The susceptibility of Salvelinus fontinalis (Mitchill) to Gyrodactylus salaris Malmberg (Platyhelminthes; Monogenea) under experimental conditions

The host specificity of Gyrodaclylus Solaris is examined experimentally with respect to its ability to infect the brook trout, Salvelinus fontinalis . The parasite readily attached to and reproduced on parr of this host and infections grew for c. 20 days from first monitoring (c. 30 days from first infection) before declining. Parasites could persist on this host for up to 70 days before finally disappearing. The pattern of infection resembled that seen in many other gyrodactylid species on their normal hosts, and suggested the action of a host response, In this respect infections of G. salaris on parr of S. fontinalis , anadromous Salvelinus alpinus, Oncorhynchus mykiss, Thymallus thymallus and Baltic Salmo salar follow a normal pattern, while infections of Norwegian S. salar are unusual in a continued unchecked growth, until the host dies, under pooled laboratory conditions.     

Susceptibility of brown trout to Gyrodactylus salaris (Monogenea) under experimental conditions

Brown trout ( Salmo trutta ) from anadromous River Lierelva, resident Lake Tunhovd, and resident Nordmarka stocks were exposed to Gyrodactylus salaris -infected salmon parr. The brown trout were fed pellets before the experiments, except for one group of the Nordmarka stock which was starved for 19 days before the experiments. The mean number of parasites declined directly and rapidly post infection for all groups of trout. There were no pronounced differences in resistance between the anadromous and the resident stocks. G. salaris infections tended to persist longer on starved than on fed trout of the Nordmarka stock. The maximum parasite persistence on trout was 50 days, and as parasite numbers increased on some fish parasite reproduction must have occurred on those trout. However, the limited susceptibility and marked innate resistance of trout to G. salaris establishment, development and reproduction, suggest parasite metapopulations will not survive on this species. Nevertheless, trout may still play a role in the dispersal of G. salaris within and between rivers.     

Arteriosclerosis in steelhead trout, Oncorhynchus mykiss (Walbaum): a developmental analysis

Arteriosclerosis in Oncorhynchur mykiss has been examined histologically in parr, smolts, spawning adults and post-spawning adults captured during downstream migration. All stages of fish from the Clearwater River in Idaho are characterized by the presence of lesions in the main vessel on the bulbus arteriosus. The incidence is lowest in parr, increases significantly in smolts, and is present in 100% of spawning fish. Regression of lesions was not observed in either hatchery or wild fish after spawning.     

秦岭山区虹鳟寄生三代虫一新纪录

记述了秦岭山区寄生于虹鳟的单殖亚纲一新纪录种：细鳞鲑三代虫Gyrodactylus brachymystacis Ergens,1978,比较了其与我国东北寄生于虹鳟的细鳞鱼三代虫Gyrodactylus lenoki Gussev,1953的区别。初步判断其来源可能一是通过水源来源于秦岭的细鳞鲑,另一种是通过早期虹鳟的引进带人的,其真实的途径及其对环境的风险性还有待于进一步研究。     

Opsonizing properties of natural antibodies of rainbow trout, Oncorhynchus mykiss (Walbaum)

In order to confirm previous observations in which a protective effect of rainbow trout natural antibodies against furunculosis was suspected, phagocytosis studies wereconducted in vitro , using combinations of rainbow trout sera with high or low levels of natural antibodies and active or inactivated complement as opsonizing factors. Opsonization was observed in all the cases where complement was present, and to a lesser degree with sera containing only natural antibodies. The results confirm the prime importance of the complement system and provide additional evidence for a possible role of natural antibodies in antimicrobial defences.     

Experimental studies on the effect of Gyrodactylus colemanensis and G. salmonis (Monogenea) on density of mucous cells in the epidermis of fry of Oncorhynchus mykiss

Fry of Oncorliynclius mykiss were experimentally infected with the ectoparasites Gyroducrylus rolenirmensis and Gyroducrylus salnionis (Monogenea) and changes in mucous cell density in the epidermis covering the caudal fin were monitored during an ensuing 42 day epidemic. At 10° C, infections of G. colemanensis rose to a recorded peak of 90 to 1 15 worms on day 27 post-infection and then declined toward extinction. Intensity of G. salmonis remained low throughout the experiment. Infection produced no clinical signs of disease and did not influence fry growth or survival. However, infected fish did have a 50% reduction in the number of mucous cells in the epidermis of the fin. The changes were first detected on day 24 post-infection and became increasingly pronounced during the subsequent 1 day period when parasite numbers declined drastically. The study hypothesizes that parasite activity on the surface indirectly leads to reduction in mucous cells through disruption of cell dynamics within the epidermis.     

Haemoglobin isomorph abundances in splenectomized rainbow trout, Oncorhynchus mykiss (Walbaum)

Rainbow trout, Oncorhynchus mykiss (Walbaum) were rendered acutely anaemic by immersion in phenylhydrazine HC1 at 5° C. Following splenectomy or sham operation erythropoiesis was induced by gradual warming. No consistent or significant differences in red cell numbers, haemoglobin, haematocrit, mean erythrocytic volume or haemoglobin or haemoglobin isomorph system organization were evident during or following recovery. It is unlikely, therefore, that reported acclimatory adjustments in haemoglobin isomorph abundances can be attributed to site-related differences in erythroid cell lineages.     

The effect of feeding hierarchy on individual variability in daily feeding of rainbow trout, Oncorhynchus mykiss (Walbaum)

Three groups of rainbow trout, Oncorhynchus mykiss , (initial weight 40 g) were fed one of three rations (low, medium or high) for 73 days. Consumption by individual fish within the three ration groups was measured on four occasions (days 27, 55, 64 and 72) using radiography. Food intake by individual fish varied between days and this variability was expressed using the coefficient of variation (CV). Dominant fish within each ration group were defined as individuals with the greater share of the group meal and these fish had low CVs for food intake indicating relatively little variation in daily consumption. By contrast, the fish which had consumed a low mean proportion of the group meal displayed high CVs for food intake, indicating considerable variability in the sizes of individual meals consumed by these fish. As group ration increased, the range of meal sizes and individual CVs in daily feeding decreased, suggesting that the strength of the feeding hierarchy and the variability in individual consumption decreased as food availability increased. It is suggested that radiography can be used to assess social relationships within groups of fish allowing the assessment of feeding hierarchies in larger groups of fish than would be possible by observational techniques.     

Uptake of vitellogenin into oocytes during early vitellogenic development in the rainbow trout, Oncorhynchus mykiss (Walbaum)

Uptake of ³H-vitellogenin (³H-VTG) into oocytes of various sizes was investigated during early vitellogenic development in the rainbow trout, Oncorhynchus mykiss (Walbaum). Females were injected with ³H-VTG and uptake into oocytes of different sizes (<0.4,0.4–0.59, 0.6–0.79, 0.8–0.99 and 1.0 1.2 mm in diameter) measured. Oocytes measuring less than 0.6 mm in diameter appeared unable to sequester VTG and were therefore considered pre-vitellogenic. Oocytes measuring 0.6 mm or more all sequestered VTG. The larger the oocyte, the more ³H-VTG it sequestered, even when uptake was expressed per unit surface area. The latter observation could be due to an increase in the number of VTG receptors per unit surface area, an increase in the rate of turnover of the VTG receptor, greater access of VTG to the receptors as oocytes grow, or a combination of any of these factors. The data suggest that the ability to sequester VTG is developmentally regulated.     

Demonstrating freedom from Gyrodactylus salaris (Monogenea: Gyrodactylidae) in farmed rainbow trout Oncorhynchus mykiss

This paper describes an approach to demonstrate freedom of individual rainbow trout farms from Gyrodactylus salaris Malmberg, 1957. The infection status of individual farms is relevant should G. salaris be introduced into a country or zone previously known to be free of the parasite. Trade from farms where G. salaris may have been introduced would be restricted until freedom had been demonstrated. Cage, fish and parasite sample sizes were calculated based on the minimum detectable prevalence (P*), test characteristics, population size, and Type I and II errors. Between 5 and 23 cages per farm would need to be sampled to demonstrate freedom at a cage level P* of 10%. The number of fish sampled per cage depended mainly on the test sensitivity (probability of correctly identifying an infected fish). Assuming a test sensitivity of 99% at the fish level, 59 fish per cage are needed (P* = 5%). Since G. salaris may exist in mixed infection with G. derjavini, testing a sample of gyrodactylid parasites may not result in the parasite being detected when present. Test sensitivity at the fish level depends on the number of gyrodactylids on the fish, the proportion of which are G. salaris and the number examined. Assuming a P* of 5% (i.e. G. salaris are at least 5% of the gyrodactylid population), between 20 and 73 parasites per fish would need to be sampled (depending on abundance) to maintain the Type I error at 0.01 (thus a fish level test sensitivity of 99%). This work identifies the critical information, and further research, needed to assess freedom from G. salaris with a known level of confidence; this is essential to provide a sound scientific basis for decision-making about disease control measures.     

Iteroparity in complex mating systems of steelhead Oncorhynchus mykiss (Walbaum)

This study investigated diverse reproductive types in complex mating systems of steelhead Oncorhynchus mykiss . Postspawned steelhead (kelts) were sampled during attempted downstream migration over Lower Granite Dam on the Snake River, U.S.A. Multilocus microsatellite genotypes (14 loci) were used to assign unknown origin, kelt individuals to upstream populations of origin. Results indicated that iteroparity is a life-history trait that remains in several tributaries of the Snake River basin despite strong selection against downstream adult passage because of hydroelectric dams. The largest populations of steelhead in the Snake River, however, were only weakly represented (Clearwater River = 7·5% and Salmon River = 9·4%, respectively) in the kelt steelhead mixture relative to the Grande Ronde River (18·2%), Imnaha River (17·4%), Pahsimeroi Hatchery (25·2%) and Asotin Creek (22·2%). A lack of correlation between population escapement size and kelt proportions ( P > 0·05) suggests that iteroparity was not uniformly expressed across populations, but was significantly negatively correlated with body size ( P < 0·05). Iteroparity may be a valuable source of genetic variability and a conservation priority, especially in years with poor recruitment or in recently bottlenecked populations.     

Lysozyme activity in rainbow trout, Oncorhynchus mykiss (Walbaum), stressed by handling, transport and water pollution

Blood lysozyme activity was determined in rainbow trout that were stressed by handling and transport, or suffered from acute ammonia poisoning. To optimize the lysoplate assay, the pH dependence of the lysozyme activity was studied. For serum and kidney lysozyme, the pH optimum was 5.5 to 6.0. Ventilation frequency and plasma glucoseconcentration of the fish were used as stress indicators. Less severely stressful situations, such as 30 min of handling, brought about either a stimulation or a suppression of the lysozyme activity. Strong stressors, such as transport lasting 2 h or acute water pollution, reduced the lysozyme level significantly. The decrease of lysozyme activity due to handling and transport persisted for 24 h, but the activity returned to normal within 2 weeks following confrontation with the stressor.     

The hypoxic threshold for maximum cardiac performance in rainbow trout Oncorhynchus mykiss (Walbaum) during simulated exercise conditions at 18° C

Perfused rainbow trout Oncorhynchus mykiss hearts exposed to simulated exercise conditions (hypoxia, hyperkalemia and acidosis) at 18° C experienced complete failure of maximum cardiac performance at oxygen tensions <5·6 kPa and partial failure at <6·7 kPa. This hypoxic threshold, which occurred in the presence of maximal adrenergic stimulation (500 nM adrenaline), is unusually high compared with previous results at a colder acclimation temperature. Cardiac failure was primarily due to significant decreases ( P < 0·05) in heart rate rather than cardiac stroke volume at all hypoxia levels tested.     

Variations of opisthaptoral hard parts of Gyrodactylus salaris Malmberg, 1957 (Monogenea: Gyrodactylidae) on rainbow trout Oncorhynchus mykiss (Walbaum, 1792) in a fish farm,with comments on the spreading of the parasite in south-eastern Norway

Studies were carried out on Gyrodactylus salaris Malmberg, 1957 on rainbow trout Oncorhynchus mykiss (Walbaum, 1792) from a fish farm in Lake Tyrifjorden, south-eastern Norway. The anchors were larger, and the shape of the anchors and the ventral bar differed slightly, as compared with the same parts of G. salaris on parr of Atlantic salmon Salmo salar L. in northern and north-western Norwegian rivers. At different water temperatures (0.8°C, 10.0°C, 18.0°C), the opisthaptoral hard parts of G. salaris on rainbow trout showed considerable variation in size, but varied only slightly in shape. It was found that the total length of the anchors of G. salaris on rainbow trout may considerably exceed the previously reported maximum of 80 m for the species. The spread of G. salaris to south-eastern Norway is described and discussed.     

Is there an optimal haematocrit for rainbow trout,Oncorhynchm mykiss (Walbaum)? An interpretation of recent data based on blood viscosity measurements

The viscosity of blood from rainbow trout was measured following manipulation of haematocrit by bleeding, hypoxia. exercise, and anaesthesia. Blood viscosity when measured at high shear rate (225 s ¹) was proportional to haematocrit, but the dependence of viscosity on shear rate was far less for swollen erythrocytes from exercised and anaesthetized trout. Erythrocyte swelling was most marked in exercised and anaesthetized trout, and is a confounding factor when considering the effect of haematocrit on viscosity.
The viscosity of blood with variable haematocrit, but constant mean cell Hb concentration, indicated that the relative oxygen transport capacity in trout was optimal at a haematocrit of 30%. Data from this, and earlier studies show that haematocrit in trout is variable and labile, yet none of the haematocrit values following manipulations are less than 85% of optimal. Optimal haematocrit is however, significantly higher than measured values from either cannulated or acutely venesected resting trout.     

Environmental influences on cardiovascular variables in rainbow trout, Oncorhynchus mykiss (Walbaum)

Two groups of wild (lakedwelling and anadromous), and a group of hatchery-reared Oncorhynchus mykiss (Walbaum) (rainbow trout) were sampled in order to measure cardiac morphometrics, haemoglobin concentration, and the DNA and protein concentration in cardiac muscle. A combination of these variables was used to distinguish wild fish from domestic ones.
The wild fish had significantly higher levels of haemoglobin (for male fish, 10.10 and 10.07 g 100 ml⁻¹ vs. 7.69 g d⁻¹) and larger relative ventricle mass (females, 0.091 and 0.099% ofbody mass vs. 0.073%; males, 0.108 and 0.134% vs. 0.102%; immatures, 0.086 and 0.094% vs. 0, 072%, respectively) than the domestic fish. The anadromous and domestic fish had significantly higher amounts of compact tissue when compared with lake fish (females, 43 and 47% of ventricle mass vs. 34%, respectively). Ventricle size distinguished wild fish from domestic fish, except male anadromous and male domestic fish which were distinguished only by haemoglobin and compact tissue values. Immature fishes from all groups had lower total protein levels in the ventricle, lower compact tissue levels, and less haemoglobin. Points regarding the potential environmental influences in determining these cardiovascular trends are discussed.     

Escape from an evolutionary dead end: a triploid clone of Gyrodactylus salaris is able to revert to sex and switch host (Platyhelminthes, Monogenea, Gyrodactylidae)

Diploid parthenogenesis, with rare sex, is considered as the basic mode of reproduction among the hermaphroditic and viviparous Gyrodactylus. A particular strain of the monogenean parasite Gyrodactylus salaris (RBT clone) was recognized by an invariable, unique mitochondrial DNA haplotype in rainbow trout (Oncorhynchus mykiss) farms. The RBT clone was shown to be triploid and asexual by analyzing a 493 bp sequence of a nuclear DNA marker. Three alleles were present as heterozygous in all 237 individuals sampled in years 2001-2005 from five isolated Finnish farms. The triploid clone probably originated from a diploid oocyte fertilized by a non-self hermaphrodite, most probably in a fish farm. Identical mitochondrial COI gene (1606 bp) was also found in G. salaris parasites on landlocked salmon (Salmo salar) in two rivers draining to the lake Kuitozero, Russian Karelia. In the river Pisto, the clone was triploid, but the diagnostic "short" nuclear allele of the RBT clone was replaced by an allele typical for salmon specific parasites in the Lake Onega. The clone in the river Kurzhma was diploid, having lost the "short" allele, but still heterozygous for the other two alleles of the RBT clone. Evidently, the triploid parthenogenetic RBT clone had produced diploid oocytes, when (as a female) stimulated by a non-self mate in the new environment. The genetic reorganization coincided with a switch to the salmon host. Participation of triploids into the gene pool of the species is rarely reported in animals, and the triploidy is generally considered as an irreversible dead-end of the evolution. Liberalism in ploidy level may significantly add to the evolutionary options available for a parasite in ever-changing environments.     

Germinal epithelium, folliculogenesis, and postovulatory follicles in ovaries of rainbow trout, Oncorhynchus mykiss (Walbaum, 1792) (Teleostei, protacanthopterygii, salmoniformes)

The rainbow trout, Oncorhynchus mykiss (Walbaum, 1792), is a salmoniform fish that spawns once per year. Ripe females that had ovulated naturally, and those induced to ovulate using salmon gonadotropin-releasing hormone, were studied to determine whether follicles were forming at the time of spawning and to describe the process of folliculogenesis. After ovulation, the ovaries of postspawned rainbow trout were examined histologically, using the periodic acid-Schiff procedure, to stain basement membranes that subtend the germinal epithelium and to interpret and define the activity of the germinal epithelium. After spawning, the ovary contained a few ripe oocytes that did not ovulate, numerous primary growth oocytes including oocytes with cortical alveoli, and postovulatory follicles. The germinal epithelium was active in postspawned rainbow trout, as determined by the presence of numerous cell nests, composed of oogonia, mitotic oogonia, early diplotene oocytes, and prefollicle cells. Cell nests were separated from the stroma by a basement membrane continuous with that subtending the germinal epithelium. Furthermore, follicles containing primary growth oocytes were connected to the germinal epithelium; the basement membrane surrounding the follicle joined that of the germinal epithelium. After ovulation, the basement membrane of the postovulatory follicle was continuous with that of the germinal epithelium. We observed consistent separation of the follicle, composed of an oocyte and surrounding follicle cells, from the ovarian stroma by a basement membrane. The follicle is derived from the germinal epithelium. As with the germinal epithelium, follicle cells derived from it never contact those of the connective tissue stroma. As with epithelia, they are always separated from connective tissue by a basement membrane.     

The susceptibility of grayling (Thymallus thymallus) to experimental infections with the monogenean Gyrodactylus salaris

The pathogenic monogenean Gyrodactylus salaris infecting Atlantic salmon (Salmo salar) is found to attach and reproduce under laboratory conditions on several species in the subfamily Salmoninae other than the Atlantic salmon. The gyrodactylid species Gyrodactylus thymalli infecting grayling (Thymallus thymallus) in another subfamily, Thymallinae, is previously said to be very similar to G. salaris based on morphometry and genetical analysis which prompted the present laboratory experiments to test the susceptibility and resistance of grayling to G. salaris. All 0+ and 1+ grayling became infected with G. salaris during the experimental infection procedure. However, both innate resistant and susceptible grayling were found. In susceptible individually isolated fish, parasite reproduction lasted for more than 35 days. Parasite reproduction also occurred among grouped grayling as judged from the duration of infection of more than 50 days. However, grayling susceptibility as judged from G. salaris reproduction, was very limited. Hence, the results indicate significant biological differences between the function of Atlantic salmon and grayling as host for G. salaris. The grayling is interpreted as unable to sustain G. salaris in nature which implies that G. thymalli is not conspecific with G. salaris. However, G. salaris dispersal by grayling cannot be excluded.