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Bacterial Coldwater Disease in Westslope Cutthroat Trout: Hatchery Epidemiology and Control

[Click to download 44 KB PDF] Final Report

Dates: 2002-2004

Background:
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Washoe Park Fish Hatchery in Anaconda, MT. Click on the photo for larger view and more info.

Bacterial coldwater disease, caused by the gram-negative bacterium Flavobacterium psychrophilum, is a septicemic infection that originated in the northwestern United States and is responsible for significant losses of hatchery-reared salmonids worldwide. It is especially problematic at hatcheries rearing fish for native species restoration, because the wild, undomesticated strains used for restoration are prone to stress in the hatchery environment and stress induces the disease. Currently, bacterial coldwater disease is the only fish disease found in State fish hatcheries in Montana. The Washoe Park State Fish Hatchery in Anaconda, Montana, is the only facility in the state that produces westslope cutthroat trout ( Oncorhynchus clarki lewisi) suitable for restoration programs. However, the severity of bacterial coldwater disease at this hatchery limits fish production and therefore implementation of the westslope cutthroat trout restoration program. Losses typically occur in three episodes among young fish from shortly after swim-up until they reach a length of about 75 millimeters. The first outbreak occurs in indoor raceways at first feeding, the second about three weeks later also indoors, and the third about three weeks after the juvenile fish are moved to outdoor raceways. In aggregate, the three outbreaks can achieve losses of about 30 to 45 percent annually. Successful control of bacterial coldwater disease at Washoe Park State Fish Hatchery would therefore facilitate restoration of native westslope cutthroat trout in Montana.

Project Goal:

To better understand the ecology of F. psychrophilum in hatcheries and thereby facilitate development and testing of control measures, eventually leading to increased and enhanced restoration efforts.

Project Objectives:
1. To determine where the pathogen occurred in the hatchery.
2. To determine avenues of transmission within the hatchery.
3.

To determine what factors cause disease outbreaks.


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Water tower with plastic koch ring.
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Female broodfish with bacterial coldwater disease.
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Inoculating agar plate to test for the presence of Flavobacterium psychrophilum.
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Collecting a sample from a cutthroat trout brain to determine the presence of Flavobacterium psychrophilum.
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Warm water spring source to the Washoe Park State Hatchery.
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Collecting a sample from the spring source to test for the presence of Flavobacterium psychrophilum.

Abstract:

Bacterial coldwater disease, caused by the gram-negative bacterium Flavobacterium psychrophilum, is responsible for significant losses of hatchery-reared salmonids worldwide. We used Washoe Park State Fish Hatchery in Anaconda, Montana, as a case study to enhance understanding of the disease in a hatchery setting and to develop practical hatchery-management strategies to better control the pathogen and the disease. Washoe Park typically loses 30 to 45 percent of its westslope cutthroat trout (Oncorhynchus clarki lewisi) production to the disease annually. Our objectives were to determine where the pathogen was located in the hatchery system, how it was transmitted, and what factors caused disease outbreaks. We found the bacterium in the warm-spring water source, in the degassing water tower, and in production and broodstock fish. It was transmitted both horizontally and vertically, with both male and female parents passing the pathogen on to their offspring. Transmission from females was vertical only, but both horizontal and vertical transmission from males occurred. Iodine surface-disinfection post-fertilization eliminated the pathogen from egg surfaces, thereby limiting horizontal transmission. Chronic and mild acute stress did not result in disease outbreaks, but a combination of acute stress events associated with moving juvenile production fish from indoor to outdoor raceways did. These fish harbored the pathogen, primarily in cranial tissues, prior to the outbreak. Measures resulting from our findings implemented at Washoe Park to reduce horizontal transmission included cleaning and sterilization of hatchery structures and iodine surface-disinfection of eggs post-fertilization. Eradication of the pathogen from the hatchery is unlikely, but efforts to reduce the frequency and intensity of stress events should reduce the frequency of disease outbreaks; management to reduce the number of fish carrying the pathogen may minimize losses during outbreaks.

Completion Date and Products
A detailed final report was produced in June of 2004 (see top of page to download).

Need for Further Study
Hatchery personnel are currently (2004) attempting to reduce prevalence of the pathogen in male broodstock through oxytetracycline and fluorofenicol injections and oxytetracycline-coated feed. These actions are aimed at reducing vertical transmission. Sperm washing techniques, as commonly used in animal husbandry of domestic mammals to eliminate pathogens, may warrant future investigation also.


Click here to read the MSU University News article, "Researcher battles roads and deadly trout disease"


Principal Investigators:
Eileen K. N. Ryce and Alexander V. Zale
Montana Cooperative Fisheries Research Unit
Department of Ecology
Montana State University
Bozeman MT 59717

 

 
Updated: June 23, 2006
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