Size-related changes in habitat selection by larval grayling (Thymallus thymallus L.)

Abstract  – In a medium-sized river in northern Finland, larval grayling shifted with growth from shallow habitats with slow velocities, fine substrata and abundant vegetation cover to deeper sites with swifter velocities, coarse substrata and sparse vegetation cover within 3 weeks. Small (17–21 mm) larvae preferred water depths 10–30 cm, substrata dominated by mud or sand (<2 mm), 10–70% vegetation cover and water velocities <10 cm · s⁻¹. Middle-sized (22–25 mm) larvae preferred 30–90 cm depths, sandy substrata, <40% vegetation cover and velocities <10 cm · s⁻¹. Large (26–31 mm) larvae preferred >50 cm depths, substrata dominated by sand or boulders, <20% vegetation cover and 10–50 cm · s⁻¹ velocities. The strict habitat requirements of the smallest larval group suggest that these habitats, in particular, are important to the early survival of grayling.     

Mass-marking of grayling, Thymallus thymallus (L.), larvae by fluorochrome tagging of otoliths

Abstract Larvae of grayling, Thymallus thymallus (L.) were immersed in tetracycline hydrochloride (TC) and alizarin red S (AL) solutions for 3–4 h at concentrations of 300 and 70 ppm, respectively. Fluorescent marks on the otoliths from the marked fish were identified using a UV-light microscope. Results obtained with AL were superior to TC in marking otoliths. Fish kept in an aquarium retained good marks for 842 days. Stocking of the Carpathian River Ochotnica with marked grayling larvae revealed good mark retention for 128 days and a survival rate of approximately 12.2%.     

Post-stocking dispersal of European grayling,Thymallus thymallus (L.), in a semi-natural experimental stream

Abstract Artificially hatched and reared juvenile European grayling, Thymallus thymallus (L.), from three natural populations were released into an experimental stream. Two of the populations were from lakes and the third population was from a river. Post-stocking dispersal was studied by recapturing fish in traps and by electric fishing after each of the stocking exercises. On all three occasions the first fish to be recaptured were caught in a trap about 400 m downstream from the release site, within 5–12 min of release. Proportions of stocked grayling juveniles recaptured by electric fishing in the fast-flowing section of the experimental stream (the rapids) were 9.9%, 46.7% and 16.6% after the first, second and third stocking, respectively. No significant population-specific difference was found between fish caught in the downstream trap and fish recaptured in the rapids. The uppermost part of the rapids (stretch 1) and the catch in the upstream trap was dominated by fish of riverine origin. Post-stocking migration was influenced by sex but not by body size. Females showed a greater propensity to stay in the rapids compared with males. Implications of the results for stocking grayling in natural streams and regulated rivers are discussed.     

Rearing of European grayling, Thymallus thymallus L., larvae using dry and live food

Abstract. European grayling, Thymallus thymallus L., larvae, after hatching, were fed solely on a dry food diet and compared to those fed live zooplankton. After 28 days of rearing at temperatures ranging from 15·0°C to 18·3°C (mean, 16·4°C) fish fed the dry diet reached 0·21 g individual wet weight and 31mm in total length; those fed zooplankton reached 0·13g and 27·5 mm respectively. The survival rate of fish fed the dry diet was higher (56·6%) than that of fish fed the live food (40·7%). Results are discussed in the light of the development of the alimentary tract in the early ontogeny of grayling.     

Evidence of enhanced bacterial invasion during Diplostomum spathaceum infection in European grayling, Thymallus thymallus (L.)

Farmed grayling, Thymallus thymallus (L.), are susceptible to atypical Aeromonas salmonicida (aAS) infections. Interactions between bacteria and parasites were studied using grayling subjected to concomitant exposure to aAS bacteria and the digenean parasite Diplostomum spathaceum. Atypical AS was detected from fish by a combination of bacterial cultivation and polymerase chain reaction techniques. A detection level of 17 aAS cells per 100 mg intestine tissue sample was obtained. Concomitant bacterial exposure did not enhance the severity of grayling eye rupture and nuclear extrusion induced by D. spathaceum, but D. spathaceum invasion into grayling increased the proportion of fish carrying aAS in their heart tissue. However, the number of aAS cells detected in heart tissue was low. Atypical AS did not cause acute disease or mortality during 15 days post-exposure. There was a higher prevalence of aAS in grayling heart samples than in intestinal samples, indicating that the intestine is not favoured by aAS. We suggest that heart tissue would be a good organ from which to isolate aAS when tracing latent carrier fish. We conclude that penetrating diplostomids can enhance bacterial infections in fish and that diplostomids can cause serious eye ruptures in grayling.     

病毒性出血性败血病

病毒性出血性败血病（Viral haemorrhagic septicaemia,VHS）是由弹状病毒引起鲑、鳟、狗鱼和大菱鲆的一种烈性传染病,表现为出血性败血症,致死率极高。本病主要流行于欧洲及北美,是世界动物卫生组织（OIE）必须申报的疫病,我国将其列为二类疫病。     

Post-stocking behaviour of hatchery-reared European grayling, Thymallus thymallus (L.), and brown trout, Salmo trutta L., in a semi-natural stream

Hatchery-reared, juvenile European grayling, Thymallus thymallus (L.), and brown trout, Salmo trutta L., were each stocked six times into an area of a semi-natural stream. The order in which the two species were released was switched after every second experimental stocking. Temporal and spatial post-stocking dispersal, effects of previously stocked species, feeding behaviour and the influences of sex and size were studied. During each 48-h experimental release period, some fish were recaptured in a trap situated 200 m downstream from the stocking site, and fish remaining in the stream after each experimental release were caught by electric fishing. Significantly more grayling than trout moved downstream and left the semi-natural stream. Proportions of stocked grayling recaptured in the trap within 2 h and from 2 to 48 h post stocking in the stream were 36.4% and 10.0%, respectively. Corresponding recapture rates for brown trout were both 1.5%. Most of the grayling and brown trout that did not leave the stream early were recaptured in deep, slow-moving water at low velocities in the release area. The presence of grayling at the time that the brown trout were stocked resulted in significantly fewer brown trout staying in the upper part of the stream. Within 48 h of their release, 33.3% and 22.8% of the grayling and brown trout, respectively, had eaten natural food items. The mean length of brown trout recaptured in the upper part of the stocking area was significantly lower than that of fish recaptured in the lower part and in the trap. Among brown trout, males showed a significantly greater propensity to eat and to stay in the upper part of the stream near the stocking site compared with females . Brown trout with natural food items in their stomachs had significantly lower mean length than trout without such items. No sex- or size-related differences were found in the spatial distribution or feeding activity of grayling.     

Short-term habitat use by young grayling, Thymallus thymallus L., under variable flow conditions in an experimental stream

Abstract  In an experimental stream containing available refuge zones, moderate and instantaneous flow variations did not cause high mortality or substantial increase in downstream migration of 2-month-old grayling, Thymallus thymallus L. Fish were able to react rapidly to spate simulations by a change in space utilization determined by changes in water velocity patterns. Reaction of fish to these experimental flow variations corresponded to a refuge strategy, i.e. seeking shelter and minimizing the distance to find it.     

Outbreak of viral haemorrhagic septicaemia (VHS) in lumpfish (Cyclopterus lumpus) in Iceland caused by VHS virus genotype IV

A novel viral haemorrhagic septicaemia virus (VHSV) of genotype IV was isolated from wild lumpfish (Cyclopterus lumpus), brought to a land‐based farm in Iceland, to serve as broodfish. Two groups of lumpfish juveniles, kept in tanks in the same facility, got infected. The virus isolated was identified as VHSV by ELISA and real‐time RT‐PCR. Phylogenetic analysis, based on the glycoprotein (G) gene sequences, may indicate a novel subgroup of VHSV genotype IV. In controlled laboratory exposure studies with this new isolate, there was 3% survival in the I.P. injection challenged group while there was 90% survival in the immersion group. VHSV was not re‐isolated from fish challenged by immersion. In a cohabitation trial, lumpfish infected I.P. (shedders) were placed in tanks with naïve lumpfish as well as naïve Atlantic salmon (Salmo salar L.). 10% of the lumpfish shedders and 43%–50% of the cohabiting lumpfish survived after 4 weeks. 80%–92% of the Atlantic salmon survived, but no viral RNA was detected by real‐time RT‐PCR nor VHSV was isolated from Atlantic salmon. This is the first isolation of a notifiable virus in Iceland and the first report of VHSV of genotype IV in European waters.     

Putative virulence factors of atypical Aeromonas salmonicida isolated from Arctic charr,Salvelinus alpinus (L.), and European grayling,Thymallus thymallus (L.)

Atypical Aeromonas salmonicida (AAS) causes generalized lethal infections in farmed Arctic charr, Salvelinus alpinus (L.), and European grayling, Thymallus thymallus (L.), and is thus a serious threat for culture of these fish species. Virulence factors were studied among isolates of AAS from Arctic charr (n = 20), European grayling (n = 19) and other fish species (n = 20), of which 48 were of Finnish and 11 of Swedish origin. All isolates produced an A-layer. Extracellular products (ECP) of the AAS isolates did not produce detectable gelatinase and caseinase activity in test assays. Analysis of the same ECP preparations with substrate sodium dodecyl sulphate-polyacrylamide gel electrophoresis showed weak proteolytic activity, indicating the different sensitivity of the detection methods used. The ECP from AAS isolates showed low cytotoxic activity against cultured cells. However, the ECP did not induce mortality in challenged Arctic charr. The results suggest that toxic components, like ECP, secreted by the bacterium may not be the major virulence factor in AAS-infection in Arctic charr and European grayling, and hence the pathogenesis also differs from the pathogenesis of AAS-infection in Atlantic salmon, Salmo salar L.     

Effects of rearing technique and fish size on post-stocking feeding, growth and survival of European grayling, Thymallus thymallus (L.)

European grayling, Thymallus thymallus (L.), were reared by four different techniques: (A) in a natural pond without a supply of artificial diet; (B) in a net pen fed artificial dry diet; (C) in tanks fed artificial dry diet; (D) in tanks, and for 16 days before stocking, transferred to a diet based on mixed frozen natural food exclusively. Fish within groups C and D were also split into subgroups of large and small fish. Feeding, growth and survival of fish in the different groups were compared after stocking into three different stocking sites during the spring of 1994. The stocking sites consisted of a barren lake (Lake L), a second lake (Lake N) containing an allopatric perch, Perca fluviatilis L., population, and a barren man-made pond. In Lake L, higher recapture rates and mean weights of stomach contents were obtained in fish reared in the natural pond and in the net pen. There were no differences in total recapture rate, or mean condition factor between small and large fish within groups C and D. In Lake N only five (0.5%) stocked grayling from rearing groups A and B were recaptured during the experiment and more fish from categories C and D were recaptured. This was probably related to size-selective predation. In the pond, there were no differences in recapture rates or in mean weight of stomach contents between fish from any of the groups. In sample 1 (Lake N, 22 days after stocking), mean weight of stomach contents was higher in the larger fish within categories C and D but there was no difference in condition factor between the groups. In sample 1, mean condition factor of fish from the different groups varied markedly, being highest in Lake L followed by Lake N and the Pond. Fourteen out of 26 dead fish in the pond showed clinical symptoms of the bacterial disease ASA. Mortality was related to low initial condition factor at the time of stocking. No stocked grayling were recaptured from the two lakes in the last sample (June 1995).     

Conservation genetics and management implications for European grayling, Thymallus thymallus: synthesis of phylogeography and population genetics

Abstract  European grayling, Thymallus thymallus (L.), populations have steadily declined since the mid-1980s, mostly because of habitat degradation and increased predation pressure. To provide guidelines for conservation strategies and future management programmes, delineation of conservation and management units is warranted. Over the last decade, several studies have investigated the phylogeography and population genetic structure of T. thymallus using mitochondrial and nuclear microsatellite DNA markers. However, no study has covered the distribution range of T. thymallus across Europe; thus, a comprehensive survey of the evolutionary history of the species is lacking. This paper synthesises the major findings of the molecular studies and provides guidelines for the conservation and management of genetic resources of T. thymallus . At least five major mtDNA lineages have evolved in geographical isolation during the Pleistocene and these lineages should be recognised as the basic evolutionary significant units for T. thymallus in northern, central and southern Europe. There is also evidence for a high level of admixture among major lineages and sublineages, especially in the contact zones of drainages (e.g. in mainland Sweden and Norway and in central Germany), most probably resulting from a complex process of post-glacial and inter-glacial colonisation and re-colonisation events from different refugia during the Pleistocene ice ages. Based on the microsatellite data, T. thymallus shows a substantial level of inter-population genetic differentiation and, compared with other freshwater fish species, a relatively low level of within-population genetic diversity. The species develops discrete population structure, both within hydrologically connected rivers or lakes on comparatively small scales as well as in large river systems.     

Angling pressure, yield and catch per effort of grayling, Thymallus thymallus (L)., and brown trout, Salmo trutta L., on the rivers Glomma and Rena, southeastern Norway

Abstract Postal questionnaires, interviews and accurate reports from anglers were used to study angling activity, yield and catch per effort of grayling, Thymallus thymallus (L.), and brown trout, Salmo trutta L., on three stretches of the regulated river Glomma and two stretches of its tributary, the river Rena in southeastern Norway. Angling activity was expressed as hours angling per kilometre of river and varied from 364 (CL 245–598) to 1337 (CL 912–1991) h km^-1. Grayling and brown trout dominated the catches. The catches were estimated between 476 (CL 334–659) and 753 (CL 517–1155) grayling and brown trout per kilometre river stretch. The mean weights were estimated between 216 and 353 g for grayling, and 260 and 395 g for brown trout. The yield of grayling and brown trout was estimated between 8.7 (CL 6.1–12.4) and 46.4 (CL 34.9–62.3) kg ha^-1 yr^-1. The quotient grayling/brown trout in the catches varied between 0.41 and 5.1, and was found to be significantly negatively correlated with angling effort ( r = - 0.838, P <0.05). Catch per effort was estimated between 132 (CL 82–218) and 382 (CL 287–513) g h^-1, and was also negatively correlated to angling effort, according to: Catch per effort (g h^-1) = - 0.210 × effort (h km^-1) + 405 ( r = and 0.899, P <0.02).
Angling activity in earlier years was determined from number of permits sold where data were available. On stretches where angling activity was moderate, there was a tendency towards increase, whereas on stretches where activity had been high for several years, the activity seemed to have stagnated, indicating that a saturation had been reached.     

Pathological changes in the midgut epithelium of grayling, Thymallus thymallus L., larvae reared on different kinds of food, and their relation to mortality and growth

Abstract. The histological differentiation of the digestive tract in grayling, Thymallus thymallus L., larvae was studied during the first 20 days after hatching. The normal development was assessed in larvae fed with nauplii of two Anemia strains. These results were compared with a starved control. Grayling larvae reared on lake zoo-plankton suffered mortality which was ten times higher than that in the Artemia-fed groups. The histopathology of the zooplankton-fed larvae was characterized by excessive fat accumulation in the apices of columnar absorptive cells, complete absence of fat transport into the villus lamina propria, and disintegration of enterocyte cell membranes. This pathology is compared with the clinical findings in human abetalipoproteinaemia. It is concluded that the histopathological manifestations and the resulting elevated mortality in larval grayling were provoked by a disturbance of the fat absorption process in the enterocytes. This disturbance is likely to be due to the inability to synthesize chylomicrons. It is further hypothesized that lake plankton either contains a toxic agent which interferes with the build-up of chylomicrons or is deficient in some lipoprotein moiety which is essential for chylomicron synthesis.     

Seasonal variation in habitat use by salmon, Salmo salar, trout, Salmo trutta and grayling, Thymallus thymallus, in a chalk stream

Abstract A portable multi‐point decoder system deployed in a tributary of the River Itchen, a southern English chalk stream, recorded the habitats used by PIT‐tagged juvenile salmon, Salmo salar L., trout, Salmo trutta L. and grayling, Thymallus thymallus L., with a high degree of spatial and temporal resolution. The fishes’ use of habitat was monitored at 350 locations throughout the stream during September/October 2001 (feeding period) and January/February 2002 (over‐wintering period). Salmon parr tended to occupy water 25–55 cm deep with a velocity between 0.4 and 1.0 m s^?1. During both autumn and winter, first year salmon (0+ group) were associated with gravel substrate during the daytime and aquatic weed at night. In autumn, 1+ salmon were strongly associated with hard mud substrates during the day and with marginal tree roots at night. In winter, they were located on gravel substrate by day and gravel and mud at night. Trout were associated with a greater range of habitats than salmon, generally occupying deeper and faster water with increasing age. During the autumn, 0+ trout were located along shallow (5–10 cm) and slow (?0.1–0.4 m s^?1) margins of the stream, amongst tree roots by day and on silty substrates at night. During winter the 0+ trout occupied silty substrates at all times. As age increased, trout increasingly used coarse substrates; hard mud, gravel and chalk, and weed at night. All age groups of grayling (0+, 1+ and 2+) tended to occupy hard gravel substrate at all times and used deeper and faster water with increasing age. The 1+ and 2+ groups were generally found in water 40–70 cm deep with a velocity between 0.3 and 0.5 ms^?1, whilst the 0+ groups showed a preference for shallower water with reduced velocity at night, particularly in the winter. There were greater differences in the habitats used between species and age groups than between the autumn and winter periods, and the distribution of fish was more strongly influenced by substrate type than water depth or velocity. The results are discussed in relation to the habitat requirements of mixed salmonid populations and habitat management.