticus supplemented the invertebrate diet with both young and small fish; adults above 80 cm total length were mainly piscivorous. L. niloticus feeds on fish prey of about one third its own length.The tendency of L. niloticus to switch from one prey item to another, depending on availability, is reported; e.g., in the Nyanza Gulf, the prey diet has shifted from the haplochromine to Caridina nilotica and L. niloticus juveniles.