Rainbow Trout Fingerlings Research Articles

Requirement for phenylalanine and replacement value of tyrosine for phenylalanine in rainbow trout ( Oncorhynchus mykiss)

The requirement for phenylalanine (Phe) and replacement value of tyrosine (Tyr) for Phe in fingerling rainbow trout were determined by feeding diets containing various levels of Phe and Tyr. Four tanks of 30 trout each were assigned per diet and fed 3 times daily to satiation at 15°C. With diets containing excess Tyr (1.33%) and 35% crude protein, weight gain of 13-g trout over a 6-week period increased linearly with increasing levels of Phe up to 0.75% of dry diet but was not different ( P>0.05) at 0.75% and 1.75% Phe. Feed efficiency and nitrogen retention improved as Phe level in the diet increased up to 0.55%, but no significant effects were found at levels above 0.55%. Carcass protein content increased with Phe level up to 0.65% and fat content decreased concurrently. Increasing Tyr level up to 0.7% in a diet containing 0.6% Phe increased weight gain of 14-g trout over a 6-week period, but a further increase in Tyr level to 0.8% had no effect ( P>0.05). Analysis of data using a broken-line model showed that the Phe requirement of fingerling rainbow trout is 0.7±0.05% (mean±s.e.) of dry diet or 2.0% of dietary protein, when a diet contains excess Tyr and 35% protein, and that the replacement value of Tyr for Phe is approximately 53% on a weight basis or 48% on a molar basis. Thus the total aromatic amino acid requirement is 1.5% of dry diet or 4.3% of dietary protein.

Rare Earth Elements as Internal Batch Marks for Rainbow Trout: Retention, Distribution, and Effects on Growth of Injected Dysprosium, Europium, and Samarium

A chloride suspension in saline with 12.7 μg of dysprosium or europium, or with 127 μg samarium, or with a mixture containing 12.7 μg Dy, 12.7 μg Eu, and 254 μg Sm, was injected interperitoneally in fingerling rainbow trout Oncorhynchus mykiss. The retention and tissue distribution of these rare-earth marks, and the effects of these marks on fish growth and development, were monitored for 763 d. Rare earth element concentrations in the fish were determined by neutron activation analysis. Mark retention and tissue distribution were similar in fish receiving the lanthanides singly or in combination. Soon after injection, over 90% of each element was associated with the gut. As the fish developed toward sexual maturation, small amounts of the injected elements were detected in the kidneys and gonads. After treatment, small amounts (<2%) of Sm were recovered from the liver during the first year but not thereafter. Levels of Dy and Eu declined by 40–50% during the first 6 months, but they remained stable for the next 1.5 years. Conversely, Sm levels in the fish were unchanged over the 2-year study period, even though this element became concentrated in different tissues over time. Physiological effects of marking were restricted to a transient increase in the weight of the gut relative to body weight and a slight edema in the kidney, especially in fish receiving the mixture of elements. Growth and sexual development were unaffected by the treatments.

Field observations on the production of rainbow trout(Oncorhynchus mykiss) under high concentrations of water-borne iron

Effluents from brown coal mines are frequently rich in iron, the water being red-brown in colour and turbid. For several years a fish farm in the Lusatia (Brandenburg, Germany) has used such mine effluents for rearing rainbow trout. The total iron content of this water varies between 5 and 10 mg/l with pH ranging from 6.7 to 7.4. Water turbidity is high with a transparency of 10 to 40 cm at the most due to the substantial ferric hydroxide concentrations. Until 1989 trout have been reared in net cages within concrete settling basins destined for settling the ferric hydroxide mud. The cages were stocked with fingerlings in autumn, reaching portion size in the spring of the following year. Specific growth rate was 0.98% per day, comparable to that of trout in another farm unpolluted by iron but stocked at higher densities. The results show that fingerling rainbow trout may live in water containing more than 5 mg/l total iron but in the absence of Fe2+. These concentrations of water-borne iron seem to have only a limited detrimental effect on fish growth and feed conversion and do not prevent trout culture in principle.

Effect of fasting or feeding diets containing different levels of protein or amino acids on the activities of the liver amino acid-degrading enzymes and amino acid oxidation in rainbow trout ( Oncorhynchus mykiss)

Experiments were done on fingerling rainbow trout at 15 ± 1 °C to determine the effect of: (1) fasting or feeding diets containing different levels of protein on liver glutamate dehydrogenase (GDH), alanine aminotransferase (AAT) and histidase activities; (2) dietary protein level on the oxidation rates of glutamate and phenylalanine in vivo; and ( 3 ) dietary phenylalanine level on the phenylalanine oxidation rate in vivo. Activities of GDH and AAT per g liver or per g protein were higher ( P < 0.05) in fasted than in fed trout. GDH activity per g liver was higher in trout fed diets containing 50% crude protein (a commercial diet) or 35% protein (a casein-based diet) than in fish fed a diet containing 10% protein (a casein-based diet), but AAT activity per g liver was not significantly different between the fed fish. The differences in GDH or AAT activity among the treatments disappeared when the activities were expressed per 100 g body weight. Regardless of method of expression, liver histidase activity was higher ( P < 0.05) in fish fasted or fed a commercial diet containing 50% protein than in fish fed diets containing 10 or 35% protein. The oxidation rates of dietary glutamate and phenylalanine in trout that had previously been fed the 35% protein diet, were about three and five times those found in fish previously fed the 10% protein diet, respectively. This difference was found when fish that had been fed the 10 or 35% protein diet were subsequently fed the 35% protein diet containing 14C-labeled glutamate or phenylalanine, indicating that trout previously fed a high protein diet have a greater capacity to catabolize these amino acids. The oxidation rate of dietary glutamate was approximately five or nine times that of phenylalanine in trout fed diets containing 35 or 10% protein, respectively. The rate of phenylalanine oxidation increased with increasing phenylalanine levels in the diet; no break point or plateau in the dose-response curve was observed. Our studies demonstrated that fasting or feeding a high protein diet increases the amino acid catabolism in trout, and suggested that the amino acid oxidation technique cannot be used to determine the amino acid requirements of trout.

Requirements for lysine and arginine by rainbow trout ( Oncorhynchus mykiss)

A series of experiments was conducted to determine lysine and arginine requirements of fingerling rainbow trout. Four groups of 30 trout each were assigned per diet and fed three times daily to satiation at 15 °C. The diets contained varying levels of lysine and arginine. Weight gain of 13.7-g trout over a 6-week feeding period increased linearly with lysine level up to 1.4% of diet, but was not different at 1.4 and 1.6% lysine. Feed efficiency and nitrogen retention improved as the lysine level in the diet increased. Carcass protein content of trout fed diets containing sufficient ivsine was higher than that of trout fed diets deficient in lysine. No significant differences in weight gain were shown between 11.6-g trout fed diets containing 0.47% arginine and either 1.5 or 3.0% lysine, indicating no apparent interactions between the dietary arginine and lysine levels. Weight gain of 12.4-g trout over a 6-week feeding period significantly ( P < 0.05) increased with dietary arginine level up to 1.3%, but further increases in the arginine level did not have a significant effect. A similar trend was noted in feed efficiency, nitrogen retention and carcass protein content. Analysis of weight gain versus dietary lysine or arginine level indicated that the lysine and arginine requirements of young trout are 1.30 (3.71) and 1.41 (4.03)% of diet (protein), respectively.

Pharmacokinetics of enrofloxacin in fingerling rainbow trout (Oncorhynchus mykiss).

The pharmacokinetics of intravenously and orally administered enrofloxacin was determined in fingerling rainbow trout (Oncorhynchus mykiss). Doses of 5 or 10 mg enrofloxacin/kg body weight were administered intravenously to 26 fish for each dose and blood was sampled over a 60-h period at 15 degrees C. Two groups of fish were treated orally with 5, 10, or 50 mg/kg (80 fish/dose at each temperature) and held at 15 degrees C or 10 degrees C during the 60-h sampling period. Following intravenous administration, the serum concentration-time data of enrofloxacin in rainbow trout were best described by a two-compartment open model for both doses of 5 and 10 mg enrofloxacin/kg. The hybrid rate constants alpha and beta did not differ between doses. The distributional phase was rapid with a half-life of 6-7 min for both doses. Overall half-lives of elimination were 24.4 h (95% CI = 20.2-30.8) and 30.4 h (24.2-41.0), respectively, for the 5- and 10-mg/kg doses. A large Vd(area) was observed following dosing of either 5 or 10 mg enrofloxacin/kg,: 3.22 and 2.56 l/kg, respectively. Whole body clearance for 5 mg/kg was 92 ml/h.kg and 58 ml/h.kg at the 10-mg/kg dose. Following oral administration, the serum concentration-time data for enrofloxacin were best described as a one-compartment open model with first-order absorption and elimination. Apparent Ka over all doses at 10 degrees C averaged 62% less than apparent Ka at 15 degrees C. Estimates of the apparent t(1/2)e over both temperatures ranged from 29.5 h (18.4-73.4) to 56.3 h (38.3-106.6). Bioavailability averaged 42% over all doses at 15 degrees C and was decreased to an average of 25% at 10 degrees C. Peak serum concentrations appeared between 6 and 8 h following dosing. A dose of 5 mg/kg/day was estimated to provide average steady-state serum concentrations at 10 degrees C that are approximately 4.5 times the highest reported MIC values for Streptococcus spp., the fish pathogen least sensitive to enrofloxacin. Owing to the long apparent half-life of elimination of enrofloxacin in fingerling trout, it would take approximately 5 to 9 days to achieve these predicted steady-state serum concentrations; this estimate is important when considering the duration of therapy in clinical trials.

Requirements for sulfur amino acids and utilization of D-methionine by rainbow trout ( Oncorhynchus mykiss)

The methionine (Met) requirement and replacement values of cystine (Cys) and D-Met for L-Met were estimated at 15 °C for fingerling rainbow trout ( Oncorhynchus mykiss) by feeding diets containing various levels of L-Met, D-Met and L-Cys. Four tanks, each containing 28 (Experiment 1) or 30 (Experiment 2) trout (11–15 g initial weight), were assigned to each diet. Weight gain and nitrogen retention during a 6-week period increased with increasing levels of L-Met up to 0.5% of diet, when the diets contained excess L-Cys (0.5%). Feed efficiency increased as L-Met increased, but no significant ( P > 0.05) differences were detected at levels above 0.4%. With diets containing 0.5% L-Met, the Cys requirement was found to be 0.3% of the diet. Increasing D-Met level above 0.27% in a diet containing 0.23% L-Met did not significantly ( P < 0.05) influence weight gain, feed efficiency or nitrogen retention. Analysis of the results showed that 1. the Met requirement of fingerling rainbow trout is 0.52 (1.49)% of diet (dietary protein), when a diet contains excess Cys; 2. the Cys replacement value (on an equal molar sulfur basis) of L-Cys for L-Met is approximately 42%; 3. the requirement of trout for total sulfur amino acids is about 0.8 (2.3)% of diet (dietary protein); and 4. rainbow trout utilize D-Met as efficiently as L-Met for weight gain but less efficiently for protein accretion.

Neutrophil, glass-adherent, nitroblue tetrazolium assay gives early indication of immunization effectiveness in rainbow trout

Neutrophil activity in rainbow trout ( Oncorhynchus mykiss) is increased upon antigenic stimulation with the Yersinia ruckeri O-antigen bacterin. The characteristics of neutrophil attachment to glass and nitroblue tetrazolium (NBT) staining were used to determine the effectiveness of immunization programs with fingerling rainbow trout. Fish immunized by intraperitoneal injection with doses of 100, 10, or 1 μg of the bacterin showed the highest responses in that order in numbers of glass adherent, NBT-positive neutrophils. Studies on the kinetics of the occurrence of numbers of glass-adherent, NBT-positive staining cells from the fish injected with the 10 μg dose showed the numbers of positive cells were largest on Day 2 after injection. The specific immune response was confirmed by demonstrating the presence of plaque-forming cells by the passive hemolytic plaque assay and the rise in humoral antibody titers by passive hemagglutination 12 days after injection. The effects of immunization in trout could be detected earlier by using the neutrophil glass adherence and NBT reduction assays than by using assays based on observations of the specific immune response.

ニジマス稚魚の成長および血液性状に及ぼす飼料のα化率と可消化エネルギー含量の影響

This study was conducted to investigate the effect of gelatinized ratio (GR) and digestible energy (DE) content on growth and blood profile of rainbow trout fingerlings (4.7g on aver-age). The fish were fed low energy (DE: 298-334kcal/100g) or high energy (DE: 334-395kcal/100g) diets with different levels of GR adjusted using raw and extruded potato starch at a 30% level. At both dietary energy levels, growth, feed efficiency, and protein efficiency ratio were all improved by elevation of dietary GR levels, and reached a virtual plateau at around 40% GR. Daily feed consumption was inversely related to the dietary GR. At the same DE level (334kcal/100g) of both low and high energy diets, growth was higher in fish fed a diet with a higher GR. The apparent digestibility of dietary protein was not influenced by dietary GR at either of the energy levels. Digestibility of lipid in the high energy diets and starch and energy in both energy levels increased with an increase of dietary GR. Blood profiles were also affected by both dietary GR and energy levels. A suitable level of GR in the diet for rainbow trout was estimated to be more than 40%, regardless of DE levels.

Dietary Excesses of Leucine Influence Growth and Body Composition of Rainbow Trout

This study examined the hypothesis that a diet containing excess leucine may promote protein deposition in the body of rainbow trout. Diets were formulated with wheat germ meal and crystalline amino acids as major nitrogen sources. In Experiment 1, diets containing 1.1, 1.5, 2.2, 2.7, 3.5, 4.5, 6.0 and 6.5% leucine in wheat germ meal-crystalline amino acid diets were fed to fingerling rainbow trout. Diets containing up to 6.5% leucine did not inhibit weight gain or food intake. Body protein concentration tended to decrease as dietary leucine increased. In Experiment 2, fish were fed similar diets containing 3.3, 6.2, 9.2 and 13.4% leucine. After 10-11 wk of feeding, gross lesions including scoliosis, deformed opercula, scale deformities, scale loss, spongiosis of epidermal cells and scale regeneration were observed in 20% of the fish fed diets containing 13.4% leucine. High dietary leucine did not depress plasma valine or isoleucine concentrations. Therefore, the gross lesions could be attributed to a toxic effect of excess dietary leucine. Polyamine concentrations, which were used as a metabolic indicator for growth, were not significantly different in the tissues of fish receiving different treatments, thus supporting the hypothesis that increasing dietary leucine did not increase body protein deposition.

Genetic variation in red cell membrane fragility in Atlantic salmon and rainbow trout

The strength and flexibility of cell membranes are of particular importance for fish which routinely experience changes in the surrounding temperature. 496 fingerlings of Atlantic salmon from 50 full-and 25 halfsib groups and 133 fingerlings of rainbow trout from 14 fullsib groups were used to study the total as well as the genetic variation in cell membrane Strength, measured as red cell membrane fragility. Fragility is defined as the salinity at which 50% of the red cells are hemolyzed, and gradient as the regression coefficient of the fragility curve at the turning point. Both parameters showed significant differences between Atlantic salmon and rainbow trout. The mean fragility curve for rainbow trout was similar to the fragility curve for the family of Atlantic salmon which had the strongest cell membranes. Heritability was high for fragility and medium for gradient. There was no significant genetic correlation between red cell fragility and susceptibility to experimental infection with furunculosis.

Purified diet development and re-evaluation of the dietary protein requirement of fingerling rainbow trout ( Oncorhynchus mykiss)

Two experiments were carried out to develop a purified diet, and to examine the protein requirement of fingerling rainbow trout. Four replicate tanks (50 fish each) of trout were assigned to a commercial salmon diet or a purified diet containing 30, 35, 40 or 45% protein; and three replicate tanks (40 fish each) to a diet containing 10, 15, 20, 25 or 35% protein (not counting crystalline dispensable amino acids which replaced casein protein to vary the protein level). Trout were fed three times a day for 6 weeks, and weight gain and feed gain ratio were monitored No significant differences were found in weight gain or feed gain ratio among 10.5-g fish fed diets containing 30, 35, 40 and 45% protein. Fish fed these diets gained about 80% as much weight as fish fed a commercial salmon diet which produced gain of 19.5 g/fish per 6 weeks. Weight gain increased with the increasing levels of casein up to 25% and the breakpoint was found at 24% intact protein, indicating that the levels of indispensable amino acids (IDAA) supplied by the 24% intact protein were sufficient to meet the requirements of trout for IDAA. Results suggested that energy sources used for trout diets play an important role in the determination of the protein requirements, and that the conventionally established protein requirement (40%) includes protein required to meet IDAA requirements (24%) and that required to meet energy needs (16%).

Experimental Production of Bacterial Gill Disease in Trout by Horizontal Transmission and by Bath Challenge

Abstract Bacterial gill disease was horizontally transmitted both within and between salmonid species by using good-quality water common to both sick and healthy fish, and by using young and mature animals. Clinical disease was produced within 24 h, and mortality resulted often within 48 h, even in 2- or 3-year-old rainbow trout Oncorhynchus mykiss. Clinical disease was correlated with the appearance of filamentous bacteria on the gill surface. These bacteria were isolated and determined to be morphologically, biochemically, and antigenically related to Flavobacterium branchiophila. Subsequently, they were used to produce bacterial gill disease in fingerling rainbow trout by adding a pure culture of 105 organisms/mL of tank water; once again, good water quality conditions were maintained. Attachment of large numbers of bacteria to the gill surface was seen immediately postexposure, followed by clinical disease within 20 h and death by 24 h.

Effect of pH on the bioaccumulation of low level, dissolved methylmercury by rainbow trout (Oncorhynchus mykiss)

An inverse relationship has been observed between pH and McHg concentration in freshwater fish. Many hypotheses exist regarding the mechanisms which lead to elevated levels of organic Hg in fish from low pH lakes. To determine if pH has a direct effect on the rate of McHg bioaccumulation in fish, rainbow trout fingerlings (Oncorhynchus mykiss) were exposed to a low concentration of aqueous methylmercuric chloride (1.38 ± 0.49 ng·L−1) at four pH-levels (8.2, 7.0, 6.3, 5.8) for eight weeks. McHg and total Hg were specifically determined on whole fish homogenates and water samples. The pH was found to have a significant: inverse effect on the rate of McHg bioaccumulation in the fish only in the lowest exposure level. Fish held at pH 5.8 had an uptake rate of 1.11 ± 0.07 ng·g−1·d−1, while those at pH ≥ 6.3 had a MeHg tissue uptake rate of ≤ 0.64 ± 0.07 ng·g−1·d−1. Total body burden of MeHg in the fish held at pH 5.8 also showed an elevated level of MeHg when compared with fish held at higher pH-levels, but the difference was less dramatic. These results suggest that a portion of the Hg burden in fish from low pH systems may be due to the direct effects of low pH on bioaccumulation, but that a threshold may exist above which pH does not play a significant role.

EFFECT OF LERNAEA CYPRINACEA (CRUSTACEA: COPEPODA) ON STOCKED RAINBOW TROUT (ONCORHYNCHUS MYKISS)

Prevalence, intensity and pathogenesis of Lernaea cyprinacea (anchorworm) in stocked rainbow trout (Oncorhynchus mykiss) fingerlings were monitored annually for 4 yr (1981 to 1984) in East Canyon Reservoir, Utah (USA). Anchorworms were first detected in midsummer each year and were most abundant in the fall. The mean parasite intensity was highest in October 1982 (19 anchorworms per fish); in other years, maximum density was 7 to 9. The dorsal and caudal areas of the fish were the most heavily parasitized. The histological response to parasite attachment included an infectious granuloma similar to that reported in other fish hosts. Bacteria were not found in the kidneys of fish before stocking, but afterward bacteria that were presumptively identified as belonging to the genus Aeromonas, were found in the kidneys of up to 45% of the parasitized fish. Most (94%) anglers noticed the anchorworms, but few (8%) discarded parasitized fish. Some 28% used special cleaning techniques to prepare fish but 49% did nothing special to clean them. Lernaeosis probably had little effect on the fishery management goals for the reservoir.

Supplemental Stocking for Estimating Population Size and First-Year Survival of Fingerling Rainbow Trout Stocked in a Utah Reservoir

The traditional method of estimating population size with mark-recapture techniques was compared to estimates made by using stocked hatchery fish as the sample of marked fish. The study was conducted on a 401-hectare reservoir with a put-grow-and-take sport fishery for rainbow trout Oncorhynchus mykiss. Populations were estimated in September 1987 and 1988 for rainbow trout (mean total lengths [TL] of 212–226 mm) that were stocked between March and June as fingerlings (89–102 mm TL). Population estimates for the mark-recapture and hatchery-fish methods, respectively, were 68,000 and 67,000 in 1987 and 51,000 and 63,000 in 1988. Survival estimates for stocked fingerlings ranged from 39 to 55% in 1987 and from 19 to 25% in 1988. The numbers of marked-and-released fish recaptured were proportionally similar to recaptures of hatchery fish used as the marked fish sample; there was no significant difference in recapture rates between methods. Confidence limits of the estimates were much narrower for the hatchery-fish method because of larger sample size. Thus, use of hatchery fish as the marked fish sample removed the limitation of low sample size that commonly restricts the use of population estimation for management. Supplemental stocking of hatchery fish also made it possible to estimate the population size of older rainbow trout of larger size (380–394 mm mean TL). Assumptions used to make estimates were evaluated, which was necessary to validate population estimates based on use of hatchery fish.

Pathology of bacterial gill disease: sequential development of lesions during natural outbreaks of disease

Abstract. This study examines the sequential pattern of bacterial colonization and the sequential development of branchial lesions during typical outbreaks of bacterial gill disease (BGD) among four groups of commercially reared rainbow trout fingerlings, Oncorhynchus mykiss (Richardson). During a 5‐month monitoring regime, prior to the onset of natural outbreaks of BGD, gill morphology of examined fish remained unaltered. Bacterial colonization of the gills was immediately preceded by several gill changes which were widely present in fish from all groups under study. These changes, which were detectable at the ultrastructural level only, included cytoplasmic blistering and degeneration of the microridges of the superficial filament epithelium, in addition to slight topographical irregularity of the filament tips suggestive of mild hyperplasia. Bacterial colonization began at these altered filament tips before spreading to more proximal regions of the filament and adjacent lamellar surfaces. Explosive increases in proportional morbidity and mortality coincided with the development of the following gill lesions: extensive bacterial colonization of lamellar surfaces, lamellar epithelial hydropic degeneration and necrosis, and lamellar oedema. In contrast, gill lesions such as lamellar fusion, epithelial hyperplasia and various metaplastic responses were detected later either as subacute (3–5 days) or chronic (7–14 days) changes. The role of epithelial necrosis as an important event during BGD is discussed as a possible mechanism leading to the development of sub‐acute and chronic gill lesions.

Multiplication of Infectious Hematopoietic Necrosis Virus in Rainbow Trout following Immersion Infection: Whole-Body Assay and Immunohistochemistry

Abstract The sites of replication of infectious hematopoietic necrosis virus (IHNV) in infected tissues were detected in fingerling rainbow trout Oncorhynchus mykiss by in situ histologic techniques following immersion infection. Virus antigens in tissues were detected by a neutralizing mouse monoclonal antibody and a one-step anti-mouse biotin-streptavidin conjugated to horseradish peroxidase. The efficiency of infection and virulence of the virus determined by mortality rates showed high virulence of the selected IHNV isolates, and viral replication in individual fish showed that virus content of the fish increased rapidly from the second day to the seventh day postinfection. The earliest viral lesions following infection were detected in the epidermis of the pectoral fins, opercula, and ventral surface of the body. Virus lesions became evident in kidneys on the third day. By the fifth day, when there was a significant increase in virus titer, foci of viral replication were detected in gill tissue and i...

Comparative evaluation of soybean meal and carob seed germ meal as dietary ingredients for rainbow trout fingerlings

Resume Carob seed gcrm meal (CSGM), and soybean meal (SBM), were tested as ingrcdients of diets for rainbow trout (Oncorhynchus mykiss) fingerlings at three lcvcls of inclusion, contributing 20 to 50% of the total protein of the diet. Fish growth decreased as the level of the plant by-product in the diet increased. Diets containing CSGM resulted in lowcr growth than comparable SBM diets, especially at inclusion levels contributing 37 and 53% of the total protein. High dietary CSGM levels decreased body protein and increased liver protcin indicating possible amino acid dcficiencies and/or imbalances at high inclusion levels. Mortality was also high at the high dietary CSGM Icvel. The results are discussed with referencc to the antinutritional factors contained in the raw materials tested and their possible effects on protein availability.

Neutralization of Egtved virus pathogenicity to cell cultures and fish by monoclonal antibodies to the viral G protein

Egtved virus, the rhabdovirus causing viral haemorrhagic septicaemia in rainbow trout, was analysed at the antigen level with a future subunit vaccine in mind. Three monoclonal antibodies to the viral G protein were characterized with respect to neutralizing activity at the cell culture level, as well as their ability to protect rainbow trout fingerlings against virus infection following passive immunization. Two antibodies showed strong protective activity in fish. Only one of these antibodies was able to neutralize viral infectivity in vitro. Reduction of disulphide bonds in the G protein abolished reactivity of this antibody in immunoblotting, whereas antigen deglycosylation did not influence the binding ability of any of the antibodies. These data suggest that the G protein contains linear as well as non-linear, carbohydrate-free epitopes, which are involved in the protection against Egtved virus. However, an indirect influence of oligosaccharide side chains on epitope formation could not be excluded, since in situ inhibition of glycosylation prevented the binding of the protecting antibodies in immunofluorescence.