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Largemouth Bass Virus

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Anglers urged to help prevent spread of bass virus

April 22, 2003

State resource officials today announced that Largemouth Bass Virus (LMBV) continues spread in southern Michigan lakes, and called upon anglers to help contain the disease and protect fish populations.

Largemouth Bass Virus is one of more than 100 naturally occurring viruses that affect fish, and is closely related to viruses found in frogs and other amphibians. Its origin and how it is spread are unknown. The virus is not known to infect humans, and infected fish are considered safe to eat. However, it is recommended that all fish be thoroughly cooked as general food safety rule.

The disease was first discovered in Michigan in the fall of 2000, by biologists from the Michigan and Indiana Departments of Natural Resources jointly investigating a die-off of largemouth bass in Lake George, on the Michigan-Indiana border near I-69. It was the furthest north that the virus had ever been detected in the United States.

The Department of Natural Resources began actively surveying lakes in Southern Michigan for LMBV in 2002. Based on these and earlier data, the virus has been confirmed in the following 9 of 19 lakes examined:

• Lake George, Branch County - found in 90% of 2000 samples but not detected in 2002 samples.
• Long Lake, Hillsdale County (near Camden) - found in 2001 samples
• Klinger Lake, St. Joseph County - found in 2001 samples
• Long Lake, St. Joseph County - found in 2001 samples
• Austin Lake, Kalamazoo County - found in 2002 samples
• Woodland Lake, Livingston County - found in 100% of 2002 samples
• Saddle Lake, Van Buren County - found in 2002 samples
• Lake Orion, Oakland County - found in 6.3% of 2002 samples
• Devils Lake, Lenawee County - found in 14.3% of 2002 samples

Michigan DNR Fish Production Manager Gary Whelan said LMBV appears to infect other fish species, including smallmouth bass, bluegill, and crappies, but has caused mortality to only largemouth bass. The disease typically kills large adult fish and usually causes mortality when fish are most stressed. Potential stressors include very hot weather, heavy angling pressure, and possibly aquatic weed or other treatments during very hot periods. Any measures that minimize stress on these fish will reduce the impact of the disease and mortality.

"The DNR cannot eradicate this virus or treat infected wild fish populations," Whelan said. "However, as we continue investigating this disease, we appreciate receiving reports of unusual fish mortalities."

Many largemouth bass mortalities reported in 2002 occurred from mid-July to mid-August, and some of these were likely LMBV related. However, many of the reports came in weeks after the mortalities, which is too long for confirmation of the disease.

Infected fish show few outward signs, although they may be lethargic, swim slowly, and are less responsive to activity around them. The virus has been found in many lakes with no reports of disease or fish mortalities.

Dying fish often are seen near the surface and have difficulty remaining upright. Upon internal examination, such fish usually will have bloated swim bladders, which accounts for the cause of swimming problems. Red sores or other lesions occasionally may be seen on the skin of the fish, but these are secondary in nature and not part of the virus infection.

Consistent with the recommendations reported from the Largemouth Bass Virus Workshops, sponsored by ESPN and BASS Federation, the DNR is calling on anglers who target largemouth bass to voluntarily help reduce angling stress on largemouth bass populations during warm weather. DNR Fisheries Division Chief Kelley Smith noted the DNR will again be monitoring lakes in central and southern Michigan this summer, in partnership with the Michigan BASS Federation.

"This is a new disease to northern lakes, and there is much for us to learn about how it works," Smith said. "For example, we still do not know how largemouth bass populations will be affected in Michigan's lakes on the long term. We urge all members of the angling community to continue to help us monitor our waters. When you see unusually high mortalities of adult largemouth bass, please contact one of our offices immediately so we can investigate the die-off. Further, we look forward to working with our partners at the Michigan BASS Federation, and appreciate their willingness to help us collect information necessary to better understand and manage this virus."

The DNR reminds anglers and boaters to take the following steps to help prevent the spread of LMBV:

• Clean boats, trailers, other equipment thoroughly between fishing trips to keep from transporting LMBV, as well as other undesirable pathogens and organisms, from one water body to another with special care to clean fishing equipment when you are done fishing known locations of the virus.
• Do not move fish or fish parts from one body of water to another, and do not release live bait into any water body. • Handle bass as gently as possible if you intend to release them and release them as quickly as possible.
• Refrain from hauling the fish for long periods in live wells if you intend to release them.
• Minimize targeting of largemouth bass during the period from mid-July to mid-August, especially during exceptionally hot weather conditions.
• Report dead or dying adult largemouth bass fish to Michigan Department of Natural Resources Fisheries Division offices.
• Volunteer to help agencies collect bass for LMBV monitoring.
• Educate other anglers about LMBV.

The Michigan DNR will continue to communicate any new information learned about the disease in Michigan. Interested anglers can get more information here.
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