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Bull trout (Salvelinus confluentus) are challenging to detect as a result of the species cryptic behavior and coloration, relatively low densities in complex habitats, and affinity for cold, high clarity, low conductivity waters. Bull trout are also closely associated with the stream bed, frequently conceal in substrate, and this concealment behavior is poorly understood. Consequently, population assessments are problematic and biologists and managers often lack quantitative information to accurately describe bull trout distributions, estimate abundance, and assess status and trends; particularly for stream-dwelling populations. During controlled laboratory trials, we recorded concealment, resting, and swimming behavior of juvenile wild bull trout in response to: (1) constant and fluctuating water temperature, (2) presence or absence of light, and (3) substrate size. Light level had the strongest influence on wild fish concealment and more fish concealed as light levels increased from darkness to daylight. Wild fish were 14.5 times less likely to conceal in constant darkness and 4.1 times more likely to conceal in 12 h light x 12 h darkness compared to constant light. Wild fish were 6.2 times less likely to conceal in small (26-51 mm) substrate compared to larger (52-102 mm) substrate. As water temperature increased, fewer wild fish concealed. Knowledge of wild bull trout concealment will improve field sampling protocols and increase detection efficiencies. These data also enhance knowledge of bull trout niche requirements which illuminates ecological differences among species and informs conservation and restoration efforts.

Virkon® Aquatic is a broad-spectrum disinfectant used in aquaculture as a disinfectant for footwear, nets, and equipment for protection from bacteria, viruses and one fungus. Recent studies provided data to support Virkon® Aquatic as a tool to kill invasive molluscs and other organisms in aquaculture and field settings. This study reports the efficacy of Virkon® Aquatic to kill New Zealand mudsnails Potamopyrgus antipodarum and quagga mussels Dreissena rostriformis bugensis from aquaculture facilities. To address the safety of Virkon® Aquatic used in aquaculture and field settings, the safety limits of Virkon® Aquatic to Steelhead trout Oncorhynchus mykiss were tested. Short-term emersion of fingerlings for up to 3.5 h in diluted concentrations of 30 mg/L showed no observable toxic effects. Smaller fry were somewhat less tolerant but survived exposures for up to 5 h in 10 mg/L. We conclude that the risks to fish are low if residues of the chemical remained on equipment, or if containers with disinfecting concentrations of 20 g/L were spilled into raceways or around fish holding systems. We suggest that this and additional studies may be helpful to support a label claim of this disinfectant for targeted invasive invertebrate species.

As the world's oldest and largest organization dedicated to the fisheries profession, the American Fisheries Society (AFS) has a unique opportunity to be an active agent of change for the fisheries...

We compared the hydrodynamics of replicate experimental mixed cell and replicate standard Burrows pond rearing systems at the Dworshak National Fish Hatchery, ID, in an effort to identify methods for improved solids removal. We measured and compared the hydraulic residence time, particle removal efficiency, and measures of velocity using several tools. Computational fluid dynamics was used first to characterize hydraulics in the proposed retrofit that included removal of the traditional Burrows pond dividing wall and establishment of four counter rotating cells with appropriate drains and inlet water jets. Hydraulic residence time was subsequently established in the four full scale test tanks using measures of conductivity of a salt tracer introduced into the systems both with and without fish present. Vertical and horizontal velocities were also measured with acoustic Doppler velocimetry in transects across each of the rearing systems. Finally, we introduced ABS sinking beads that simulated fish solids then followed the kinetics of their removal via the drains to establish relative purge rates. The mixed cell raceway provided higher mean velocities and a more uniform velocity distribution than did the Burrows pond. Vectors revealed well-defined, counter-rotating cells in the mixed cell raceway, and were likely contributing factors in achieving a relatively high particle removal efficiency-88.6% versus 8.0% during the test period. We speculate retrofits of rearing ponds to mixed cell systems will improve both the rearing environments for the fish and solids removal, improving the efficiency and bio-security of fish culture. We recommend further testing in hatchery production trials to evaluate fish physiology and growth.

Effective and economical tools are needed for treating ship ballast to meet new regulatory requirements designed to reduce the introduction of invasive aquatic species from ship traffic. We tested the efficacy of hydroxide stabilization as a ballast disinfection tool in replicated, sequential field trials on board the M/V Ranger III in waters of Lake Superior. Ballast water was introduced into each of four identical 1,320 L stainless steel tanks during a simulated ballasting operation. Two tanks were treated with NaOH to elevate the pH to 11.7 and the remaining two tanks were held as controls without pH alteration. After retention on board for 14–18 h, CO2-rich gas recovered from one of two diesel propulsion engines was sparged into tanks treated with NaOH for 2 h to force conversion of NaOH ultimately to sodium bicarbonate, thereby lowering pH to about 7.1. Prior to gas sparging, the engine exhaust was treated by a unique catalytic converter/wet scrubber process train to remove unwanted combustion byproducts and to provide cooling. The contents of each tank were then drained and filtered through 35-µm mesh plankton nets to collect all zooplankton. The composition and relative survival of zooplankton in each tank were evaluated by microscopy. Zooplankton populations were dominated by rotifers, but copepods and cladocerans were also observed. Hydroxide stabilization was 100% effective in killing all zooplankton present at the start of the tests. Our results suggest hydroxide stabilization has potential to be an effective and practical tool to disinfect ship ballast. Further, using CO2 released from the ship engine reduces emissions and the neutralized by product, sodium bicarbonate, can have beneficial impacts on the aquatic environment.

The American Fisheries Society (AFS) has routinely assessed the contributions of hatcheries to natural resource management and issued recommendations to guide natural resource managers in the best...

Researchers have cautioned against the borrowing of consumption and growth parameters from other species and life stages in bioenergetics growth models. In particular, the function that dictates temperature dependence in maximum consumption (Cmax) within the Wisconsin bioenergetics model for Chinook Salmon Oncorhynchus tshawytscha produces estimates that are lower than those measured in published laboratory feeding trials. We used published and unpublished data from laboratory feeding trials with subyearling Chinook Salmon from three stocks (Snake, Nechako, and Big Qualicum rivers) to estimate and adjust the model parameters for temperature dependence in Cmax. The data included growth measures in fish ranging from 1.5 to 7.2 g that were held at temperatures from 14°C to 26°C. Parameters for temperature dependence in Cmax were estimated based on relative differences in food consumption, and bootstrapping techniques were then used to estimate the error about the parameters. We found that at temperat...

The profiles of specific fatty acids (FA) in white muscle and liver of fasting steelhead trout Oncorhynchus mykiss were evaluated at three periods during their prespawning migration and at kelt emigration in the Snake-Columbia River of Washington, Oregon and Idaho, to improve the understanding of energy change. Twenty-seven FAs were identified; depletion of 10 of these was positively correlated in liver and white muscle of prespawning O. mykiss. To observe relative changes in FA content more accurately over sampling intervals, the lipid fraction of tissues was used to normalize the quantity of individual FA to an equivalent tissue wet mass. Saturated and monounsaturated FAs were depleted between upstream migration in September and kelt emigration in June, whereas polyunsaturated FAs were more conserved. Liver was depleted of FAs more rapidly than muscle. Three FAs were detected across all sampling intervals: 16:0, 18:1 and 22:6n3, which are probably structurally important to membranes. When structurally important FAs of O. mykiss are depleted to provide energy, physiological performance and survival may be affected.

workshop, collectively referred to as “Propagated Fishes in Resource Management (PFIRM).” Each of the previous cycles yielded a proceedings book (Fish Culture in Fisheries Management [Stroud 1986], Uses and Effects of Cultured Fishes in Aquatic Ecosystems [Schramm and Piper 1995], and Propagated Fishes in Resource Management [Nickum et al. 2004]), and most recently a guidance document, “Considerations for the Use of Propagated Fishes in Resource Management.” The so-called “PFIRM Considerations” guide, published by AFS in 2005 (Mudrak and Carmichael 2005), provided resource managers with general recommendations for decision making and bilitation, and restoration programs.

In anadromous salmonids, muscle tissue provides the primary source of energy to support spawning migrations and spawning. We examined changes in the lipid content, protein content, and energy density of white muscle collected from stream-maturing Snake River–Columbia River steelhead Oncorhynchus mykiss during several phases of reproduction. At a basinwide scale, we estimated that between early freshwater entry and postspawning (kelt) emigration, the lipid content of white muscle was reduced by 94% to levels less than 1% of wet tissue weight. Lipid was depleted more rapidly than protein during the reproductive cycle and afterward provided the only remaining somatic energy source for the postspawning migration. We found that protein content was consistently higher in sexually mature male steelhead than in females, suggesting that energy allocation prior to reproduction varies between the sexes. In kelts, the lipid content, protein content, and energy density of white muscle were significantly higher...

Management goals to improve the recovery of steelhead (Oncorhynchus mykiss) stocks at risk of extinction include increasing the proportion of postspawning fish that survive and spawn again. To be successful, postspawning steelhead (kelts) migrating downstream to the ocean must prepare physiologically and physically for a seawater transition. We sampled blood, gill filaments, and evaluated the external condition of migrating kelts from an ESA-listed population in the Snake/Columbia River system over two consecutive years to evaluate their physiological readiness for transition to seawater. We chose attributes often considered as measures of preparation for seawater in juveniles, including gill Na+,K+ ATPase activity, plasma electrolytes and hormones to consider factors related to external condition, size and sex. We found kelts in good external condition had plasma profiles similar to downstream-migrating smolts. In addition, we found more than 80% of kelts ranked in good external condition had smolt-like body silvering. We compared measures from migrating kelts with samples obtained from hatchery fish at the time of spawning to confirm that Na+, K+ ATPase activity in kelts was significantly elevated over spawning fish. We found significant differences in gill Na+, K+ ATPase activity in migrating kelts between the years of sampling, but little indication of influence of fish condition. We conclude that the postspawning steelhead sampled exhibited a suite of behaviours, condition and physiology characteristic of fish prepared for successful transition to a seawater environment.

Steelhead trout (Oncorhynchus mykiss) are anadromous and iteroparous, but repeat-spawning rates are generally low. Like other anadromous salmonids, steelhead trout fast during freshwater spawning migrations, but little is known about the changes that occur in vital organs and tissues. We hypothesized that fish capable of repeat-spawning would not undergo the same irreversible degeneration and cellular necrosis documented in semelparous salmon. Using Snake River steelhead trout as a model we used histological analysis to assess the cellular architecture in the pyloric stomach, ovary, liver, and spleen in sexually mature and kelt steelhead trout. We observed 38 % of emigrating kelts with food or fecal material in the gastrointestinal tract. Evidence of feeding was more likely in good condition kelts, and feeding was associated with a significant renewal of villi in the pyloric stomach. No vitellogenic oocytes were observed in sections of kelt ovaries, but perinu- cleolar and early/late stage cortical alveolus oocytes were present suggesting iteroparity was possible. We documented a negative correlation between the quan- tity of perinucleolar oocytes in ovarian tissues and fork length of kelts suggesting that larger steelhead trout may invest more into a single spawning event. Liver and spleen tissues of both mature and kelt steelhead trout had minimal cellular necroses. Our findings indicate that the physiological processes causing rapid senescence and death in semelparous salmon are not evident in steelhead trout, and recovery begins in fresh water. Future management efforts to increase itero- parity in steelhead trout and Atlantic salmon must consider the physiological processes that influence post-spawning recovery.

We studied the growth rate, downstream movement, and size of naturally produced fall Chinook Salmon Oncorhynchus tshawytscha subyearlings (age 0) for 20 years in an 8th-order river landscape with regulated riverine upstream rearing areas and an impounded downstream migration corridor. The population transitioned from low to high abundance in association with U.S. Endangered Species Act and other federally mandated recovery efforts. The mean growth rate of parr in the river did not decline with increasing abundance, but during the period of higher abundance the timing of dispersal from riverine habitat into the reservoir averaged 17 d earlier and the average size at the time of downstream dispersal was smaller by 10 mm and 1.8 g. Changes in apparent abundance, measured by catch per unit effort, largely explained the time of dispersal, measured by median day of capture, in riverine habitat. The growth rate of smolts in the reservoir declined from an average of 0.6 to 0.2 g/d between the abundance p...

New Zealand mudsnails Potamopyrgus antipodarum (NZMS) have been introduced into many continents and are easily transported live while attached to wading and other field gear. We quantified the relative attachment by different life stages of NZMS to felt, neoprene, and rubber-soled boots exposed to two densities of NZMS in experimental exposure totes. Attachment by NZMS occurred on boots of all surfaces, but the highest numbers of all life stages occurred on boots with felt surfaces. We found a 15–20-min bath application of 20 g/L Virkon Aquatic was a reliable tool to disinfect boot surfaces infested with NZMS and other aquatic invertebrates. Our studies support that spray application of this disinfectant was not reliable to provide complete mortality of attached adult NZMS or neonates. Wading gear surfaces exposed to repeated bath disinfections showed little deterioration. Our results provide strong evidence that bath disinfections with Virkon Aquatic are helpful to assure biosecurity in field an...

We explored the resilience of the invasive New Zealand mudsnail Potamopyrgus antipodarum to fluctuating winter freezing and near-freezing temperature cycles in laboratory tests. Our goal was to provide data to confirm field observations of mortality and presumed mortality in stream habitats with fluctuating freezing to near-freezing temperatures. We tested individuals from 2 locations with distinctly different thermal regimes and population densities. One location had low snail densities and water temperatures with strong diel and seasonal water variation. The other location had high snail densities and nearly constant water temperatures. Groups of individuals from both locations were tested in each of 3 laboratory-created diel thermal cycles around nominal temperatures of 0, 2, or 4°C. Mortality occurred in cycles around 0°C in both populations, and little to no mortality occurred at temperatures >0°C. Individuals from both sources held in diel 0°C cycles for 72 h showed 100% mortality. Our findi...

Dams can be equipped with a bypass that routes a portion of the fish that enter the turbine intakes away from the powerhouse into flumes, where they can be counted. Daily passage abundance can be estimated by dividing the number of fish counted in the bypass by the sampling rate and then dividing the resulting quotient by the collection probability (i.e., the proportion of the fish population passing the dam that is bypassed). We used multistate mark–recapture modeling to evaluate six candidate models for predicting the collection probabilities of radio-tagged subyearling fall Chinook salmon (n = 3,852) as a function of 1–2-d time periods (general model), four different combinations of outflow (i.e., the total volume of water passing the dam) and turbine allocation (i.e., the proportion of outflow directed through the turbines), and a null (intercept only) model. The best-fit model was the additive combination of turbine allocation and outflow, which explained 71% of the null deviance. Cross vali...

Groundwater scientists use fluorescein dyes to trace ground water resources that supply springs which may contain threatened or endangered mollusk species. To estimate risks of a commonly used groundwater tracer to the threatened Bliss Rapids snail Taylorcocha serpenticola we tested the toxicity of sodium fluorescein solutions to a surrogate species, the ashy pebblesnail Fluminicola fuscus. Trials were conducted in static 24-h exposures to several levels of treatment as well as controls with no sodium fluorescein at temperatures similar to those in the habitat of concern. We estimated 377 mg L-1 as the median lethal concentration for the ashy pebblesnails. Using these data, we concluded that risks to mollusks from the proposed groundwater testing were likely low.

The authors tested the efficacy of elevated partial pressures of CO2 to kill invasive New Zealand mudsnails. The New Zealand mudsnails were exposed to 100 kPa at three water temperatures, and the survival was modeled versus dose as cumulative °C-h. We estimated an LD50 of 59.4°C-h for adult and juvenile New Zealand mudsnails. The results suggest that CO2 may be an effective and inexpensive lethal tool to treat substrates, tanks, or materials infested with New Zealand mudsnails. Environ. Toxicol. Chem. 2012; 31: 1838–1842. © 2012 SETAC

Invasive New Zealand mudsnails ( Potamopyrgus antipodarum , NZMS) have infested freshwater aquaculture facilities in the western United States and disrupted stocking or fish transportation activities because of the risk of transporting NZMS to naive locations. We tested the efficacy of a gravity-fed, hydrocyclonic separation system to remove NZMS from an aquaculture water source at two design flows: 367 L/min and 257 L/min. The hydrocyclone effectively filtered all sizes of snails (including newly emerged neonates) from inflows. We modeled cumulative recovery of three sizes of snails, and determined that both juvenile and adult sized snails were transported similarly through the filtration system, but the transit of neonates was faster and similar to the transport of water particles. We found that transit times through the filtration system were different between the two flows regardless of snail size, and the hydrocyclone filter operated more as a plug flow system with dispersion, especially when transporting and removing the larger sized adult and juvenile sized snails. Our study supports hydrocyclonic filtration as an important tool to provide snail free water for aquaculture operations that require uninfested water sources.

Following the discovery of New Zealand mudsnails, Potamopyrgus antipodarum, in the Silver Creek watershed in Idaho, we investigated the distribution and dynamics of the snail populations over two years in field surveys. Despite extensive fishing and recreational activities in the watershed, the infestations appeared limited in extent. As with other published studies, densities of P. antipodarum were highest during summer months, but the distribution in Silver Creek was patchy. We found that near-to-below freezing winter water temperatures in localized reaches of the watershed were related to reduced populations or lack of detection. Distributions observed in winter were associated with regions of groundwater releases, or downstream of impoundments that dampened the temperature extremes observed in locations elsewhere in the watershed. We speculate that the population has remained restricted because thermal conditions are not conducive to yearround survival and growth. However, these relationships could be altered with watershed alterations or global climate change.

SUMMARY 1. Aggregations of fine sediments are a suitable proxy for the presence and abundance of Tubifex tubifex, one of the obligate hosts in the parasitic life cycle that causes salmonid whirling disease (Myxobolus cerebralis). 2. To determine and evaluate practical approaches to predict fine sediments (

To assess the risk of transferring alien, invasive New Zealand mudsnails (NZMS) Potamopyrgus antipodarum with shipments of live rainbow or steelhead trout Oncorhynchus mykiss, we conducted laboratory trials to quantify and determine the volitional ingestion of snails from the bottom of laboratory tanks. Approximately, 2000 snails were placed on the bottom of a test tank and groups of 10 fish were added to each tank. After 48 h, the fish were removed, euthanized and the snails in the gastrointestinal tract were counted, and the proportion of snails remaining in each tank was measured. We found that both rainbow trout and steelhead consumed NZMS, but rainbow trout consumed nearly twice the number of snails that were consumed by steelhead trout. Feeding fish a maintenance diet increased the total consumption of snails by fish to an average of 64.5 snails per rainbow trout. Our study suggests that management strategies that depurate fish without feed for 48 h to reduce the risk of transfer of NZMS are not viable in preventing the transport of snails if rearing waters contain snails.

Within their native range in western North America, resident redband trout Oncorhynchus mykiss gairdneri occupy stream habitat from high mountains to low desert. To better understand the temperature tolerance, growth, and stress physiology of native redband trout populations and compare the resilience and responses to reciprocal environments of stocks adapted to desert or montane conditions, we conducted controlled laboratory trials. We tested groups of age-0 progeny from naturally reproducing desert and montane fish stocks in temperature cycles that simulated the summer conditions typical in regional desert and montane stream habitats. The diel cycles ranged from 8°C to 16°C for montane treatments and from 18°C to 26°C for desert treatments, and our tests were repeated over 2 years. We evaluated survival, growth, feed efficiency, plasma cortisol, heat shock protein levels, and body proximate composition in samples of fish collected during and at the completion of the trials. All of the stocks te...

A native of Michigan, Bouck served in the U.S. Air Force as a survival instructor from 1953 to 1957. Following his military service, Bouck enrolled in Central Michigan University, graduating in 196...

We conducted laboratory trials to determine the transit time and survival of New Zealand mudsnails Potamopyrgus antipodarum in the gastrointestinal tract of rainbow trout Oncorhynchus mykiss. To assess the rate of snail passage, we force-fed groups of fish a known quantity of snails and then held them in tanks. At selected intervals we removed individual fish from the test tanks and recorded the number of snails, their condition (live or dead), and their location in the gastrointestinal tract (stomach, anterior intestine, and posterior intestine). Feces were removed from tanks and examined for live snails. We repeated evaluations of passage rate and snail survival to determine the effects of varying the number of snails ingested, fish size, snail size, and feeding a commercial diet to fish after snail ingestion. We plotted and modeled gut evacuation using a stochastic model for ordinal data to consider each test variable. Snail passage rates were faster in fish that were fed smaller snails. Surpr...

We used digital data sets, aerial photos and direct field observations in a geographical information system to evaluate the stream habitat in an Idaho watershed affected by agriculture. We found that the scale of the digital data sets affected the outcome of the assessment due to the presence of dewatered stream channels in the drainage. We analysed the spatial configuration of irrigation canals in the watershed to determine if the contemporary stream network connectivity could be attributed to human- caused or to natural hydrological processes. Many irrigation canals were significantly longer than would have been expected if these canals were constructed to capture water from the closest portion of the abandoned stream channels. Our findings provide evidence that some of these tributary streams had reaches that were likely ephemeral or intermittent at the time of canal construction. Our approach for assessing stream and irrigation network connectivity in pastoral and agricultural lands should aid managers in prioritizing the effective and appropriate reconnection efforts. Published in 2008 by John Wiley & Sons, Ltd.

Habitat assessments were conducted in an intermountain watershed at three spatial extents to explore ways to predict the presence of tubificid oligochaetes likely to support the parasite Myxobolus cerebralis, cause of salmonid whirling disease. Stream reaches with six different reach slope characteristics were selected using GIS. The aquatic habitat in 60 reaches selected at random was measured and classified into distinct habitat units. Within the habitat units, areas of microhabitat with depositional fine sediments were chosen, measured, and core samples were removed to characterize the sediments and benthic oligochaetes. Two tubificids, Tubifex spp. and Limnodrilus hoffmeisteri, were abundant and co-occurred in silt-clay and fine sand sediments in these habitats. Models were posed and tested to predict the presence and relative abundance of tubificids using habitat characteristics from the three spatial extents: reach, habitat unit, and microhabitat. At the reach extent, tubificids were associated with low-reach slope and with slow water habitats. Within habitat units, tubificids were associated with higher percentages of fine sediments and higher stream width:depth ratios. In microhabitat cores, the presence of silt-clay sediments was positively associated with higher average stream width:depth ratios. Since ecological relationships are often scale dependent and stream systems have a natural hierarchy, predictive habitat models such as these that use measures from several scales may help researchers and managers more efficiently identify and quantify aquatic communities at highest risk of infection by the M. cerebralis parasite.

Resource managers considering restoration and reconnection of watersheds to protect and enhance threatened populations of bull trout Salvelinus confluentus have little information about the consequences of bacterial kidney disease (BKD) caused by Renibacterium salmoninarum. To better understand the response of bull trout to R. salmoninarum challenge, we conducted several laboratory experiments at two water temperatures. The extent, severity, and lethality of BKD in bull trout were compared with those of similarly challenged lake trout S. namaycush, Arctic char S. alpinus, Chinook salmon Oncorhynchus tshawytscha, and rainbow trout O. mykiss. The lethal dose of bacterial cells necessary to induce 50% mortality (LD50) was 10-fold lower at the 15°C challenge than at the 9°C challenge. Of the species tested, bull trout were relatively resistant to BKD, Arctic char were the most susceptible among Salvelinus species, and Chinook salmon were the most susceptible among Oncorhynchus species tested. Mean ti...

We describe the resident heterotrophic aerobic microflora of the salmonid posterior intestine before, during, and after the administration of rations with erythromycin in a hatchery raceway environment. We compare the profiles of medicated Chinook salmon Oncorhynchus tshawytscha with those of control fish that were not fed erythromycin. The combined counts of bacteria and yeasts per gram of fish intestine originating from four upstream raceways ranged from 3.0 × 102 to 9.6 × 105 colony-forming units (CFU) over the study period. Yeasts were commonly identified in the gut, and abundances ranged from 0% to more than 80% of the CFU. Erythromycin therapy decreased the total microbial population and altered the bacterial diversity in the gut during treatment. The intestinal microbial populations in fish medicated with erythromycin increased rapidly after treatment ceased, and by 25 d after treatment the CFU were similar in samples from both medicated and control fish populations. Of 325 isolates from f...

We provide the computer program code to estimate pathogen prevalence and calculate confidence intervals for estimates based on maximum likelihood methods for the open-source statistical and graphics package R and a commercially licensed statistical package, the Statistical Analysis System (SAS). We correct a previously published SAS program to allow use of newer versions of the SAS software and provide a second SAS program that will work in either version of SAS. All of the programs allow users to calculate prevalence from any number of pooled samples representing different numbers of individuals, and two of these programs allow users to make estimates from data pools that are entirely test-positive or test-negative.

We tested the swimming endurance of juvenile bull trout Salvelinus confluentus, lake trout S. namaycush, Arctic char S. alpinus, and rainbow trout Oncorhynchus mykiss at 9°C and 15°C to determine whether sublethal infection from a moderate challenge of Renibacterium salmoninarum administered months before testing affected the length of time fish could maintain a swimming speed of 5–6 body lengths per second in an experimental flume. Rainbow trout and Arctic char swam longer in trials than did bull trout or lake trout, regardless of challenge treatment. When we tested fish 14–23 weeks postchallenge, we found no measurable effect of R. salmoninarum on the swimming endurance of the study species except for bull trout, which showed a mixed response. We conducted additional trials with bull trout 5–8 weeks postchallenge to determine whether increasing the challenge dose would affect swimming endurance and hematocrit. In those tests, bull trout with clinical signs of disease and those exposed to the hi...

We quantified and modeled the 96-h toxicity of commercially available injectable erythromycin, Erythro-200, to Atlantic salmon (Salmo salar), cutthroat trout (Oncorhynchus clarki), rainbow trout (O. mykiss) and spring chinook salmon (O. tshawytscha) in several trials to analyze the response within and across different temperatures and ages (sizes) of fish. Probit analyses of the dose response across these separate trials, including those conducted at different water temperatures, revealed no significant differences among the slopes of the mortality response for each species group. Median lethal doses (LD50) of erythromycin for the species and temperatures tested ranged from 350 to 1041 mg kg 1 . We provided a general dose-response model for injectable erythromycin in all test species and size groups, with the differences in probit intercepts modified by species, temperature and size of fish. The toxicity–body size relationship for injectable erythromycin fits an allometric equation with a body weight exponent of 0.79 to 0.80. We observed some mortality at high dosages of the injectable drug carrier (PEG-400, ethyl acetate and absolute ethanol) in all species tested and we estimated the LD50=4.71 ml kg 1 for yearling chinook salmon at 12 jC. D 2002 Elsevier Science B.V. All rights reserved.

In a hatchery field trial, we evaluated the efficacy and safety of two sequential injections of erythromycin phosphate in prespawning spring chinook salmon Oncorhynchus tshawytscha. For dosing purposes, we used fish fork length to estimate weight based on a weight−length relationship for spring chinook salmon. All fish in the hatchery population (N = 1,568) were injected with one of two doses of erythromycin phosphate (20 mg/kg body weight or 40 mg/kg), followed 1 month later by a second injection of 20 mg/kg. We monitored the survival of fish after injection and determined the concentrations of erythromycin and soluble antigen of Renibacterium salmoninarum in the kidneys of each fish that survived to spawning. After the first injection, mortality was significantly higher among female fish administered 40 mg/kg than among female fish administered 20 mg/kg. The concentration of erythromycin in the kidneys of females at spawning was significantly higher than that in the kidneys of males, as was the...

We used maximum likelihood methods to estimate an observed or apparent prevalence for a pathogen in a pooled sample of fish and here provide the program code for such calculations using commonly available statistical software. To illustrate the characteristics and variability of prevalence estimates from pooled samples, we explored the relationships among pathogen prevalence, sample size, and method of pooling samples. We calculated the average width of confidence intervals and the mean square error of the prevalence estimator for samples from populations with pathogen prevalence ranging from 1% to 90% using several pooling strategies for samples of 30 and 60 fish. As an illustration, we calculated the confidence interval and apparent prevalence of Myxobolus cerebralis in samples of fish from Utah screened with pooled sampling strategies. When all pools were positive, the apparent prevalence was 100%, but the bounds of the confidence interval ranged from 8% to 100%. Interpretations of data sets t...

We evaluated the toxicity of commercial injectable erythromycin (Erythro-200) and the injectable drug carrier alone (PEG-400, ethyl alcohol and ethyl acetate) in sub-adult spring chinook salmon, Oncorhynchus tshawytscha (Walbaum). Both compounds were toxic to salmon when administered in high dosages. The 50% lethal dose (LD50) of the erythromycin within its carrier (Erythro-200) at 12 °C was estimated at 429 mg erythromycin kg–1 or 2.145 mL kg–1 when measured by volume. The LD50 of the drug carrier alone was 2.95 mL kg–1 by volume or equivalent to 3.21 g kg–1 of the carrier. We evaluated and scored the histopathology in kidney sections removed from fish 96 h after injection with Erythro-200, the drug carrier alone, or sterile phosphate buffered saline (PBS) that served as a reference control. Kidneys from fish injected with saline had lowest average pathology scores; those injected with the carrier had moderate scores; and the pathology score was highest in tissues from erythromycin-injected fish. In tests at 12 °C, vacuoles were more prominent in proximal and distal tubules of fish injected with higher dosages of 403 and 545 mg erythromycin kg–1 than in tissues from fish injected with 299 mg kg–1.

We evaluated the severity and extent of infection by Myxobolus cerebralis in replicated groups of same-age Yellowstone cutthroat trout Oncorhynchus clarki bouvieri and Kamloops rainbow trout O. mykiss exposed to natural river conditions at four sites along a 25-km reach of the South Fork of the Boise River. Groups of fry were each exposed for 10 d in June, July, or August and then removed from the river and reared in specific-pathogen-free water under controlled laboratory conditions for 84 d. At the end of the study, surviving fish were examined for clinical signs of disease, and fish heads were evaluated by histology for the presence of characteristic pathology and then scored for the extent of pathology on a five-point scale. The mean histological scores and percent of fish testing positive in both species were similar within each site but varied across the study sites from no infection in fish at the uppermost site to 90–100% testing positive at the lowermost site. Cutthroat trout with modera...

Maturing chinook salmon Oncorhynchus tshawytscha were injected with 20, 60, 160, 435, or 665 mg erythromycin/kg body weight and held at 8°C or 13°C for 18 d to quantify the lethal dose (LD50) at two different water temperatures and to evaluate the health status of fish that survived. Mortality was highest in fish administered erythromycin at 13°C (LD50 = 369 mg/kg). Among fish that survived to the end of trials, the presence of yellow-colored livers and ascites was correlated positively with increasing drug dosage. Erythromycin residues in perfused tissues, in plasma, and in skin and muscle of surviving fish were significantly higher in fish held at 8°C than in fish at 13°C, but erythromycin in the ova increased with dosage regardless of water temperatures. Hematoxylin- and eosin-stained kidney tissues from surviving fish that were injected with erythromycin had significantly more necrosis and cloudy swelling in the tubules than did those of uninjected fish, and tissue damage and vacuolation were...

A new bioassay procedure was developed for the detection of erythromycin in aquaculture samples using a strain of a Stenotrophomonas as an indicator organism. Conventional disk-plate and well-plate radial diffusion assay procedures were developed, as well as a third procedure using the same indicator organism in Luria-Bertani (LB) broth, supplemented with the indicator dye Brilliant Black (40 μg/mL) in a multi-well microtiter plate. For both the disk-plate and well-plate radial diffusion assays, the response reflected in the size (width) of the growth inhibition zone, which was linear over the tested concentration range of 0.05 to 2.0 μg erythromycidml. The limit of quantitation of the bioassay was at 0.05 μg erythro-mycidml. Among the three methods of assay tested with Stenotrophomonas sp., the semi-quantitative dye reduction method is easy to read and is not diffusion dependent. This method allows for processing of more samples and more replication on a single titer plate. This new indicator organism is specific for erythromycin when tested in the presence of other antibacterial agents, i.e., oxytetracycline (Terramycin®) and/or Romet-Ma. This new bioassay procedure is suitable for quantitation of low concentrations of erythromycin in aquaculture water and sediment samples.

We conducted tests with maturing spring chinook salmon Oncorhynchus tshawytscha to determine the effects of 10, 20, or 40 mg erythromycin (Erythro-200)/kg body weight injected every 20 d for up to six times before sexual maturity. We examined the effect of repeated erythromycin injection on the survival and teratogenesis of progeny of fish surviving the trial. To provide controls, some fish were not injected with erythromycin and some fish were injected with the erythromycin carrier only. We detected no elevated total bilirubin in plasma samples from any groups of fish, although more than 70% of the erythromycin-injected fish surviving the trial had yellow-colored livers. Erythromycin was effective in significantly reducing the mortality from an unexpected outbreak from two bacterial pathogens, Renibacterium salmoninarum and Aeromonas salmonicida. Surviving fish injected with erythromycin had significantly lower levels of infection, determined by clinical and immunologic tests. Progeny from fish ...

A dose (10 mg/kg) of erythromycin phosphate or erythromycin base was injected intraperitoneally into maturing spring chinook salmon Oncorhynchus tshawytscha on one of three dates before sexual maturity to observe if differences in the concentrations of erythromycin remaining in tissues 14 d after injection could be attributed to the sex of fish, drug formulation, or timing of injection. No differences due to formulation of erythromycin or date of injection were detected in kidneys or spleens, but females injected with erythromycin phosphate had significantly higher levels of erythromycin/mL in the maturing ova than did fish injected with erythromycin base. Mean concentration of erythromycin in kidneys of fish was 1.6 μg erythromycin/g wet weight. Thirty-six of 37 females and 18 of 29 males had detectable erythromycin in the spleens. Only six fish had detectable erythromycin in the liver, and no muscle tissues collected had detectable erythromycin.

A two-day conference was convened to address the status of information about pathogens and diseases of free-ranging fish populations. In this introduction, we provide background information about the conference, and a summary of the presentations and panel discussions.