Immunogenicity of amoebic antigens in rainbow trout, Oncorhynchus mykiss (Walbaum)

Abstract. Rainbow trout developed a humoral immune response against numerous antigens of sonicated amoebae which were emulsified with Freund's complete adjuvant and injected into the peritoneum. The amoebae were cultured from the gills of Atlantic salmon, Salmo salar L., affected by amoebic gill disease. Antibodies in fish sera were detected by both enzyme-linked immunosorbent assay (ELISA) and immunoblotting. Non-spedfie reactivity in fish serum against Escherichia coli, the bacterium used in co-cultivation of amoebae in vitro, was removed by immunoadsorption. Results obtained using ELISA and immunoblotting were comparable and indicated no significant difference in response to immunization with 10, 50 or 250 μg of sonieatcd amoebic protein. Amoebae contained immunogenie components of > 100, 100, 89, 49, 37 and 34kDa.     

A comparative tissue distribution study of oxytetracycline in rainbow trout, Oncorhynchus mykiss (Walbaum), and chinook salmon, Oncorhynchus tshawytscha (Walbaum)

Oxytetracycline (OTC), a broad-spectrum antibiotic, is used widely to treat bacterial diseases in farmed fish. In the present study, the time course of OTC concentrations in freshwater rainbow trout, Oncorhynchus mykiss (Walbaum), and seawater chinook salmon, Oncorhynchus tshawytscha (Walbaum), were compared, tissue by tissue, after receiving a bolus dose of the antibiotic (5 mg kg^–1 or 50 mg kg^–1) intra-arterially (i.a.). The OTC concentration–time profiles of rainbow trout tissues were found to be very similar to those of the corresponding tissues in chinook salmon. Therefore, neither water salinity nor fish species seemed to play an important role in the disposition and elimination of OTC in these salmonids. In a separate experiment, rainbow trout were implanted surgically with a urinary cannula and received a single dose of OTC (50 mg kg^–1) i.a. Urine was collected from the cannula daily for 13 days. The amount of OTC excreted into the bile was found to be larger than that eliminated by the urine. These results show the similarity of OTC pharmacokinetics in freshwater rainbow trout and seawater chinook salmon and render support in using a single fish species to study the pharmacokinetics of a drug for other species in the same taxon.     

Oxygen-induced gas bubble disease in rainbow trout, Oncorhynchus mykiss (Walbaum)

Abstract. An Aquatector oxygen injection unit was used to supersaturate a hatchery water supply to 200% oxygen saturation (18–20mg/l) and increase the total gas pressure to 120% of saturation. Nitrogen saturation was reduced to near 100%. Rainbow trout, Oncorhynchus mykiss (Walbaum), held in the treated water developed signs of gas bubble disease in 4 days, and 50% died within 20 days. We demonstrated that supersaturated total gas pressure due to excessive oxygen saturation causes gas bubble disease in the absence of supersaturated nitrogen gas. It is recommended that users of oxygen injection systems closely adjust the amount of oxygen added to the water to keep the total gas pressure near saturation.     

Continuous exposure to infectious pancreatic necrosis virus during early life stages of rainbow trout, Oncorhynchus mykiss (Walbaum)

Rainbow trout, Oncorhynchus mykiss (Walbaum), were exposed continuously to infectious pancreatic necrosis virus (IPNV) at 0, 10¹, 10³ or 10⁵ plaque forming units (pfu) L⁻¹ of water to estimate the effects of chronic IPNV exposure on early life stages. Fish density averaged 35 fish L⁻¹ (low density) or 140 fish L⁻¹ (high density), and the tank flow rate was 250 mL⁻¹ min. Virus exposure began at 6 days before hatch and continued until fish were 44 days old. Cumulative per cent mortality, analysis of survival and hazard functions, and discrete-time event analysis were used to explore the patterns of survival and mortality. In eggs and fish exposed to IPNV, mortality significantly greater than in the 0 pfu L⁻¹ exposure did not occur until IPNV concentration was 10⁵ pfu L⁻¹ at low fish density and 10³ pfu IPNV L⁻¹ at high fish density. These results suggest that in the natural aquatic environment, where rainbow trout densities are likely to be considerably lower than in this study, mortality resulting from infection with IPNV will very likely not occur when ambient concentrations of virus are ≤10³ pfu IPNV L⁻¹. In aquaculture rearing units, trout density is likely to be as high or higher than the densities used in this study. Therefore, continuous inputs of virus at concentrations greater than 10¹ pfu L⁻¹ may result in IPN epidemics in aquaculture facilities.     

Indirect estimation of stomach volume of rainbow trout Oncorhynchus mykiss (Walbaum)

The main hypothesis of this study was that if stomach volume is correlated with food intake it can be estimated without laborious and destructive direct measurement. Rainbow trout, Oncorhynchus mykiss (Walbaum), ca. 500–1300 g, were starved for 1, 4, 8 or 16 days at 15°C after which they were fed in excess with dry pellets containing known amounts of X‐ray‐dense markers. Immediately after feeding the fish were killed, X rayed and weighed. Then the stomach was dissected, its contents removed and weighed, and stomach volume was measured. X‐ray plates were developed and feed intake was estimated based on the amount of marker. All measured variables correlated positively with stomach volume. The best fit for linear regression models was obtained for fish starved for 4 days, where stomach content (dry mass) explained 94%, food intake (based on X‐ray measurement) 77% and fish mass 62% of the variation in stomach volume. However, as stomach content measurement can be a lethal, or at least very stressful, event for the fish, the accuracy of food intake measurement (X‐ray) could be increased using multiple regression. In multiple linear regressions, R²‐values varied between 0.79 (16‐day starvation) and 0.91 (1‐day starvation) with food intake and fish mass as explanatory variables for stomach volume. These results indicate that the stomach volume in rainbow trout can be estimated satisfactorily using indirect methods, which are not detrimental to the fish, although feeding history may affect the accuracy of the estimates.     

Initiation of tetraploid breeding line development in rainbow trout, Oncorhynchus mykiss (Walbaum)

Abstract. For initiating the development of tetraploid breeding lines in rainbow trout, Oncorhynchus mykiss (Walbaum), using different populations as a genetic basis, four populations out of a total of 11 were selected according to gene marker differences. Egg batches resulting from single-pair and group matings and representing purebred and crossbred progenies of these populations were exposed to a late pressure shock for inhibition of first mitosis. In six out of 26 treated egg batches tetraploid embryos were identified by chromosome examinations. In the case of successful ploidization single-pair matings yielded exclusively tetraploid embryos, while group matings contained some diploids. Successful tetraploidization was dominated by mating partners from one population. In total 413 putative tetraploid fry were available for further rearing. Besides diploids, triploids obtained by thermally induced retention of the second polar body were kept as controls mainly for methods to identify genome manipulated trout. Survival rates of tetraploids were low and loss frequencies did not stabilize till the age of 9 months. Thirty-three tetraploid fish, individually checked by measurements of DNA content, nuclear cell size and number of nucleoli per cell, were found at the age of 17 months. Body weights of survivors were similar for tetraploids and diploids throughout the whole testing procedure.     

Malachite green: a pharmacokinetic study in rainbow trout, Oncorhynchus mykiss (Walbaum)

Abstract. Pharmacokinetic studies on uptake, distribution and climination of malachite green in rainbow trout, Oncorhynchus mykiss (Walbaum), are described. All three processes were found to be temperature-dependent, with levels and rates increasing with increases in water temperature. Tissues investigated were serum, liver, kidney, muscle and viscera. First-order kinetics were found to apply and, in particular, kidney was observed to be a deep compartment. The long-term persistence of kidney residues may explain the efficacy of malachite green for the control of proliferative kidney disease in rainbow trout. Residue levels in muscle were never excessive and an extrapolation from the experimental data suggests that a suitable withholding period might be 600 degree days.     

Attachment of Vibrio pathogens to cells of rainbow trout, Oncorhynchus mykiss (Walbaum)

Abstract. The attachment of Vibrio pathogens to cells of rainbow trout, Oncorhynchus mykiss (Walbaum), was studied by use of species-specific monoclonal antibodies and indirect FITC- immunofluorescence microscopy. Vibrio anguillarum, V. ordalii and V. parahaemolyticus attached to cultured cells of rainbow trout gonads, various tissues of cryostat sections of whole fish, and smears of gills, intestine, buccal mucosa and skin. The attachment was inhibited by prior incubation of bacteria with monoclonal antibodies at titres of 1:32, or less. Other Vibrio pathogens used in this study did not attach to any trout cells. The research provides approaches to study the mechanisms of bacterial attachment in the onset of Vibrio infections.     

Electrophoretic characterization of three hatchery-reared strains of rainbow trout, Oncorhynchus mykiss (Walbaum)

Abstract. Significant differences were found in allelic frequency distribution among three strains of rainbow trout, Oncorhynchus mykiss (Walbaum). The isocitrate dehydrogenase ( IDH-3, 4 * 1.1.1.42) locus was fixed in two of the strains while the superoxide dismutase ( SOD-I * 1.15.1.1) locus was absent in one of the strains.
Similarity indices ranged from 0·973 to 0·988 on the scale of 1·000 among pairwise comparisons of the strains while Nei's genetic distance was 0·012 between Mount Lassen and Manx, 0·025 between Mount Lassen and Tagwerker and 0·027 between Manx and Tagwerker. Gene diversity was highest in the Mount Lassen strain, followed by Manx, with Tagwerker having the least. These differences in gene diversity were attributed to breeding and management practices used at the hatchery. The need for characterization of strains of rainbow trout, especially in crossbreeding programmes that aim to increase genetic variation, was emphasized by these results.     

Individual variation in progeny size and quality in rainbow trout, Oncorhynchus mykiss (Walbaum)

Early life history traits of fish are very variable as a result of both genetic and environmental factors. In this study, we examine individual variation in early life history traits in progeny of seven females crossed with one male rainbow trout, Oncorhynchus mykiss (Walbaum). Individual eggs were followed from fertilization through hatching until death of the larvae by starvation. Larvae and yolksac size (size was shown to be a good indicator of energy content) increased with increasing egg size, but there were still differences between families after variation in egg size was accounted for. Incubation time was not correlated with egg size, and did not differ between families. The progeny from the different families utilized the available energy differently, as both longevity and growth-rate without food was independent of yolk-sac size, but strongly dependent on family. The observed between-family differences in early life history traits in rainbow trout were mainly caused by genetically based effects; egg size and thus probably egg quality, which differed strongly bet ween females, could not adequately explain these differences.     

Efficacy of injection vaccines against Flavobacterium psychrophilum in rainbow trout, Oncorhynchus mykiss (Walbaum)

Efficacy of mineral oil-based experimental injection vaccines against Flavobacterium psychrophilum were tested in rainbow trout, Oncorhynchus mykiss (Walbaum), under laboratory and field conditions. The vaccines consisted of formalin- or heat-inactivated whole bacterium cell preparations of two different serotypes (Fd and Th) or a combination of serologically different F. psychrophilum (Fd and/or Th and/or Fp(T);Th). Specific antibody responses against the bacterium in plasma and skin mucus were evaluated post-vaccination with enzyme-linked immunosorbent assay. Efficacy of the vaccinations was determined by challenge trials to F. psychrophilum with the vaccinated rainbow trout. Significantly higher antibody levels in plasma were detected in vaccinated fish compared with mock-vaccinated fish. Injection vaccination did not trigger specific antibody production in the skin mucus. Significantly higher survival of i.p. vaccinated fish compared with non-vaccinated fish was observed during the challenge. The results suggest that mineral oil-based injectable vaccines containing formalin- or heat-inactivated virulent cells of F. psychrophilum effectively triggered specific antibody production and protected the fish against bacterial cold water disease.     

Effects of skeleton photoperiods on melatonin secretion in the rainbow trout, Oncorhynchus mykiss (Walbaum)

Abstract Plasma melatonin levels were measured at hourly intervals in rainbow trout, Oncorhynchus mykiss (Walbaum), maitained under three different skeleton photoperiods: 8L:2D:2L:12D;8L:7D:2L:7D; and 8L:12D:2L:2D. Blood samples were taken before, during and after the 2-h light pulses. Melatonin levels increased rapidly to mean scotophase (dark period) values of 261 ± 7 pg/ml after the first light-dark transition and had returned to mean photophase (light period) values of 51 ± 2 pg/ml 30min after the end of the scotophase. Light pulses during the early, mid- and late scotophase elicited similar reductions in melatonin levels, followed by rapid increases after the light pulse, reaching previous scotophase values within 90 min. The inability of any of the light pulses to modulate or truncate melatonin secretion under a LD 8:16 photoperiod provides further support for the hypothesis that melatonin secretion in the rainbow trout is a direct response to darkness, and is not under endogenous circadian control as in other vertebrates examined.     

Effect of esterification on the absorption of astaxanthin in rainbow trout, Oncorhynchus mykiss (Walbaum)

Gastrointestinal and serum absorption of astaxanthin was studied in rainbow trout, Oncorhynchus mykiss (Walbaum) (217 ± 2 g) fed diets supplemented with either esterified astaxanthin (from Haematococcus pluvialis) or free astaxanthin (synthetic, as 8% w/w beadlets) at similar levels (50 mg kg^?1). After 56 days of feeding, there was a significant difference (P = 0.0582) between steady‐state serum astaxanthin concentrations for fish fed free (2.0 ± 0.3 μg mL^?1) or esterified astaxanthin (1.3 ± 0.1 μg mL^?1) at the 90% confidence level. However, following ingestion of a single meal supplemented with free or esterified astaxanthin, the rates of astaxanthin absorption into serum were not significantly different (P > 0.1) (0.8 ± 0.2 µg mL^?1 h^?1 and 1.0 ± 0.4 µg mL^?1 h^?1 respectively). In fish fed both free or esterified astaxanthin, higher absorption (P < 0.05) of astaxanthin by the ileal (0.8 ± 0.14 μg g^?1 and 0.9 ± 0.15 μg g^?1 respectively) compared with the posterior (0.2 ± 0.01 μg g^?1 and 0.3 ± 0.14 μg g^?1 respectively) intestine was recorded. This confirmed the role of the anterior intestine in carotenoid absorption. Non‐detectable levels of esters in digesta taken from the hind intestine suggest the anterior intestine is also the primary region for ester hydrolysis.