This is a questions and picture recently submitted by one of our readers. These marking are due to a pigment called melanin and are found typically in bass over 12 inches and most often in cold water.
These bass tend to be common in the Susquehanna River as well as a number of other Pennsylvania Rivers. They have also been reported in at least 11 other states.
According to the Pennsylvania Fish and Boat Commission, this condition has been reported since the 1980s and the cause is unknown. The spots appear to go away when the water temperature increases and the smallmouth bass appear to be otherwise healthy.
For more information, see this press release from 2012:
“The condition is commonly known as ‘blotchy bass’ or ‘black spot’ and has been documented occasionally in various Pennsylvania waters since as early as 1980,” said Andy Shiels, PFBC deputy director of operations. “In fact, in 1986, a nationwide survey found that the condition was present in 11 other states, including N.Y., Delaware and Maryland.”Source
There is some speculation that this change is due to a parasite, also known as “Black spot” but the lesions from this disease are smaller an there are small little cysts found in the tissue. When these marks are examined, there is pigment change.
There is also speculation that these can be related to hormonal effects on the bass but that is not likely to be the cause because there is definitely a seasonality to it. If this were due to hormones in the water, it would probably not be so prevalent in cold water as compared to summer.
If you have found these black markings on the smalllmouth bass in your state, please comment below and let us know where you fish!