Effects of Acute Stress on Blood Clotting and Yeast Killing by Phagocytes of Rainbow Trout

Abstract

The effects of different levels of acute stress on blood parameters in rainbow trout Oncorhynchus mykiss were examined. Experimental fish were exposed to a combination of stressors, including handling, confinement, and thermal shock, and were bled 30 s to 30 min later. Control fish were sampled quickly without intentional stress. Two activities of blood were investigated: (1) whole-blood clotting (coagulation) times and (2) the ability of heparinized plasma to influence the survival of yeast soaked in this plasma and subsequently exposed to trout phagocytes. The speed of blood clotting increased with increasing levels of stress, whereas blood from many resting fish failed to clot. Yeast incubated in plasma from stressed fish before exposure to pronephric phagocytes experienced a greater reduction in colony-forming units (cfus) than yeast incubated in plasma from resting fish. Different levels of stress did not induce significant differences in reduction of yeast cfus. These data support the idea that acute stress may actually stimulate primary defense mechanisms rather than suppress them. Furthermore, whole-blood clotting times seemed to be reliable indicators of the severity of stressors to which fish were exposed. We postulate that factors appearing in blood very quickly, such as catecholamines and specific plasma proteins, play important roles in both the killing activities of phagocytes and the blood-clotting process.     

Effects of Salinity on Yersinia ruckeri Infection of Rainbow Trout and Brown Trout

Abstract

Juvenile rainbow trout Oncorhynchus mykiss and brown trout Salmo trutta acclimated to freshwater or salinities of 9.0‰ or less were exposed to Yersinia ruckeri, the bacterial pathogen that causes enteric redmouth disease (ERM). Both species of fish were kept in the same recirculating systems after bacterial exposure. Rainbow trout mortality was significantly (P < 0.05) different in each salinity: 96.5% in freshwater, 89.5% in water of 1.1‰ salinity, 81.3% in 3.0‰ salinity, and 75.0% in 9.0‰ salinity (model SE = 1.0). All deaths occurred between 3 and 12 d after exposure to Y. ruckeri. Only 2.3% of brown trout in all salinities died, and differences among treatments were not significant. For both fish species, Y. ruckeri was isolated from liver, spleen, and trunk kidney of fish dying during this experiment, and lesions of rainbow trout were consistent with ERM. Yersinia ruckeri was not isolated from brown trout surviving for 21 d after bacterial exposure but was isolated from 3 of 24 surviving rainbow trout; a polymerase chain reaction assay detected the DNA of Y. ruckeri in 3 additional rainbow trout survivors. Neither the lesions of fish with ERM nor the percentage of surviving fish subclinically infected with Y. ruckeri was affected by salinity. Bacterial growth in vitro was not affected by low (≤9.0‰) salinity; however, bacterial adhesion to polystyrene was significantly reduced as salinity increased. Although mortality caused by Y. ruckeri was significantly lower for rainbow trout in water with slightly increased salinity, none of the salinities tested was effective in preventing serious losses caused by this pathogen in recirculating systems.     

Regulatory Effects of Water Temperature on Loma salmonae (Microspora) Development in Rainbow Trout

Abstract

Water temperature, a pivotal factor influencing interactions between teleosts and pathogens, was examined to determine its effects on the kinetics of xenoma formation and dissolution subsequent to experimental exposure of rainbow trout Oncorhynchus mykiss to the microsporidian gill pathogen Loma salmonae. The permissive water temperature range in which xenomas developed was between 9° and 20°C. Parasite development was arrested at temperatures outside this range, as indicated by the absence of visible xenomas among exposed fish. In addition, when these trout were subsequently moved to temperatures within the permissive range, xenomas failed to develop. Water temperature, within the permissive range, had no significant effect on either the number of xenomas that formed or the proportion of fish that developed xenomas following gastric intubation with a standard dose of spores. The relationship between water temperature and xenoma onset-time was best described (R ² = 88.3%) by polynomial regression analysis: onset = 320 ? 33.4T + 0.9547T ², where T is temperature (°C). Xenoma onset rate was also described through a modified degree-days model, yielding a predictive equation appropriate for use under conditions of fluctuating temperature. The thermal units, expressed as days × (°C above 7°C) necessary for xenoma onset were 298.6 on average. Xenoma dissolution rates, from the time of onset, also appeared to have a trend; more rapid dissolution occurred as temperatures increased. However, this trend correlated minimally with regression models.     

Branchial Ionic Flux Responses in Rainbow Trout to Chloramine-T after Acclimation to Different Levels of Water Hardness

Abstract

Whole-body unidirectional and net Na⁺ and Cl^– fluxes were determined in rainbow trout Oncorhynchus mykiss exposed to chloramine-T (9 mg/L) alone or in combination with 17 mmol NaCl/L (0.1 % weight per volume, w/v) after acclimation to either city of Ottawa tap water (TW: Na⁺ 0.121 ± 0.004 mmol/L; K⁺ 0.015 ± <0.001 mmol/L; Ca²⁺ 0.351 ± 0.004 mmol/L; Cl^– 0.141 ± 0.002 mmol/L) or artificial soft water (ASW: Na⁺ 0.043 ± 0.003 mmol/L; K⁺ 0.005 ± <0.001 mmol/L; Ca²⁺ 0.063 ± 0.002 mmol/L; Cl^– 0.026 ± 0.002 mmol/L). Preexposure fluxes revealed that ASW-acclimated fish had reduced rates of Na⁺ uptake and efflux compared with TW-acclimated fish. However, rates of Cl^– uptake among ASW-acclimated fish were comparable with those of TW-acclimated fish. Exposure to chloramine-T in TW resulted in significant increases in the Cl^– efflux, giving rise to a significant and negative net flux. Exposure of ASW-acclimated fish to chloramine-T caused a significant negative increase in the Na⁺ efflux and net flux but only a negative increase in the Cl^– net flux. The addition of 17 mmol NaCl/L (0.1% w/v) eliminated the negative net ionic fluxes, resulting in significant increases in influx. The combined use of chloramine-T with NaCl is a potential method of remediating ionic disturbances that may be caused by treating trout with chloramine-T.     

Blood Parameters and Immune Status of Rainbow Trout with Proliferative Kidney Disease

Abstract

Blood parameters, disease resistance, and the immune response were sequentially evaluated in rainbow trout Oncorhynchus mykiss with proliferative kidney disease (PKD). The fish were maintained under laboratory conditions, and the study group went through a full cycle of the disease. Hematological and serological changes occurred primarily in those fish with severe kidney lesions. Fish infected with the parasite that causes PKD demonstrated a greater resistance to bacterial challenge, and their immune responses were heightened when compared with those of uninfected fish. These data suggest that PKD alone is not a predisposing factor for secondary infections if the fish does not incur severe renal lesions.     

Physiological and Immunological Effects of Adjuvanted Aeromonas salmonicida Vaccines on Juvenile Rainbow Trout

Abstract

The effects of injectable vaccines against Aeromonas salmonicida on oxygen consumption, growth, kidney lysozyme activity, and anti-A. salmonicida plasma antibody titers of juvenile rainbow trout Oncorhynchus mykiss were examined. The vaccines were A. salmonicida bacterin only, bacterin adjuvanted with levamisole, bacterin in emulsified oil, microencapsulated bacterin, microencapsulated bacterin with muramyl dipeptide, microencapsulated bacterin with β-1,3-glucan, and microencapsulated bacterin with Vibrio anguillarum lipopolysaccharide (LPS). The greatest and broadest ranges of responses were caused by the microencapsulated bacterin with V. anguillarum LPS. Oxygen consumption rates and specific growth rates were significantly higher over the course of 1 month among fish treated with the LPS vaccine. These fish also maintained a higher anti-A. salmonicida plasma antibody titer and kidney lysozyme activity for a substantially longer period than fish receiving the other treatments.     

Plasma and Tissue Levels of Trimethoprim in The Rainbow Trout,Salmo Gairdneri,After Absorption From Fresh and Salt Water

This is a comparative study of uptake of trimethoprim from 1) fresh water, 2) salt water after 7 days of adaption and 3) salt water without previous adaptation. The rate and extent of absorption were found to vary significantly. The salt water adapted group reached a plasma concentration of approx. 1 µg/ml after 10 h, the unadapted salt water group after 24 to 48 h and the fresh water group did not reach this concentration. The results are discussed in relation to the non-ionic diffusion theory and to the alterations taking place in euryhaline species of fish during adaptation to salt water.     

牛磺酸对虹鳟仔鱼生长、体成分和免疫指标的影响

本试验研究了牛磺酸对虹鳟仔鱼生长、体成分和免疫指标的影响。试验共分4个处理,分别添加0、500、1 000和1 500 mg/kg牛磺酸,试验共进行12周。结果表明:与未添加牛磺酸组相比,12周时添加1 000 mg/kg组体重、体长和平均日增重显著下降(P<0.0 5),饵料系数极显著提高(P<0.0 1)。添加1 5 0 0 mg/kg组体脂肪显著下降(P<0.0 5)。4周时,添加1 000和1 500 mg/kg组提高超氧化物岐化酶(superoxide dismutase,SOD)和总抗氧化能力(P<0.05),但酸性磷酸酶却下降(P<0.05)。添加500(P<0.05)和1 500 mg/kg组(P<0.01)丙二醛也升高。添加牛磺酸各组碱性磷酸酶活性显著提高(P<0.05)。12周龄时,添加1 500 mg/kg组显著降低SOD水平(P<0.05),添加1 000和1 500 mg/kg组总抗氧化能力也显著下降(P<0.05),添加牛磺酸各组丙二醛、碱性磷酸酶和酸性磷酸酶显著下降(P<0.05)。本试验得出,添加500~1 500 mg/kg牛磺酸不能改善虹鳟仔鱼生长性能,但可提高4周时免疫力,但长时间添加(16周)却降低免疫力。     

茶多酚对虹鳟生长性能、生化指标和非特异性免疫指标的影响

本试验研究了不同水平茶多酚对虹鳟生长、肉质、血浆生化指标和免疫指标的影响.选取2日龄初始体重(152.23±4.90)g的虹鳟560尾,随机分为6组,分别饲喂添加0、25、50、100、500、1 000 mg/kg茶多酚饲料,每组3个重复,每个重复30尾鱼,试验进行8周.结果显示:与对照组相比,添加不同水平的茶多酚对虹鳟增重率、特定生长率和饵料系数未产生显著的影响(P>0.05),但显著提高肝体比和脏体比(P<0.05).除茶多酚25 mg/kg组外其它试验组鱼体水分含量显著降低(P<0.05),茶多酚1 000 mg/kg组显著降低鱼体粗脂肪含量(P<0.05),茶多酚25 mg/kg组显著升高鱼体粗蛋白质含量(P<0.05).随着茶多酚添加量的增加,谷草转氨酶(GOT)逐渐下降,除25 mg/kg组外各试验组均显著低于对照组(P<0.05);谷丙转氨酶(GPT)、总胆固醇(CHO)、低密度脂蛋白(LDL)和高密度脂蛋白(HDL)差异不显著(P>0.05).随着茶多酚从25 mg/kg提高到100 mg/kg,肝脏和肌肉中的SOD活性升高,茶多酚25 mg/kg组肌肉SOD和50 mg/kg组肝脏SOD显著升高(P<0.05);肌肉和血浆中MDA下降,25、100 mg/kg组肌肉和50、100 mg/kg组血浆MDA显著(P<0.05或P<0.01)下降;茶多酚50 mg/kg组肝脏、鳃中AKP、ACP活性与对照组相比显著升高(P<0.05),但500和1 000 mg/kg组却未表现出明显的作用.以上结果表明:添加50～100 mg/kg茶多酚可改善虹鳟非特异性免疫和肌肉品质;但茶多酚超过500 mg/kg对虹鳟未产生有益作用.     

Effects of a Prophylactic Chloramine-T Treatment on Growth Performance and Condition Indices of Rainbow Trout

Abstract

Chloramine-T has been commonly used as a therapeutic agent for the treatment of bacterial gill disease and related phenomena in the salmonid aquaculture industry, and it is now commonly used as a prophylactic agent to prevent diseases in commercial salmonid hatcheries. Rainbow trout Oncorhynchus mykiss (average weight 98 g) were exposed to chloramine-T at 10 mg/L for 1 h twice weekly (a commonly used regime in the aquaculture industry) for 11 weeks. Fish were fed ad libitum without feed wastage throughout the 11 -week trial. Body weight, specific growth rate, feed conversion index, and appetite were assessed at the end of weeks 3, 6, 9, and 11. Chloramine-T treatment was not associated with either clinical disease or mortality. However, by the end of the trial, growth (based on body weight) of treated fish was significantly suppressed (δ = 7.3%) compared with control fish. Growth suppression was attributed to a significant reduction (δ = 9.3%) of feed conversion efficiency in treated fish. Based on specific growth rates, chloramine-T had an early negative effect on growth. The effect was diminished in later weeks (although not completely lost), which suggests some degree of compensation by the fish to the chemical agent.     

Effects of Ferrous Fumarate on Growth Performance, Blood Biochemical Parameters and Trace Element Contents of Juvenile Rainbow Trout ( Oncorhynchus mykiss )

本试验旨在研究富马酸亚铁对虹鳟幼鱼生长性能、血液生化指标及微量元素含量的影响.选择初始体重为(89.2±0.2)g的虹鳟幼鱼360尾,随机分成6组,每组3个重复,每个重复20尾鱼,分别饲喂添加富马酸亚铁0、20、40、80、160、480 mg/kg(以铁计)的试验饲料,各组分别为D0、D20、D40、D80、D160和D480组,D0组为对照组.各组铁实测含量分别为62.6、79.5、99.6、139.3、215.2、538.4mg/kg,试验期60 d.结果表明:1)饲料中不同铁水平对虹鳟幼鱼生长性能无显著影响(P＞0.05).2)血红蛋白含量和红细胞数量呈现先上升后平稳的趋势;各组血细胞比容无显著差异(P＞0.05).3)各组全鱼、脊椎骨和肌肉铁含量随着饲料中铁水平的升高而升高,肝脏铁含量呈现先上升后平稳的趋势;各组全鱼锌含量无显著差异(P＞0.05),D0组脊椎骨和肌肉锌含量显著高于其他各组(P＜0.05),D0和D20组肝脏锌含量显著高于其他各组(P＜0.05);全鱼铜含量随饲料铁含量的升高而升高,各组脊椎骨铜含量无显著差异(P＞0.05),D0组肌肉铜含量显著低于其他各组(P＜0.05).4) D0组血清溶菌酶活性显著低于D20、D40、D80和D160组(P＜0.05),但与D480组无显著差异(P＞0.05);过氧化氢酶活性呈现先上升后下降趋势,D40组最高.由此可见,本试验条件下,以富马酸亚铁作为铁源,通过折线模型分析发现,以血红蛋白含量和肝脏铁含量为评价指标的虹鳟幼鱼对饲料中铁需要量分别为99.8和100.4 mg/kg.     

富马酸亚铁对虹鳟幼鱼生长性能、血液生化指标及微量元素含量的影响

本试验旨在研究富马酸亚铁对虹鳟幼鱼生长性能、血液生化指标及微量元素含量的影响。选择初始体重为（８９．２±０．２）ｇ的虹鳟幼鱼 ３６０尾,随机分成 ６组,每组 ３个重复,每个重复 ２０尾鱼,分别饲喂添加富马酸亚铁 ０、２０、４０、８０、１６０、４８０ｍｇ／ｋｇ（以铁计）的试验饲料,各组分别为 Ｄ０、Ｄ２０、Ｄ４０、Ｄ８０、Ｄ１６０和 Ｄ４８０组,Ｄ０组为对照组。各组铁实测含量分别为 ６２．６、７９．５、９９．６、１３９．３、２１５．２、５３８．４ｍｇ／ｋｇ,试验期６０ｄ。结果表明：１）饲料中不同铁水平对虹鳟幼鱼生长性能无显著影响（Ｐ＞０．０５）。２）血红蛋白含量和红细胞数量呈现先上升后平稳的趋势;各组血细胞比容无显著差异（Ｐ＞０．０５）。３）各组全鱼、脊椎骨和肌肉铁含量随着饲料中铁水平的升高而升高,肝脏铁含量呈现先上升后平稳的趋势;各组全鱼锌含量无显著差异（Ｐ＞０．０５）,Ｄ０组脊椎骨和肌肉锌含量显著高于其他各组（Ｐ＜０．０５）,Ｄ０和 Ｄ２０组肝脏锌含量显著高于其他各组（Ｐ＜０．０５）;全鱼铜含量随饲料铁含量的升高而升高,各组脊椎骨铜含量无显著差异（Ｐ＞０．０５）,Ｄ０组肌肉铜含量显著低于其他各组（Ｐ＜０．０５）。４）Ｄ０组血清溶菌酶活性显著低于 Ｄ２０、Ｄ４０、Ｄ８０和 Ｄ１６０组（Ｐ＜０．０５）,但与 Ｄ４８０组无显著差异（Ｐ＞０．０５）;过氧化氢酶活性呈现先上升后下降趋势,Ｄ４０组最高。由此可见,本试验条件下,以富马酸亚铁作为铁源,通过折线模型分析发现,以血红蛋白含量和肝脏铁含量为评价指标的虹鳟幼鱼对饲料中铁需要量分别为 ９９．８和 １００．４ｍｇ／ｋｇ。     

Gill Ectoparasites of Juvenile Rainbow Trout and Brown Trout in the Upper Colorado River

Abstract

During 1996 and 1997, 112 rainbow trout Oncorhynchus mykiss and 204 brown trout Salmo trutta, all young of the year, were sampled from a 40-km study area of the upper Colorado River and were examined for gill parasites. Ambiphrya, Chilodonella, Ichthyobodo, Apiosoma, Trichodina, Trichodinella, Tripartiella, Epistylis, and an unidentified cochliopodid amoeba were the representative protozoan genera observed on fish examined. Significant month–year–species interactions (P = 0.0295) were revealed, reflecting the changes in infestation prevalence among months, years, and species of salmonid. Greater ectoparasite richness was observed in downstream sections of the study area, most notably near Hot Sulphur Springs, Colorado. Peaks of infestation intensity and ectoparasite richness occurred in August and September of both years, presumably because of high mean water temperatures and low flows during that time.     

Experimental Infection of Rainbow Trout with Saprolegnia parasitica

Abstract

A method was developed to experimentally induce saprolegniasis in rainbow trout Oncorhynchus mykiss. The development of a reliable method to produce infected fish is essential to efforts to determine the efficacy of various antifungal treatments. Three methods for inducing saprolegniasis were evaluated in waters containing known concentrations of Saprolegnia parasitica zoospores. These methods included application of the following stressors to fish: (1) abrasion and dewatering, (2) water temperature increase, and (3) a combination of abrasion, dewatering, and temperature increase. Neither physical abrasion nor temperature increase stress alone was effective for inducing saprolegniasis. Only 25.9% of fish stressed by abrasion and dewatering alone became infected. Application of both abrasion and temperature stress, however, induced saprolegniasis in 77.8% of fish tested. Most of these fish became infected after 5 d of stress treatments. No fish became infected or died in the positive control group (not stressed but exposed to S. parasitica zoospores) or the negative control group (not stressed or challenged). This method should enable researchers to induce saprolegniasis in rainbow trout to study its pathogenesis or to test the efficacy of antifungal treatments. In conducting efficacy studies, it is important that therapeutic treatments begin promptly after the first signs of saprolegniasis are observed because the disease can progress very quickly and often results in mortality.     

Purification and Partial Characterization of a Rainbow Trout Egg Lectin

Abstract

A lectin that agglutinates rabbit red blood cells (RBCs) and human type B RBCs was isolated from ova of rainbow trout Oncorhynchus mykiss. Hemagglutination of rabbit RBCs was inhibited completely by 10 mM L-rhamnose but not by certain other sugars, 100 mM EDTA, or 100 mM 2-mercaptoethanol. Partial purification of this hemagglutinating material was achieved by affinity chromatography of an H₂O-dialyzed yolk homogenate on rhamnose-linked Sepharose. A polyacrylamide gel electrophoresis (SDS-PAGE) performed on this sample revealed two polypeptides with approximate molecular masses of 19 kilodaltons (kDa) and 30 kDa. By fast-phase liquid chromatography, proteins with a molecular mass of less than 20 kDa were separated from other elements of the affinity-purified hemagglutinating material. These proteins were found to lack hemagglutinating activity. When a western blot with rabbit antilectin antiserum was performed against yolk extract, rainbow trout serum, or yolk from larvae, a 30-kDa polypeptide was detected within all three samples. If the rainbow trout serum and egg lectins are the same molecule, then the biological function of the rainbow trout egg lectin may include host defense or perhaps a basic, homeostatic mechanism such as glycoprotein transport     

The Effects of Whirling Disease on Growth and Survival of Snake River Cutthroat and Colorado River Rainbow Trout Fingerlings

Abstract

Two sizes of fingerling Snake River cutthroat trout Oncorhynchus clarkii behnkei and Colorado River rainbow trout O. mykiss were raised at hatcheries testing negative for Myxobolus cerebralis and stocked into the Dolores and Cache la Poudre rivers from 1999 to 2001. Populations were resampled over a 2-year period to determine which species and size combination had the highest growth and survival rates. Fish were tested for M. cerebralis via polymerase chain reaction and pepsin?trypsin digest analyses. Growth and survival rates between the species and size groups were not significantly different in either river. In the Dolores River, annual survival for both species and sizes of fish combined ranged from 0.063 to 0.12. In the Cache la Poudre River, survival for both sizes of rainbow trout was 0.004; survival for cutthroat trout ranged from 0.182 to 0.53. Larger fish had higher growth rates than smaller fish, and cutthroat trout had higher rates than similar sizes of rainbow trout. In both rivers, a higher percentage of the rainbow trout sample was infected than in the cutthroat trout sample. Rainbow trout also had a higher mean number of spores per head than cutthroat trout, and small rainbow trout had higher spore counts than large rainbow trout. Survival rates for cutthroat trout in the Cache la Poudre River were the highest of any of the groups, suggesting a difference that is biologically significant. Raising fingerlings to sizes greater than 100 mm can improve poststocking survival. If rainbow trout are stocked into contaminated waters, raising fingerlings to a larger size does not appear to improve growth or survival rates. Stocking rainbow trout in the spring could maximize growth rates but will expose fish to greater triactinomyxon densities, resulting in higher intensities of infection.     

The Effects of Myxobolus cerebralis on the Physiological Performance of Whirling Disease Resistant and Susceptible Strains of Rainbow Trout

Abstract

The development of rainbow trout Oncorhynchus mykiss strains that are resistant to whirling disease has shown promise as a management tool for populations in areas where Myxobolus cerebralis is present. However, the physiological effects of the disease on characteristics necessary for fish survival in natural river conditions have not been tested in many of these strains. Five rainbow trout strains were evaluated for their swimming ability and growth characteristics in relation to M. cerebralis exposure: the resistant German rainbow trout (GR) strain (Hofer strain), the susceptible Colorado River rainbow trout (CRR) strain, and three intermediate (hybrid) strains (F₁ = GR × CRR; F₂ = F₁ × F₁; B₂ = backcross of F₁ × CRR). Three broad response patterns among strain and exposure were evident in our study. First, exposure metrics, growth performance, and swimming ability differed among strains. Second, exposure to the parasite did not necessarily produce differences in growth or swimming ability. Exposure to M. cerebralis did not affect batch weight for any strain, and critical swimming velocity did not differ between exposed and unexposed families. Third, although exposure did not necessarily affect growth or swimming ability, individuals that exhibited clinical deformities did show reduced growth and swimming performance; fish with clinical deformities were significantly smaller and had lower critical swimming velocities than exposed fish without clinical deformities. Research and management have focused on GR × CRR hybrid strains; however, given the performance of the GR strain in our study, it should not be discounted as a potential broodstock. Additional field trials comparing the GR and F₁ strains should be conducted before wholesale adoption of the GR strain to reestablish rainbow trout populations in Colorado.

Received September 9, 2010; accepted May 27, 2011     

Characteristics of Peripheral Blood Cells from Rainbow Trout Evaluated by Particle Counter,Image Analysis,and Hemocytometric Techniques

Abstract

Hematological assessments of peripheral blood cells from rainbow trout Oncorhynchus mykiss were compared by using a hemocytometer, image analysis, and an electronic particle counter. Although statistical differences were noted, the diameters of white blood cells (WBCs) were comparable among the methods used, but lengths and widths of red blood cells (RBCs) were remarkably different among the three methods. Red blood cells were significantly smaller when the particle counter was used compared with sizes obtained by using the hemocytometer or image analysis. Counts of whole blood and isolated RBC concentrations were similar between the hemocytometer and the particle counter in the automatic calculation mode; yet they were significantly different with the manual calculation method, which used the raw data from the particle counter. White blood cell counts from peripheral whole blood could not be obtained with the particle counter; it converted all particle volumes into spherical equivalents and, thus, interpreted rainbow trout RBCs and WBCs to be similar in size, resulting in an inability to accurately count the number of WBCs in mixed-cell populations. In addition, a lysing agent, which is commonly used to count WBCs in mammalian peripheral blood, did not completely remove all RBCs from samples of trout peripheral whole blood because of the presence of nuclei within the fish RBCs. These results indicate that assessing fish hematological characteristics with the particle counter may have limitations.     

Relation of Water Temperature to Bacterial Cold-Water Disease in Coho Salmon,Chinook Salmon,and Rainbow Trout

Abstract

The effect of water temperature on the progress of experimentally induced Cytophaga psychrophila infection was investigated in juveniles of coho salmon Oncorhynchus kisutch, chinook salmon O. tshawytscha, and rainbow trout O. mykiss (formerly Salmo gairdneri). A virulent strain of C. psychrophila was administered to fish by subcutaneous injection. Infected fish were held in tanks containing pathogen-free well water at temperatures ranging from 3 to 23°C. Mean times from infection to death of the fish were shortest at 12–15°C, which were the temperatures associated with the shortest time for doubling the population of this bacterium in vitro. Juvenile steelhead (anadromous rainbow trout) injected with viable C. psychrophila cells and held in 22°C water did not become diseased.     

Toxicity of Copper Sulfate to Flavobacterium psychrophilum and Rainbow Trout Eggs

Abstract

Tests were conducted to determine the concentrations of copper sulfate needed to kill Flavobacterium psychrophilum, the cause of bacterial coldwater disease, either in vitro or on Rainbow Trout Oncorhynchus mykiss eggs. For the in vitro test, a plastic strip dipped in a solution of F. psychrophilum was exposed for 15 min to copper sulfate solutions of 0, 1, 5, 10, 20, 35, 50, 75, or 100 mg/L. Bacteria were “too numerous to count” at concentrations ≤10 mg/L CuSO₄; significant reductions in prevalence relative to untreated controls were noted for concentrations ≥35 mg/L. However, CFUs were still observed at 50 and 75 mg/L (20% of plates with tryptone yeast extract salts media). No yellow-pigmented CFUs typical of F. psychrophilum were observed at 100 mg/L CuSO₄. For the in vivo test, eggs were exposed for 15 min to 100, 300, 500, and 700 mg/L CuSO₄ or 100 mg/L iodine (control). Survival to hatch was significantly lower at 500 (44.3 ± 15.2%, mean ± SD) or 700 mg/L CuSO₄ (1.7 ± 0.8%) than for controls treated with 100 mg/L iodine (93.6 ± 0.9%) or at copper sulfate concentrations ≤300 mg/L. The 15-min LD₅₀ and LD₁₀ for copper sulfate were 461 mg/L (95% confidence interval: 457–466 mg/L) and 259 mg/L (251–266 mg/L). The prevalence of yellow CFUs at 100 mg/L CuSO₄ (40.0%) was significantly higher than in untreated controls. Significant reductions in yellow CFUs were achieved using 300, 500, or 700 mg/L CuSO₄ (7.5, 2.5, or 0.0% of plates with CFUs, respectively) or 100 mg/L iodine (2.5%), relative to untreated control eggs. Overall, since the concentrations of copper sulfate required to eliminate F. psychrophilum were toxic to the eggs, copper sulfate is not recommended for coldwater disease control in Rainbow Trout eggs based on conditions and parameters in this study.

Received July 7, 2011; accepted March 17, 2013