Tuesday, 02 January 2024 / Published in Bass Fishing Articles, Smallmouth Bass

Smallmouth Melanosis

I remember catching my first smallmouth with black blotches on it in 2010, and this oddity has perplexed me ever since.

Never a worry, but catches like these have become more common to my boat in recent years. It only seems to occur on select inland fisheries too.

I’m most commonly finding these fish on the Wisconsin River system and its flowages. But I’ve also captured these blotchy bass on lakes, too.

At this time last year, I inquired about melanosis with Vilas County biologist, Eric Wegleitner. Unsure whether he could help tackle my questions, I was then referred to the WI-DNR veterinarians in Madison, WI for answers. Unfortunately, my inquiry received zero response from them.

I’ve caught plenty of smallmouths with this skin condition. What is melanosis and how do some fish develop it? What causes the pigment disorder?

So, I played investigation reporter.

Melanosis is a seldom-studied pigmentation disorder

Melanosis (botchy bass syndrome) is the condition when a skin cell is unable to regulate pigment production. Commonly, fish with the disorder have black, ink-like spots on their skin.

We’ve only caught smallmouths with this disorder. Most specimens are of adult fish, measuring between 14 to 18 inches length. It is unclear whether these black botches are produced at birth, or develop at some point later in life.

For many years, fisheries biologists thought it was a stress response.

According to the USGS, the only agency taking the initiative to study this subject extensively, pathologists have recently identified that melanosis is associated with a novel family of viruses known as Adomavirus.

Smallmouths with this skin hyper-pigmentation were first observed in the United States, on eastern rivers, during the 1980’s. At that time, it was speculated that the condition was caused by environmental contaminants and stressors (pollution, sun exposure, stress from spawning). However, it turns out this is much more, and likely genetic.

Skin cells on smallmouths are known to naturally change color depending on sunlight exposure, stress, time of year, water depth and turbidity. This is why smallmouths can change colors so quickly upon capture. Fish with melanosis are unable to regulate their pigmentation.

Is melanosis genetic, or a water quality / habitat issue? I’ve sometimes thought it could be a result from fish stress or over-competition amongst the population members.

If it is stress-induced, then my documentation correlates and supports this theory. Each of the fish I’ve documented have been caught during the spawn, and late fall wintering seasons. No question that fisheries are stressed during these times. Additionally, I also believe over-crowding and water temperature are factors as well.

Lastly, is melanosis a skin cancer like melanoma in humans? This could then be a skin cancer in wild fish.

Causes and Reactions

Melanosis can occur as a result of a fishery’s genetics, injury to fish, and poor water quality resulting in stress. It is not a disease, but a cellular deformity.

Due to the WI-DNR’s avoidance of my questions, I would presume they haven’t studied smallmouth bass tissue samples under the microscope.

Does melanosis occur in every fishery? To me, this seems to be more isolated, or region specific.

Smallmouth bass exhibiting melanosis are being observed in several fisheries throughout the country, and it appears to be increasing in frequency. Genetic melanosis would explain why we see certain regions of the country experience it more than others – and why in Wisconsin’s Northwoods I’m only catching these fish with regularity from one watershed (Wisconsin River) and three inland lakes (Plum, Star, Big Saint Germain). Captured from colder, and clearer waters, the black blotches become most evident due to better light penetration.

My unsuccessful interview with the WI-DNR concluded before ever starting, without receiving any answers. We should believe Wisconsin hasn’t taken the initiative to study this growing phenomenon, now occurring with regularity on specific state-wide waters, while other states and agencies have recently begun to do so.