The largemouth bass virus has been found in two additional lakes in Michigan’s northern Lower Peninsula, following an initial discovery this past October in Cedar Lake (Iosco County). This time LMBV was found in smallmouth bass in Beaver Lake (Alpena County) during the investigation of a fish kill and in Avalon Lake (Montmorency County). These latest discoveries indicate the virus is spreading northward in Michigan.
Prior to this fall, the virus previously affected adult largemouth bass in the early 2000s in southern Michigan lakes. In those lakes the pathogen killed 10 to 20 percent of the larger adult largemouth bass at first exposure, with populations recovering in a few years. The virus has been found in other species including smallmouth bass, bluegill and black crappie in that part of the state. Bluegill and black crappie are known to be carriers but do not die from this virus.
“The largemouth bass virus likely compromised the immune system of smallmouth bass in Beaver Lake, causing secondary bacterial infections to become more lethal and allowed the virus to be a direct factor in the fish kill,” said Gary Whelan, the DNR Fisheries Division’s research manager. “Because these latest detections are at the northern edge of where LMBV has been found, we may see different responses than what was documented in southern Michigan.”
LMBV 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 methods of spreading are unknown, but anglers are considered a likely transmission path. Anyone moving live, infected fish from one body of water to another, or using contaminated or uncleaned gear or boats in uninfected waters could easily spread the virus. LMBV is not known to infect humans, and infected fish – if thoroughly cooked – is safe to eat.
LMBV usually causes fish kills during periods when fish are most stressed. Potential stressors include very hot weather, increased recreational fishing, and possibly aquatic weed or other treatments made during hot weather. Anything that can be done to minimize stress on fish will reduce the effects of this virus and subsequent fish deaths.
There are few outward signs that a fish has LMBV. The virus has been found in lakes where no disease or fish kills have been reported. Affected fish usually appear normal, although they may be lethargic, swim slowly and be less responsive to activity around them. Dying fish often are seen near the surface and have difficulty remaining upright. Upon internal examination, infected fish usually have bloated and yellowish swim bladders.
Largemouth bass virus can’t be eradicated from lakes and infected fish cannot be treated. The best way to halt the virus is by anglers and boaters properly cleaning their equipment and doing their part to prevent the spread. Simple steps include:
Cleaning all fishing equipment between trips.
Not moving fish or fish parts from one body of water to another.
Handling bass gently if you intend to release them.
Refraining from keeping bass in live wells for long periods of time if you plan to release them.
Minimizing the targeting of largemouth bass during very hot weather.
Reporting dead or dying adult largemouth bass, particularly when they’re in numbers of 25 or larger – reports can be made online at Michigan.gov/EyesInTheField.
